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HENRY  HOLT  &  CO. 

29  West  23d  Street,  New  York  378  Wabash  Avenue,  Chicago 

(i,  1900) 


AMERICAN  SCIENCE  SERIES— ADVANCED   COURSE 

THE  HUMAN  BODY 

AN   ACCOUNT  OF 

ITS  STRUCTURE  AND  ACTIVITIES 

AND  THE  CONDITIONS  OF  ITS 

HEAITIIY  WORKING 


BY 

H.  NEWELL  MARTIN,  D.Sc,  M.A.,  M.D.,  F.R.S. 

Late  Professor  of  Biology  in  the  Johns  Hopkins  University 

and  of  Physiology  in  the  Medical  Faculty 

of  the  same 


EIGHTH    EDITION.  REVISED 


NEW   YORK 

IIKNWY    EOLT    AND   COMPANY 

1  898 


Copyright,  1881,  1836, 

BY 

HENRY    HOLT   &    CO. 


PUBLISHEES'   NOTE. 

The  Appendix  on  Keproduction  of  the  earlier  editions 
appears  in  this  as  Chapter  XXXIX.  Copies  of  the  book 
without  the  chapter  can  be  had  when  specially  ordered. 


PREFACE  TO  THE  SEVENTH  EDITION. 


This  edition  has  been  very  thoroughly  worked  over  and, 
I  trust,  improved.  A  considerable  amount  of  new  matter 
has  been  added,  especially  in  connection  with  the  cardiac 
and  vascular  nerves,  and  the  physiology  of  the  brain;  but 
throughout  the  whole  book  many  paragraphs  have  been  re- 
Avritten;  and  many  corrections,  rendered  necessary  by  the 
discoveries  of  the  last  three  or  four  years,  have  been  made. 
I  hope  therefore  that  the  edition  will  be  found  as  well  up  to 
date  as  it  is  possible  for  a  text-book  to  be:  for  a  text-book 
must  always  incline  to  the  conservative  side,  and  deal  with 
well-established  facts  rather  than  with  even  the  most  fasci- 
nating novelties.  Still,  as  in  previous  editions,  I  have  tried 
to  show  where  the  outposts  and  the  outlooks  of  Physiology 
are. 

H.  X.  M. 

May  1>  1896. 


PEEFACE   TO   THE  FIKST   EDITION. 


In  the  following  pages  I  have  endeavored  to  give  an 
account  of  the  structure  and  activities  of  the  Human  Body, 
which,  while  intelligible  to  the  general  reader,  shall  be  accu- 
rate, and  sufficiently  minute  in  details  to  meet  the  require- 
ments of  students  who  are  not  making  Human  Anatomy  and 
Physiology  subjects  of  special  advanced  study.  Wherever  it 
seemed  to  me  really  profitable,  hygienic  topics  have  also  been 
discussed,  though  at  first  glance  they  may  seem  less  fully 
treated  of  than  in  many  School  or  College  Text-books  of 
Physiology.  Whoever  will  take  the  trouble,  however,  to 
examine  critically  what  passes  for  Hygiene  in  the  majority  of 
such  cases  will,  I  think,  find  that,  when  correct,  much  of  it  is 
platitude  or  truism:  since  there  is  so  much  that  is  of  impor- 
tance and  interest  to  be  said  it  seems  hardly  worth  while  to 
occupy  space  with  insisting  on  the  commonplace  or  obvious. 

It  is  hard  to  write  a  book,  not  designed  for  specialists, 
without  running  the  risk  of  being  accused  of  dogmatism,  and 
some  readers  will,  no  doubt,  be  inclined  to  think  that,  in 
several  instances,  I  have  treated  as  established  facts  matters 
which  are  still  open  to  discussion.  General  readers  and 
students  are,  however,  only  bewildered  by  the  production  of 
an  array  of  observations  and  arguments  on  each  side  of  every 
question,  and,  in  the  majority  of  cases,  the  chief  responsi- 
bility under  which  the  author  of  a  text-book  lies  is  to  select 
what  seem  to  him  the  best  supported  views,  and  then  to  state 
them  simply  and  concisely:  how  wise  the  choice  of  a  side  has 
been  in  each  case  can  only  be  determined  by  the  discoveries 
of  the  future. 

Others  will,  I  am  inclined  to  think,  raise  the  contrary 
objection   that  too  many  disputed    matters    have    been  dis- 


VI  PREFACE  TO   THE  FIRST  EDITION. 

cussed:  this  was  deliberately  done  as  the  result  of  an  experi- 
ence in  teaching  Physiology  which  now  extends  over  more 
than  ten  years.  It  would  have  been  comparatively  easy  to 
slip  over  things  still  uncertain  and  subjects  as  yet  unin- 
vestigated, and  to  represent  our  knowledge  of  the  workings 
of  the  animal  body  as  neatly  rounded  off  at  all  its  contours 
and  complete  in  all  its  details — tutus,  teres,  et  rotundus. 
But  by  so  doing  no  adequate  idea  of  the  present  state  of 
physiological  science  would  have  been  conveyed;  in  many 
directions  it  is  much  farther  travelled  and  more  completely 
known  than  in  others;  and,  as  ever,  exactly  the  most  inter- 
esting points  are  those  which  lie  on  the  boundary  between 
what  Ave  know  and  what  we  hope  to  know.  In  gross  Anatomy 
there  are  now  but  few  points  calling  for  a  suspension  of  judg- 
ment; with  respect  to  Microscopic  Anatomy  there  are  more; 
but  a  treatise  on  Physiology  which  would  pass  by,  unmen- 
tioned,  all  things  not  known  but  sought,  would  convey  an 
utterly  unfaithful  and  untrue  idea.  Physiology  has  not  fin- 
ished its  course.  It  is  not  cut  and  dried,  and  ready  to  be 
laid  aside  for  reference  like  a  specimen  in  an  Herbarium,  but 
is  comparable  rather  to  a  living,  growing  plant,  with  some 
stout  and  useful  branches  well  raised  into  the  light,  others 
but  part  grown,  and  many  still  represented  by  unfolded  buds. 
To  the  teacher,  moreover,  no  pupil  is  more  discouraging  than 
the  one  who  thinks  there  is  nothing  to  learn;  and  the  boy 
who  has  "  finished  "  Latin  and  "  done  "  Geometry  finds  some- 
times his  counterpart  in  the  lad  who  has  "  gone  through  " 
Physiology.  For  this  ttnfortunate  state  of  mind  many  Text- 
books are,  I  believe,  much  to  blame:  difficulties  are  too  often 
ignored,  or  opening  vistas  of  knowledge  resolutely  kept  out  of 
view:  the  forbidden  regions  may  be,  it  is  true,  too  rough  for 
the  young  student  to  be  guided  through,  or  as  yet  pathless 
for  the  pioneers  of  thought;  but  the  opportunity  to  arouse 
the  receptive  mental  attitude  apt  to  be  produced  by  the  rec- 
ognition of  the  fact  that  much  more  still  remains  to  be  learned 
— to  excite  the  exercise  of  the  reasoning  faculties  upon  dis- 
puted matters — and,  in  some  of  the  better  minds,  to  arouse 
the  longing  to  assist  in  adding  to  knowledge,  is  an  inesti- 
mable advantage,  not  to  be  lightly  thrown  aside  through  the 
desire  to  make  an  elegantly  symmetrical  book.  While  I 
trust,  therefore,  that  this  volume  contains  all  the  more  impor- 
tant facts  at  present  known  about  the  working  of  our  Bodies, 


PREFACE  TO   THE  FIRST  EDITION.  vil 

I  as  earnestly  hope  that  it  makes  plain  that  very  much  is  yet 
to  be  discovered. 

A  work  of  the  scope  of  the  present  volume  is,  of  course, 
not  the  proper  medium  for  the  publication  of  novel  facts; 
but,  while  the  "  Human  Body,"  accordingly,  professes  to  be 
merely  a  compilation,  the  introduction  of  constant  references 
to  authorities  would  have  been  out  of  place.  I  trust,  how- 
ever, that  it  will  be  found  throughout  imbued  with  the  influ- 
ence of  my  beloved  master,  Michael  Foster;  and  on  various 
hygienic  topics  I  have  to  acknowledge  a  special  indebtedness 
to  the  excellent  series  entitled  Health  Primers. 

The  majority  of  the  anatomical  illustrations  are  from 
Henle's  Anatomie  des  Mensclien,  and  a  few  from  Arendt's 
Schulatlas,  the  publishers  of  each  furnishing  electrotypes. 
A  considerable  number,  mainly  histological,  are  from  Quain's 
Anatomy ,  and  a  few  figures  are  after  Bernstein,  Carpenter, 
Frey,  Haeckel,  Helmholtz,  Huxley,  McKendrick,  and  Wundt. 
About  thirty,  chiefly  diagrammatic,  were  drawn  specially  for 
the  work. 

Quantities  are  throughout  expressed  first  on  the  metric  sys- 
tem, their  approximate  equivalents  in  American  weights  and 
measures  being  added  in  brackets, 

H.  Newell  Martin. 

Baltimore,  October,  1880. 


CONTENTS. 


CHAPTER  I. 

THE  GENERAL  STRUCTURE  AND  COMPOSITION  OF  THE 
HUMAN  BODY. 

PAGE 

Definitions.  Tissues  and  organs.  Histology.  Zoological  position  of 
man.  The  vertebrate  plan  of  structure.  The  mammalia. 
Chemical  composition  of  the  Body 1 

CHAPTER  II. 

THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS, 

The  properties  of  the  living  Body.  Physiological  properties.  Cells. 
Cell  division.  Indirect,  karyokinetic  or  mitotic  cell  division. 
Assimilation ;  reproduction.  Contractility.  Irritability.  Con- 
ductivity. Spontaneity.  Protoplasm.  The  fundamental  physi- 
ological properties „     15 

CHAPTER  III. 

THE  DIFFERENTIATION  OF  THE   TISSUES  AND  THE 
PHYSIOLOGICAL  DIVISION  OF  EMPLOYMENTS. 

Development.  The  physiological  division  of  labor.  Classification 
of  the  tissues.  Undifferentiated  tissues.  Supporting  tissues. 
Nutritive  tissues.  Storage  tissues.  Irritable  tissues.  Coordi- 
nating and  automatic  tissues.  Motor  tissues.  Conductive 
tissues.  Protective  tissues.  Reproductive  tissues.  Organs. 
Physiological  mechanisms.  Anatomical  systems.  The  Body 
as  a  working  whole 29 

CHAPTER  IV. 

THE   INTERNAL    MEDIUM. 

The  external  medium.  The  internal  medium.  The  blood.  The 
lymph.  Histology  of  blood.  Blood  crystals.  Histology  of 
lymph 40 


X  CONTENTS. 

CHAPTER  V. 
THE  CLOTTING   OF  BLOOD. 

PAG  ft 

Coagulation  of  the  blood.  Cause  of  coagulation.  Whipped  blood. 
The  buffy  coat.  The  source  of  blood  fibrin.  Artificial  clot. 
Fibrin  ferment.  Proximate  causes  of  normal  blood  coagulation. 
Relation  of  the  blood-vessels  to  coagulation.  Chemical  com- 
position of  the  blood.  Quantity  of  blood.  The  life-history  of 
the  blood-corpuscles.     Chemical  composition  of  lymph 51 

CHAPTER  VI. 

THE   SKELETON. 

Exoskeleton  and  endoskeleton.  The  bony  skeleton.  Segmentation 
of  the  skeleton.  Homologies  of  the  bones  of  the  anterior  and 
posterior  limbs.     Peculiarities  of  the  human  skeleton 63 

CHAPTER  VII. 

THE  STRUCTURE  AND  COMPOSITION  OF  BONE.    JOINTS. 

Gross  structure  of  the  bones.  Microscopic  structure  of  bone.  Chem- 
ical composition  of  bone.  Articulations.  Joints.  Hygiene  of 
the  joints 85 

CHAPTER  VIII. 

CARTILAGE  AND  CONNECTIVE  TISSUE. 

Temporary  and  permanent  cartilages.  Varieties  of  cartilage.  The 
connective  tissues.  Elastic  cartilage  and  fibro-cartilage.  Ho- 
mologies of  the  supporting  tissues.  Hygiene  of  the  developing 
skeleton.     Adipose  tissue 98 

CHAPTER  IX. 

THE  STRUCTURE  OF  THE  MOTOR  ORGANS. 

Motion  in  animals  and  plants.  Amoeboid  cells.  Ciliated  cells.  The 
muscles.  Histology  of  striated  muscle.  Structure  of  un- 
striated  muscular  tissue.  Cardiac  muscular  tissue.  The  chem- 
istry of  muscular  tissue.  Beef-tea  and  Liebig's  extract 100 

CHAPTER  X. 

THE  PROPERTIES  OF  MUSCULAR  TISSUE. 

Contractility.  Irritability.  A  simple  muscular  contraction.  Phys- 
iological tetanus.  Causes  affecting  degree  of  contraction. 
Measure  of  muscular  work.  Muscular  elasticity.  Electrical 
currents  of  muscle.  Secondary  contraction.  Secondary  tetanus. 
Source  of  muscular  energy.  Physiology  of  plain  muscular 
tissue 127 


CONTENTS.  xi 

CHAPTER  XI. 
MOTION  AND  LOCOMOTION.    HYGIENE  OF  MUSCLES. 

PAGE 

Special  physiology  of  the  muscles.  Levers  in  the  Body.  Postures. 
Walking.  Running.  Hygiene  of  muscles.  Exercise.  Train- 
ing  


144 


CHAPTER  XII. 

ANATOMY   OF   THE  NERVOUS  SYSTEM. 

Nerve-trunks.  Nerve-centres.  Cerebro-spinal  centre  and  its  mem- 
branes. Spinal  cord.  Spinal  nerves.  Brain.  Cranial  nerves. 
Sympathetic  system.  Sporadic  ganglia.  Histology  of  nerve- 
fibres.  Histology  of  nerve-cells.  Neuroglia.  Histology  of 
spinal  cord.     Structure  of  a  spinal  ganglion 158 

CHAPTER  XIII. 

THE  GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM. 

The  properties  of  the  nervous  system.  Functions  of  nerve-centres 
and  nerve-trunks.  Excitant  and  inhibitory  nerves.  Classifica- 
tion of  nerve-fibres.  Electrical  phenomena  of  nerves.  Stimuli 
of  nerve-fibres.  General  nerve  stimuli.  Special  nerve  stimuli. 
Specific  nerve  energies.  Proof  that  all  nerve-fibres  are  physio- 
logically alike.  The  nature  of  a  nervous  impulse.  Rate  of  trans- 
mission of  a  nervous  impulse.  Functions  of  special  nerve  roots. 
Cranial  nerves.  Intercommunication  of  nerve-centres.  Degen- 
eration of  nerve-fibres  when  separated  from  their  centre 186 

CHAPTER  XIV. 
THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS. 
General  statement.  Position  of  heart.  Membranes  of  heart.  Anat- 
omy of  heart.  Valves  of  heart.  The  arterial  system.  Aorta 
and  its  branches.  The  capillaries.  The  veins.  The  pulmo- 
nary circulation.  The  portal  circulation.  Arterial  and  venous 
blood.  Structure  of  the  arteries,  of  the  capillaries,  and  of  the 
veins 211 

CHAPTER  XV. 
THE   WORKING   OF  THE  HEART  AND  BLOOD-VESSELS. 
The  beat  of  the  heart.     Cardiac  impulse.     Use  of  papillary  muscles. 
Sounds  of  the  heart.     A  cardiac  cycle.     Work  done  by  the  heart. 
The  circulation  in  the  blood-vessels.     Conversion  of  intermit- 
tent into  a  continuous  Mow 221! 

CHAPTER  XVI. 
ARTERIAL  PRESSURE.     THE  PULSE. 
Weber's  schema.     Arterial  pressure.     The  pulse.     The  rate  of  the 
blood  how.     Secondary  causes  of  the  circulation.     Aspiration 
of  the  thorax.     Proofs  of  the  circulation  of  the  blood 240 


xii  co XT ■/■:\ts. 

CHAPTER  XVII. 

THE  NERrES  HE    THE  HEART  AND    SOME  PHYSIOLOGICAL 
PECULIARITIES   OF  CARDIAC  MUSCLE. 

PAGE 

The  coordination  of  heart  and  blood-vessels.  Physiological  peculi- 
arities of  cardiac  muscle.  The  beat  of  the  frog's  heart.  Heart- 
beat not  tetanic.  Ventricular  contraction  always  maximal. 
Extrinsic  nerves  of  mammalian  heart.  Cardio-inhibitory  fibres. 
The  arterial  manometer.  The  cardio-inhibitory  centre.  The 
cardio-accelerator  nerves.  The  influence  of  temperature 
changes  and  of  calcium  salts  on  the  heart-beat 253 

CHAPTER  XVIII. 

THE  VASO-MOTOR  SERVES   AND  NERVE-CENTRES. 

The  nerves  of  the  blood-vessels.  Vaso-constrictor  nerves.  Vaso- 
constrictor centre.  Blushing.  Taking  cold.  Vaso-dilator 
nerves  and  centre.     Vascular  phenomena  of  inflammation  .....    273 

CHAPTER  XIX. 

THE  SECRETORY  TISSUES  AND  ORGANS. 

Organs  of  secretion.  Glands.  Physical  processes  in  secretion. 
Chemical  processes  of  secretion.  Mode  of  activity  of  secretory 
cells.  Influence  of  nervous  system  on  secretion.  Secretion  by 
the  submaxillary'  and  parotid  glands.  .  .  , 282 

CHAPTER  XX. 

THE  INCOME  AND  EXPENDITURE  OF   THE   BODY. 

The  material  losses  of  the  Body.  The  losses  of  the  Body  in  energy. 
The  conservation  of  energy.  Potential  and  kinetic  energy.  The 
energy  of  chemical  affinity.  Relation  between  matters  removed 
from  the  Body  and  energy  spent  by  it.  Conditions  of  oxidation 
in  the  living  Body.  The  fuel  of  the  Body.  Utilization  of  en- 
ergy in  the  living  body 299 

CHAPTER  XXI. 

FOODS. 

Foods  as  tissue-formers.  The  food  of  plants.  Non-oxidizable  foods. 
Definition  of  foods.  Conditions  which  a  food  must  fulfil.  Pro- 
teid  or  albuminous  alimentary  principles.  Albuminoid  or  gela- 
tinoid  alimentary  principles.  Hydrocarbons.  Carbohydrates. 
Inorganic  foods.  Mixed  foods.  Flesh.  Eggs.  Milk.  Veg- 
etable foods.     Alcohol.     Advantage  of  a  mixed  diet 313 

CHAPTER  XXII. 

THE  ALIMENTARY  CANAL   AM'   ITS  APPENDAGES. 

General  arrangement.  Subdivisions  of  alimentary  canal.  Mouth. 
Teeth.     Tongue.     Salivary  glands.     Fauces.     Pharynx.   GSsoph- 


CONTENTS.  xiii 

PAGE 

agus.  Stomach.  Small  intestine.  Large  intestine.  Liver. 
Pancreas.  Blood-vessels  of  alimentary  canal,  liver,  spleen,  and 
pancreas.     Blood-vessels  of  the  alimentary  canal 328 

CHAPTER  XXIII. 

THE  LYMPHATIC  SYSTEM  AND  THE  DUCTLESS  GLANDS. 

Lymphatics  or  absorbents.  Structure  of  lymph  vessels.  Thoracic 
duct.  Serous  cavities.  Lymphoid  or  adenoid  tissue.  Lym- 
phatic glands.  Movement  of  lymph.  Ductless  glands.  Spleen. 
Thyroid.     Thymus.     Pituitary  body.     Supra-renals 349 

CHAPTER  XXIV. 

DIGESTION. 

The  object  of  digestion.  Saliva.  Amylolytic  action.  Deglutition. 
Gastric  juice.  Gastric  digestion.  Chyle.  Pancreatic  secretion. 
Trypsin.  Bile.  Bile  tests  of  Pettenkofer  and  Gmelin.  Intesti- 
nal secretions  or  succus  entericus.  Intestinal  digestion.  Ab- 
sorption from  small  intestine.  Digestion  in  large  intestine. 
Digestion  of  an  ordinary  meal.  Dyspepsia.  Movements  of  the 
intestines 361 

CHAPTER  XXV. 
THE  RESPIRATORY  MECHANISM. 
Respiratory  organs.  Air-passages  and  lungs.  Trachea  and  bronchi. 
Structure  of  lungs.  Respiratory  movements.  Anatomy  of 
thorax.  Expiration.  Forced  respiration.  Respiratory  sounds. 
Capacity  of  lungs.  Hygiene  of  respiration.  Aspiration  of 
thorax.  Influence  of  respiratory  movements  upon  the  blood 
circulation.     Influence  of  respiratory  movements  on  lymph-flow  380 

CHAPTER  XXVI. 

THE  CHEMISTRY  OF  RESPIRATION. 
Changes  produced  in  air  by  being  once  breathed.  Ventilation. 
Changes  undergone  by  the  blood  in  the  lungs.  The  blood 
gases.  The  laws  governing  the  absorption  of  gases  by  a  liquid. 
The  absorption  of  oxygen  by  the  blood.  The  oxygen  inter- 
changes in  the  blood.  The  carbon  dioxide  of  the  blood.  Inter- 
nal respiration , 398 

CHAPTER  XXVII. 

THE  VTEBVOU8  FACTORS  OF  THE  RESPIRATORY  MECHANISM. 

ASPHYXIA. 

The  respiratory  centre.  The  rhythm  of  respiratory  discharges.  Re- 
lation at  pnenmogastric  nerves  to  respiratory  centre.  Expiratory 
centre.  Asphyxia.  Carbon-monoxide  haemoglobin.  Tin-  phe- 
oomena  of  asphyxia.     Modified  respiratory  movements 414 


xiv  CONTENTS. 

CHAPTEB   W'VIir. 
THE  KIDNEY8  AND    THE  8KIN. 

PAGE 

General  arrangement  <>f  the  urinary  organs.  Naked  •■}  e  si  rud  are  of 
kidneys.  .Minute  structure  of  the  kidney.  Blood-flow 
through  kidney.  Benal  secretion.  Secretory  functions  of  dif- 
ferent parts  of  an  uriniferous  tubule.  Excretion  of  urea.  In- 
fluence of  renal  1>1<><h1-M<>\v  on  quantity  of  urine.  The  skin. 
The  epidermis.  The  corium  or  cutis  vera.  Hairs.  Nails. 
Glands  of  the  skin.  Secretions  of  the  skin.  Hygiene  of  the 
skin.     Bathing 427 

CHAPTEB  XXIX. 

NUTRITION. 

The  problems  of  animal  nutrition.  The  seat  of  oxidations  in  the 
Body.  Tissue-building  and  energy-yielding  foods.  Source  of 
the  energy  expended  in  muscular  work.  Luxus  consumption. 
Antecedents  of  urea.  Circulating  and  fixed  proteid.  The  stor- 
age tissues.     Glycogen.     Diabetes.     Fats.     Dietetics 451 

CHAPTEB  XXX. 

THE  PRODUCTION  AND  REGULATION  OF  THE  HEAT  OF  THE  BODY. 

Cold  and  warm-blooded  animals.  The  temperature  of  the  Body. 
The  maintenance  of  an  average  temperature.  Local  tempera- 
tures.    Thermogenic  nerves.     Fever  or  pyrexia.     Clothing....   477 

CHAPTEB  XXXI. 
SENSATION  AND  SENSE  ORGANS. 
The  subjective  functions  of  the  nervous  system.  Common  sensa- 
tion and  organs  of  special  sense.  Peripheral  reference  of  sensa- 
tions. Differences  between  sensations.  Essential  structure  of 
a  sense  organ.  Modality  of  sensations.  The  psycho-physical 
law.     Perceptions.     Illusions 488 

CHAPTEB  XXXII. 

THE  EYE  AS  AN  OPTICAL  INSTRUMENT. 
The  essential  structure  of  an  eye.  The  appendages  of  the  eye.  The 
lachrymal  apparatus.  The  muscles  of  the  eye.  Anatc  ty  of 
the  eyeball.  Optic  nerves,  commissure,  and  tracts.  The  n  ina. 
Befracting  media  of  the  eye.  The  ciliary  muscle.  Properties 
of  light.  Refraction  of  light.  Accommodation.  Short  sight 
and  long  sight.     Hygiene  of  the  eyes.     Optical  defects  of  the  eye  504 

CHAPTEB  XXXIII. 
THE  EYE  AS   A    SENSORY  APPARATUS. 
The  excitation  of  the  visual  apparatus.     Vision  purple.     Intensity 
of  visual  sensations.     Duration  of  luminous  sensations.     Local 


CONTENTS.  XV 

PAGE 

izing  power  of  retina.  Color  vision.  Color  blindness.  Fatigue 
of  retina.  Contrasts.  Hering's  theory  of  vision.  Visual  per- 
ceptions. Single  vision  with  two  eyes.  Perception  of  solidity. 
Stereoscope.     Perception  of  shine 530 

CHAPTER  XXXIV. 
THE  EAR  AND  HEARING. 
The  external  ear.  The  tympanum.  Eustachian  tube.  Auditory 
ossicles.  Internal  ear.  Bony  labyrinth.  Membranous  laby- 
rinth. Organ  of  Corti.  Nerve  endings  in  semicircular  canals 
and  vestibule.  Loudness,  pitch,  and  timbre  of  sounds.  Pen- 
dular  vibrations.  Composition  of  vibrations.  Sympathetic 
resonance.  Functions  of  tympanic  membrane.  Functions  of 
auditory  ossicles.  Function  of  the  cochlea  Function  of  the 
vestibule  and  semicircular  canals.     Auditory  perceptions 557 

CHAPTER  XXXV. 

TOUCH.     TEMPERATURE   SENSATIONS.    PAIN.     COMMON  SENSATIONS. 
SMELL.     TASTE.     THE   MUSCULAR    SENSE. 

Nerve  endings  in  the  skin.  Tactile  cells.  End  bulbs.  Tactile 
corpuscles.  Pacinian  bodies.  Touch  or  the  pressure  sense. 
The  localizing  power  of  the  skin.  The  temperature  sense. 
Comparison  of  tactile  and  temperature  sensations.  Pain  and 
common  sensibility.  Common  sensations.  Hunger  and  thirst. 
Smell.     Taste.     Muscular  sense 576 

CHAPTER  XXXVI. 
THE  SPINAL  CORD  AND  REFLEX  ACTIONS. 
The  special  physiology  of  nerve-centres.  Conduction  in  the  spinal 
cord.  Ascending  and  descending  tracts  of  degeneration.  The 
spinal  cord  as  a  reflex  centre.  The  spinal  reflex  movements  of 
the  frog.  Disorderly  reflexes.  The  least-resistance  hypothesis. 
The  inhibition  of  reflexes.  Psychical  activities  of  the  cord. 
Reflex  time 594 

CHAPTER  XXXVII. 
THE  PHYSIOLOGY  OF  THE  BRAIN. 
The  functions  of  the  brain  in  general.  The  medulla  oblongata. 
Cerebellum  and  pons  Varolii.  Equilibrium  sensations.  Mid- 
brain. Forebrain.  Anatomical  connections  of  cerebral  convo- 
lutions. Functions  of  cerebral  cortex.  Cerebral  localization. 
Aphasia.     Mental  habits 609 

CHAPTER  XXXVIII. 
VOICE  AND   SPEECH. 
Voice.     The  larynx.      The  vocal  cords.      Muscles  of    the  larynx. 

Vowel         C       onants 633 


PACK 


XVI  CONTENTS. 

CHAPTER  XXXIX. 

REPRODUCTION. 

Reproduction  in  general.  Sexual  reproduction.  Male  reproductive 
organs.  Seminal  fluid.  Female  reproductive  organs.  Histol- 
ogy of  ovary.  The  mammalian  ovum.  Maturation  of  the  ovum. 
Ovulation.  Menstruation.  Fertilization.  Pregnancy.  Intra- 
uterine nutrition  of  embryo.  Parturition.  Lactation.  Puberty. 
The  stages  of  life.     Death 644 


THE  HUMAN  BODY. 


CHAPTER   I. 


THE   GENERAL  STRUCTURE  AND   COMPOSITION   OF  THE 
HUMAN   BODY. 

Definitions.  The  living  Human  Body  may  be  considered 
from  either  of  two  aspects.  Its  structure  may  be  especially 
examined,  and  the  forms,  connections  and  mode  of  growth  of 
its  parts  be  studied,  as  also  the  resemblances  or  differences  in 
such  respects  which  appear  when  it  is  compared  with  other 
animal  bodies.  Or  the  living  Body  may  be  more  especially 
studied  as  an  organism  presenting  definite  properties  and 
performing  certaiu  actions;  and  then  its  parts  will  be  investi- 
gated with  a  view  to  discovering  what  duty,  if  any,  each  ful- 
fils. The  former  group  of  studies  constitutes  the  science  of 
Anatomy,  and  in  so  far  as  it  deals  with  the  Human  Body 
alone,  of  Human  Anatomy;  while  the  latter,  the  science  con- 
cerned with  the  uses — or  in  technical  language  the  functions 
— of  eacb  part  is  known  as  Physiology.  Closely  connected 
witli  physiology  is  the  science  of  Hygiene,  which  is  concerned 
with  the  conditions  which  are  favorable  to  the  healthy  action 
of  the  various  parts  of  the  Body;  while  the  activities  and 
structure  of  the  diseased  body  form  the  subject-matters  of 
the  science  of  Pathology  and  Pathological  Anatomy. 

Tissues  and  Organs.  Histology.  Examined  merely  from 
the  outside  our  Bodies  present  a  considerable  complexity  of 
structure.  We  easily  recognize  distinct  parts  as  head,  neck, 
trunk  and  limbs;  and  in  these  again  smaller  constituent 
parts,  as  eyes,  nose,  ears,  mouth;  arm,  forearm,  hand;  thigh, 
leg  and  foot.  We  can,  with  such  an  external  examination, 
go  even  farther  and  recognize  different  materials  as  entering 
into  tin-  formation  of  the  larger  parts.     Skin,  hair,  nails  and 

teeth  are  obviously  different  substances;  simple  examination 


2  THE  HUMAN  BODY. 

by  pressure  proves  that  internally  there  are  harder  and  softer 
solid  parts;  while  the  blood  that  Sows  from  a  cut  linger  shows 
that  liquid  constituents  also  exist  in  the  Body.  The  concep- 
tion of  complexity  which  may  he  thus  arrived  at  from  exter- 
nal observation  of  the  living,  is  greatly  extended  by  dissection 
of  the  dead  Body,  which  makes  manifest  that  it  consists  of  a 
great  number  of  diverse  parts  or  organs,  which  in  turn  are 
built  up  of  a  limited  number  of  materials;  the  same  material 
often  entering  into  the  composition  of  many  different  organs. 
These  primary  building  materials  are  known  as  the  tissues, 
and  that  branch  of  anatomy  which  deals  with  the  characters 
of  the  tissues  and  their  arrangement  in  various  organs  is 
known  as  Histology;  or,  since  it  is  mainly  carried  on  with  the 
aid  of  the  microscope,  as  Microscopic  Anatomy.  If,  with  the 
poet,  wTe  compare  the  Body  to  a  house,  we  may  go  on  to  liken 
the  tissues  to  the  bricks,  stone,  mortar,  wood,  iron,  glass  and 
so  on,  used  in  building;  and  then  walls  and  floors,  stairs  and 
windows,  formed  by  the  combination  of  these,  would  answer 
to  anatomical  organs. 

Zoological  Position  of  Man.  External  examination  of  the 
human  Body  shows  also  that  it  presents  certain  resemblances 
to  the  bodies  of  many  other  animals:  head  and  neck,  trunk 
and  limbs,  and  various  minor  parts  entering  into  them,  are 
not  at  all  peculiar  to  it.  Closer  study  and  the  investigation 
of  internal  structure  demonstrates  further  that  these  resem- 
blances are  in  many  cases  not  superficial  only,  but  that  our 
Bodies  maybe  regarded  as  built  upon  a  plan  common  to  them 
and  the  bodies  of  many  other  creatures:  and  it  soon  becomes 
further  apparent  that  this  resemblance  is  greater  between  the 
Human  Body  and  the  bodies  of  ordinary  four-footed  beasts, 
than  between  it  and  the  bodies  of  birds,  reptiles  or  fishes. 
Hence,  from  a  zoological  point  of  view,  man's  Body  marks 
him  out  as  belonging  to  the  group  of  Mammalia  (see  Zoology), 
which  includes  all  animals  in  which  the  female  suckles  the 
young  ;  and  among  mammals  the  anatomical  resemblances 
are  closer  and  the  differences  less  between  man  and  certain 
apes  than  between  man  and  the  other  mammals;  so  that 
zoologists  still,  with  Linnaeus,  include  man  with  the  monkeys 
and  apes  in  one  subdivision  of  the  Mammalia,  known  as  the 
Primates.  That  civilized  man  is  mentally  far  superior  to 
any  other  animal  is  no  valid  objection  to  such  a  classification, 
for  zoological  groups  are  defined  by  anatomical  and  not  by 


GENERAL   STRUCTURE  AND   COMPOSITION.  3 

physiological  characters;  and  mental  traits,  since  we  know 
that  their  manifestation  depends  upon  the  structural  integ- 
rity of  certain  organs,  are  especially  phenomena  of  function 
and  therefore  not  available  for  purposes  of  zoological  ar- 
rangement. 

As  man  walks  erect  with  the  head  upward,  while  the  great 
majority  of  Mammals  go  on  all  fours  with  the  head  forward 
and  the  back  upward,  and  various  apes  adopt  intermediate 
positions,  confusion  is  apt  to  arise  in  considering  correspond- 
ing parts  in  man  and  other  animals  unless  a  precise  mean- 
ing be  given  to  such  terms  as  "anterior''  and  "posterior." 
Anatomists  therefore  give  those  words  definite  arbitrary  sig- 
nifications. The  head  end  is  always  anterior  whatever  the 
natural  position  of  the  animal,  and  the  opposite  end  posterior; 
the  belly  side  is  spoken  of  as  ventral,  and  the  opposite  side  as 
dorsal;  right  and  left  of  course  present  no  difficulty:  the 
terms  cephalic  and  caudal  as  equivalent,  respectively,  to  ante- 
rior and  posterior,  are  sometimes  used.  Moreover,  that  end 
of  a  limb  nearer  the  trunk  is  spoken  of  as  proximal  with  refer- 
ence to  the  other  or  distal  end.  The  words  upper  and  lower 
may  be  conveniently  used  for  the  relative  position  of  parts  in 
the  natural  standing  position  of  the  animal. 

The  Vertebrate  Plan  of  Structure.  Neglecting  such 
merely  apparent  differences  as  arise  from  the  differences  of 
normal  posture  above  pointed  out,  we  find  that  man's  own 
zoological  class,  the  Mammals,  differs  very  widely  in  its  broad 
structural  plan  from  the  groups  including  sea-anemones,  in- 

-  or  oysters,  but  agrees  in  many  points  with  the  groups  of 
fishes,  amphibians,  reptiles  and  birds.  These  four  are  there- 
fore placed  with  man  and  all  other  Mammals  in  one  great 
division  of  the  animal  kingdom  known  as  the  Vertebral  a. 
The  main  anatomical  character  of  all  vertebrate  animals  is 
the  presence  in  the  trunk  of  the  body  of  two  cavities,  a  dorsal 
and  a  ventral,  separated  by  a  solid  partition;  in  the  adults  of 
nearly  all  vertebrate  animals  a  hard  axis,  the  vertebral  column 
{backbone  or  spine),  develops  in  this  partition  and  forms  a 
central  support  for  the  rest  of  the  body  (Fig.  2,  ee).  The 
dorsal  cavity  is  continued  through  the  neck,  when  there  is 
one,  into  the  head,  and  there  widens  out.  The  bony  axis  is 
also  continued  through  the  neck  and  extends  into  the  head 
in  a  modified  form.  The  ventral  cavity,  on  the  other  hand, 
is  confined  to  the  trunk,     it  contains  the  main  organs  con- 


THE  HUMAN  BODY. 


nected  with  the  blood-flow  and  is  often  called  the  hmnal 
cavil  //. 

Upon  the  venl nil  side  of  the  head  is  the  mouth-opening 
leading  into  a  tube,  the  alimentary  canal,  f,  which  passes 
back  through  the  neck  and  trunk  and  opens  again  on  the 
outside  at  the  posterior  part  of  the  latter.  In  its  passage 
through  the  trunk-region  this  canal  lies  in  the  ventral  cavity. 

The  Mammalia.  In  many  vertebrate  animals  the  ven- 
tral cavity  is  not  subdivided,  but   in  the  Mammalia  it  is;  a 

....        ,1 


Fig.  1.— The  Bml.v  opened  from  the  front  to  show  the  contents  of  its  ventral 
cavity,  hi,  lungs;  /(,  heart,  partly  covered  by  other  things;  le,  le',  right  and  left 
liver-lobes  respectively  ;  ma,  stomach  ;  ne,  the  great  omentum,  a  membrane  con- 
taining fai  which  hangs  down  from  the  posterior  border  of  the  stomach  and  covers 
the  intestines. 

membranous  transverse  partition,  the  midriff  or  diaphragm 
(Fig.  ],  z),  separating  it  into   an  anterior  chest  or  thoracic 


GENERAL  STRUCTURE  AND    COMPOSITION. 


cavity,  and  a  posterior  or  abdominal  cavity.  The  alimentary 
canal  and  whatever  else  passes  from  one  of  these  cavities  to 
the  other  must  therefore  perforate 
the  diaphragm. 

In  the  chest,  besides  part  of  the 
alimentary  canal,  lie  important  or- 
gans, the  heart,  h,  and  lungs,  lu; 
the  heart  being  on  the  ventral  side 
of  the  alimentary  canal.  The  ab- 
dominal cavity  is  mainly  occupied  by 
the  alimentary  canal  and  organs  con- 
nected with  it  and  concerned  in  the 
digestion  of  food,  as  the  stomach, 
ma,  the  liver,  le,  the  pancreas,  and 
the  intestines.  Among  the  other 
more  prominent  organs  in  it  are  the 
kidneys  and  the  spleen. 

In  the  dorsal  or  neural  cavity  lie 
the  brain  and  spinal  cord,  the  former 
occupying  its  anterior  enlargement 
in  the  head.  Brain  and  spinal  cord 
together  form  the  cerebrospinal 
nervous  centre;  in  addition  to  this 
there  are  found  in  the  ventral  cavity 
a  number  of  small  nerve-centres 
united  together  by  connecting  cords, 
and  with  their  offshoots  forming  the 
sympathetic  nervous  system.  tudmai  section  of  the  body.  «, 

'  .9  tne  »eural  tube,  with  its  upper 

I  he  Avails  of  the  three  main  Cavi-    en'argement  in  the  skull-cavity 

,.  at  a';   N,  the  spinal  cord:  N', 

ties    are    lined     by    smooth,    moist  \he  .^rain:,./e'  vertebrae  form- 

serous  membranes.     That  lining  the 

dorsal  cavity  is  the  arachnoid;  that 

lining   the   chest  the  pleura;    that 

lining  the  abdomen  the  peritoneum; 

the   abdominal   cavity  is   in    conse- 

qnence   often  called  "the   peritoneal  f  ffiffi??$k^S3S 

cavity.    Externally  the  walls  of  these  nem>uschain.    From  the  stom- 

J  acta,/,  the  intestinal  tube  leads 

'■;i \  l  ties    are    covered     by    the    skin     through  the  abdominal    cavity 
,  .    ,  .    .        „  ,         .  'to  the  posterior  opening  of  the 

wnicli  consists  of  two  layers:  an  outer   alimentary  canal. 

horny  layer  called  the  epidermis,  which  is  constantly  being 
shed  on  the  surface  and  renewed   from  below;  and  a  deeper 

layer,  called    the   dermis   and    containing    blood,   which     the 


ng  the  solid  partition  between 
the  dorsal  and  ventral  cavities; 
b,  the  pleural,  and  c,  the  abdom- 
inal divisiou  of  the  ventral  cav- 
ity, separated  from  one  another 
by  the  diaphragm,  d ;  i,  the 
nasal,  and  o,  the  month  cham- 
ber, opening  behind  into  the 
pharynx,  from  which  one  tube 
leads  to  the  lungs.  I,  and  another 


6  THE  HUMAN  BODY. 

epidermis  does  not.  Between  the  skin  and  the  lining  serous 
membranes  are  bones,  muscles  (the  lean  of  meat),  and  a  greai 
number  of  other  structures  which  we  shall  have  to  consider 
hereafter.  All  cavities  inside  the  body,  as  the  alimentary 
canal  and  the  air-passages,  winch  open  directly  or  indirectlj 
on  the  surface  are  lined  by  soft  and  moist  prolongations  of 
the  skin  known  as  mucous  membranes.  In  these  two  layers 
are  found  as  in  the  skin,  but  the  superficial  bloodless  one  is 
called  epithelium  and  the  deeper  vascular  one  corium. 

Diagrammatically  we  may  represent  the  Human  Body 
in  longitudinal  section  as  in  Fig.  2,  where  aa'  is  the  dorsal 
or  neural  cavity,  and  b  and  c,  respectively,  the  thoracic  and 
abdominal  subdivisions  of  the  ventral  cavity;  d  represents 
the  diaphragm  separating  them;  ee  is  the  vertebral  column 
with  its  mod iiied  prolongation  into  the  head  beneath  the 
anterior  enlargement  of  the  dorsal  cavity;  /  is  the  ali- 
mentary canal  opening  in  front  through  the  nose,  i,  and 
mouth,  o;  h  is  the  heart,  I  a  lung,  s  the  sympathetic  nervous 
system,  and  /.•  a  kidney. 

A  transverse  section  through  the  chdst  is  represented  by  t  he 
diagram  Fig.  3,  where  x  is  the  neural  canal  containing  the 
spinal  cord.     In  the  thoracic   cavity  are  seen  the  heart,  //, 


Fig  3 —A  diagrammatic  section  across  the  Body  in  the  chest  region,  x,  the 
dorsal  tube  which  contains  the  spinal  cord;  the  black  mass  surrounding  ll  is  a 
vertebra:  a,  the  gullet,  a  part  of  the  alimentary  canal;  h,  the  heart:  sv,  sympa- 
thetic nervous  system;  ll,  Ihqks;  the  dotted  lines  around  them  are  the  pleura-;  it, 
ribs;  at,  the  breast-bone. 

the  lungs,  ll,  part  of  the  alimentary  canal,  a,  and  the  sympa- 
thetic nerve-centres,  sy  ;  the  dotted  line  on  each  side  covering 
the  inside  of  the  chest-wall  and  the  outside  of  the  lung 
represents  the  pleura. 

Sections  through  corresponding  parts  of  any  other  Mam- 
mal would  agree  in  all  essential  points  with  those  represented 
in  Figs.  2  and  3. 


GENERAL  STRUCTURE  AND   COMPOSITION.  7 

The  Limbs.  The  limbs  present  no  such  arrangement  of 
canities  on  each  side  of  a  bony  axis  as  is  seen  in  the  trunk. 
They  have  an  axis  formed  at  different  parts  of  one  or  more 
bones  (as  seen  at  U  and  R  in  Fig.  -A,  which  represents  a  cross- 
section  of  the  forearm  near  the  elbow-joint),  but  around  this 
are  closely-jacked  soft  parts,  chiefly  muscles,  and  the  whole 
is  enveloped  in  skin.  The  only  cavities  in  the  limbs  are 
branching  tubes  which  are  filled  with  liquids  during  life, 
either  blood  or  a  watery-looking  fluid  known  as  lymph.  These 
tubes,  the  blood  and  lymph  vessels  respectively,  are  not,  how- 


TL 

Fig.  4.— A  section  across  the  forearm  a  short  distance  below  the  elbow- joint.  R 
and  U,  its  two  supporting  bones,  the  radius  and  ulna;  e,  the  epidermis,  and  ii.  the 
dermis  of  the  skin;  the  latter  is  continuous  below  with  bands  of  connective  tissue, 
s,  which  penetrate  between  and  invest  the  muscles,  which  are  indicated  by  nuiu 
bers;  u    n.  nerves  and  vessels. 

ever,  characteristic  of  the  limbs,  for  they  are  present  in 
abundance  in  the  dorsal  and  ventral  cavities  and  in  their 
walls. 

Chemical  Composition  of  the  Body.  In  addition  to  the 
study  of  the  Body  as  composed  of  tissues  and  organs  which 
are  optically  recognizable,  we  may  consider  it  as  composed  of 
a  number  of  different  chemical  substances.  This  branch  of 
knowledge,  which  is  still  very  incomplete,  really  presents  two 
classe  of  problems.  On  the  one  hand  we  may  limit  ourselves 
to  tin-  examination  of  the  chemical  substances  which  exist  in 
or  may  be  derived  from  the  dead  Body,  or,  if  such  a  tiling 
were  possible,  from  the  living  Body  entirely  at  rest;  such  a 
study  is  essentially  one  of  structure  and  may  be  called  Chem- 
ical Anatomy.  But  as  long  as  the  Body  is  alive  it  is  the  seat 
of  constant  chemical  transformations  in  its  material,  and 
these  arc  inseparably  connected  with  its  functions,  the  great 
majority  of  which  are  in  tin;  long-run  dependent,  upon  chem- 
ical changes.  Prom  this  point,  of  view,  then,  the  chemical 
study  of  the  Body  presents  physiological  problems,  and  it  is 

Usual  to  include  all  the  fads  known  as  to  the  chemical  com- 
panion and  metamorphoses  of  living  matter  under  the  name 


8  THE  HUMAN  BODY. 

of  Physiological  Chemistry.     For  the  present  we  may  confine 

ourselves  to  the  more  important  substances  derived  from  or 
known  to  exist  in  the  Body,  leaving  questions  concerning  the 
chemical  changes  taking  place  within  it  for  consideration 
along  with  those  functions  which  are  performed  in  connection 
with  them. 

Elements  Composing  the  Body.  Of  the  elements  known 
to  chemists  only  sixteen  have  been  found  to  take  part  in  the 
formation  of  the  human  Body.  These  are  carbon,  hydrogen, 
nitrogen,  oxygen,  sulphur,  phosphorus,  chlorine,  fluorine, 
silicon,  sodium,  potassium,  lithium,  calcium,  magnesium,  iron 
and  manganese.  Copper  and  lead  have  sometimes  been  found 
in  small  quantities,  but  are  probably  accidental  and  occa- 
sional. 

Uncombined  Elements.  Only  a  very  small  number  of  the 
above  elements  exist  in  the  Body  uncombined.  Oxygen  is 
found  in  small  quantity  dissolved  in  the  blood ;  but  even  there 
most  of  it  is  in  a  state  of  loose  chemical  combination.  It  is 
also  found  in  the  cavities  of  the  lungs  and  alimentary  canal, 
being  derived  from  the  inspired  air  or  swallowed  with  food 
and  saliva;  but  while  contained  in  these  spaces  it  can  hardly 
be  said  to  form  a  part  of  the  Body.  Nitrogen  also  exists  un- 
combined in  the  lungs  and  alimentary  canal,  and  in  small 
quantity  in  solution  in  the  blood.  Free  hydrogen  has  also 
been  found  in  the  alimentary  canal,  being  there  evolved  by 
the  fermentation  of  certain  foods. 

Chemical  Compounds.  The  number  of  these  which  may 
be  obtained  from  the  Body  is  very  great;  but  with  regard  to 
very  many  of  them  we  do  not  know  that  the  form  in  which 
we  extract  them  is  really  that  in  which  the  elements  they 
contain  were  united  while  in  the  living  Body;  since  the 
methods  of  chemical  analysis  are  such  as  always  break  down  the 
more  complex  forms  of  living  matter  and  leave  us  only  its  de- 
bris for  examination.  We  know  in  fact,  tolerably  accurately, 
what  compounds  enter  the  Body  as  food  and  what  finally 
leave  it  as  waste;  but  the  intermediate  conditions  of  the  ele- 
ments contained  in  these  compounds  during  their  sojourn 
inside  the  Body  we  know  very  little  about;  more  especially 
their  state  of  combination  during  that  part  of  their  stay  when 
they  do  not  exist  dissolved  in  the  bodily  liquids,  but  form 
part  of  a  solid  living  tissue. 

For  present  purposes  the  chemical  compounds  existing  in 


GENERAL  STRUCTURE  AND   COMPOSITION.  9 

or  derived  from  the  Body  may  be  classified  as  organic  and  in- 
organic, and  the  former  be  subdivided  into  those  which  con- 
tain nitrogen  and  those  which  do  not. 

Nitrogenous  or  Azotized  Organic  Compounds.  These 
fall  into  several  main  groups:  proteids,  peptones,  albuminoids, 
enzymes,  crystalline  substances,  and  coloring  ■matters. 

Proteids  are  by  far  the  most  characteristic  substances 
obtained  from  the  Body,  since  they  are  only  known  as  exist- 
ing in  or  derived  from  living  things,  either  animals  or  plants. 
The  type  of  this  class  of  bodies  may  be  found  in  the  white  of 
an  egg,  where  it  is  stored  up  as  food  for  the  developing  chick; 
from  this  typical  form,  which  is  called  egg-albumin,  the  pro- 
teids in  general  are  often  called  albuminous  bodies.  Each 
of  them  contains  carbon,  hydrogen,  oxygen,  sulphur  and 
nitrogen  united  to  form  a  very  complex  molecule,  and  although 
different  members  of  the  family  differ  from  one  another  in 
minor  points  they  all  agree  in  their  broad  features  and  have 
a  similar  percentage  composition.  The  latter  in  different 
examples  varies  within  the  following  limits: 

Carbon 50  to     55  per  cent. 

Hydrogen 6.8  to    7.3      " 

Oxygen 22.8  to  24.1      " 

Nitrogen 15  4  to  18.2      " 

Sulphur 0.4to    5.0      " 

In  addition  a  small  quantity  of  ash  is  usually  left  when  a 
proteid  is  burnt. 

Proteids  are  recognized  by  the  following  characters: 

1 .  Boiled,  either  in  the  solid  state  or  in  solution,  with  strong 
nitric  acid  they  give  a  yellow  liquid  which  becomes  orange  on 
neutralization  with  ammonia.     This  is  the  xantho-proteic  test. 

2.  Boiled  with  a  solution  containing  subnitrate  and  per- 
nitrate  of  mercury  they  give  a  pink  precipitate,  or,  if  in  very 
small  quantity,  a  pink-colored  solution.  This  is  known  as 
Milton's  test. 

'■'>.  J  f  ;i  solution  containing  a  proteid  be  strongly  acidulated 
with  acetic  acid  and  be  boiled  after  the  addition  of  an  equal 
bulk  of  ;i  saturated  watery  solution  of  sodium  sulphate,  the 
proteid  will  be  precipitated. 

Among  the  more  important  proteids  obtained  from  the 
Human  Body  are  the  following: 

Serum-albumin.     This  exists  in  solution  in  the  blood  and 


10  THE  HUMAN  BODY. 

is  verj  Like  egg-albumin  in  its  properties.  It  is  coagulated 
(like  the  white  of  an  egg)  when  boiled,  and  then  passes  into 
the  state  nf  coagulated  proteid  which  is.  unlike  the  original 
serum-albumin,  insoluble  in  dilute  acids  or  alkalies  or  in 
water  containing  neutral  salts  in  solution.  All  other  proteids 
can  by  appropriate  treatment  be  turned  into  coagulated 
proteid. 

Fibrin.  'This  forms  in  blood  when  it  "clots/"  either  in- 
side or  outside  of  the  Body;  it  is  insoluble  in  water  and  dilute 
acids  or  alkalies;  soluble  in  strong  acids  and  alkalies  and, 
though  slowly,  in  ten  per  cent  neutral  saline  solutions. 

Myosin.  This  is  derived  from  the  muscles,  in  which  it 
develops  and  solidifies  after  death,  causing  the  " death-stiffen- 
ing." 

Globulin  exists  in  the  red  globules  of  the  blood  and  dis- 
solved in  some  other  liquids  of  the  body.  In  the  blood-cor- 
pnscles  it  is  combined  with  a  colored  non-proteid  substance 
to  form  licernoglobin,  which  is  crystallizable.  Allied  sub- 
stances, paraglobulin  and  fibrinogen,  are  found  dissolved  in 
the  blood-liquid.  When  blood  clots  the  fibrinogen  gives  rise 
to  fibrin. 

Casein  or,  as  it  is  better  named,  caseinogen  exists  in 
in  ilk.  Its  solutions  do  not  coagulate  spontaneously  or,  like 
that  of  serum-albumin,  on  boiling.  When  milk  turns  sour  on 
keeping,  or  when  it  is  very  slightly  acidulated  with  dilute 
acetic  acid,  the  casein  is  precipitated.  The  clot  or  curd  which 
forms  when  milk  is  gently  warmed  with  gastric  juice  or  with 
rennet,  is  also  derived  from  caseinogen;  it  differs  from  true 
casein  and  is  named  tyrein  :  it  is  the  chief  constituent  of 
cheese. 

Peptones.  These  are  formed  in  the  alimentary  canal  by 
the  action  of  some  of  the  digestive  liquids  upon  the  proteids 
swallowed  as  food.  They  contain  the  same  elements  as  the 
proteids  and  give  the  xantho-proteic  and  Millon's  reactions, 
but  are  not  precipitated  by  boiling  with  acetic  acid  and 
-odium  sulphate.  Their  great  distinctive  character  is.  how- 
ever, their  diffusibility.  The  proteids  proper  will  not  dialyze 
(see  Physics),  but  the  peptones  in  solution  pass  readily 
through  moist  animal  membranes. 

Albuminoids  or  Gelatinoids.  These  contain  carbon, 
hydrogen,  oxygen  and  nitrogen,  but  rarely  any  sulphur.  Like 
the  proteids,  the  nearest  chemical  allies  of  which  they  seem 


GENERAL  STRUCTURE  AND    COMPOSITION.  11 

to  be,  they  are  only  known  in  or  derived  from  living  beings. 
Gelatin,  obtained  from  bones  and  ligaments  by  boiling,  is  a 
typical  albuminoid;  as  is  chondrin,  which  is  obtained  similarly 
from  gristle.  Mucin,  which  gives  their  glairy  tenacious  char- 
acter to  the  secretions  of  the  mouth  and  nose,  is  anothei 
albuminoid. 

Enzymes  or  Soluble  Ferments  ;ire  a  group  of  substances 
which  seem  to  be  allied  in  chemical  composition  to  the  true 
proteids,  but  it  is  so  difficult  to  be  sure  of  the  purity  of  any 
specimen  that  their  composition  is  still  in  doubt.  The 
enzymes  have  the  power,  even  when  present  in  very  small 
quantity,  of  bringing  about  extensive  changes  in  other  sub- 
stances, and  they  are  not  themselves  necessarily  used  up  or 
destroyed  in  the  process.  Many  enzymes  of  great  physiologi- 
cal  importance  exist  in  the  digestive  fluids  and  play  a  part  in 
fitting  food  for  absorption  from  the  alimentary  canal.  For 
example,  pepsin  found  in  the  gastric  juice  and  trypsin  found 
in  the  pancreatic  secretion  convert,  under  suitable  conditions, 
albuminous  substances  into  peptones;  and  ptyalin,  found  in 
the  saliva,  converts  starch  into  sugar.  Other  ferments 
cause  the  clotting  of  various  animal  liquids:  rennin  from  the 
gastric  juice  clots  the  caseinogen  of  milk  preparatory  to  its 
digestion;  and  a  ferment  which  forms  in  drawn  blood  con- 
verts fibrinogen  into  fibrin.  We  shall  have  occasion  later  to 
study  several  enzymes  more  in  detail  in  connection  with  their 
physiological  uses. 

Crystalline  Nitrogenous  Substances.  These  are  a  heter- 
ogeneous group,  the  great  majority  of  them  being  materials 
which  have  done  their  work  in  the  Body  and  are  about  to  be 
got  rid  of.  Nitrogen  enters  the  Body  in  foods  for  the  most 
part  in  the  chemically  complex  form  of  some  proteid.  In  the 
vital  processes  these  proteids  are  broken  down  into  simpler 
substances,  their  carbon  being  partly  combined  with  oxygen 
and  passed  out  through  the  lungs  as  carbon  dioxide;  their 
hydrogen  is  similarly  in  large  part  combined  with  oxygen  and 
passed  out  as  water;  while  their  nitrogen,  with  some  carbon 
and  hydrogen  and  oxygen,  is  usually  passed  out  in  the  form 
of  a  crystalline  compound,  containing  what  chemists  call  an 
"ammonium  residue/'  Of  these  tin;  most  important  is  urea 
(Carbamide,  2NH,,CO),  which  is  eliminated  through  the  kid- 
neys. Uric  acid  is  another  nitrogenous  waste  product,  and 
many  others,  such  as  kreal'ui  and  leucin,  seem   to  be  inter- 


12  THE  HUMAN  BODY. 

mediate  stages  between  the  proteids  which  enter  the  body  and 
the  urea  and  uric  acid  which  leave  it. 

In  the  bile  or  gall,  t wo  crystallizable  nitrogen-containing 
bodies,  glycocholic  and  taurocholic  acid,  are  found  combined 

with  soda. 

Nitrogenous  Coloring  Matters.  These  form  an  artificial 
group  whose  constitution  and  origin  are  ill  known.  Among 
the  most  important  are  the  following: 

Ihrniiitin,  derived  from  the  red  corpuscles  of  the  blood  in 
which  a  residue  of  it  is  combined  with  a  proteid  residue  to 
form  hcemoglobin. 

Bilirubin  and  biliverdin,  which  exist  in  the  bile;  the 
former  predominating  in  the  bile  of  man  and  of  carnivorous 
animals  and  giving  it  a  reddish-yellow  color,  while  biliverdin 
predominates  in  the  bile  of  Herbivora,  which  is  green. 

Non-Nitrogenous  Organic  Compounds.  These  may  be 
conveniently  grouped  as  hydrocarbons  or  fatty  bodies;  carbo- 
hydrates or  amyloids  ;  and  certain  non-azotized  acids. 

Fats.  The  fats  all  contain  carbon,  hydrogen  and  oxygen, 
the  oxygen  being  present  in  small  proportion  as  compared 
with  the  hydrogen.  Three  fats  occur  in  the  Body  in  large 
quantities,  viz.:  palmatin  (C^H^O,.),  stearin  (C51H110O,.), 
and  olein  (C67H104O6).  The  two  former  when  pure  are  solid 
at  the  temperature  of  the  Body,  but  in  it  are  mixed  with 
olein  (which  is  liquid)  in  such  proportions  as  to  be  kept  fluid. 
The  total  quantity  of  fat  in  the  Body  is  subject  to  great  vari- 
ations, but  its  average  quantity  in  a  man  weighing  75  kilo- 
grams (iri:>  pounds)  is  about  2.75  kilograms  (G  pounds). 

Each  of  these  fats  when  heated  with  a  caustic  alkali,  in 
the  presence  of  water,  breaks  up  into  a  fatty  acid  {stearic, 
palmitic  or  oleic  as  the  case  may  be),  and  glycerin.  The 
fatty  acid  unites  with  the  alkali  present  to  form  a  soap. 

Carbohydrates.  These  also  contain  carbon,  hydrogen 
and  oxygen,  but  there  is  one  atom  of  oxygen  present  for 
every  two  of  hydrogen  in  the  molecule  of  each  of  them. 
Chemically  they  are  related  to  starch.  The  more  important 
of  them  found  in  the  Body  are  the  following: 

Glycogen  (CB1I1005),  found  in  large  quantities  in  the  liver, 
where  it  seems  to  be  a  reserve  of  material  answering  to  the 
starch  stored  up  by  many  plants.  It  exists  in  smaller  quanti- 
ties in  the  muscles. 

(•'///rose,  or  grape-sugar  (C,H„Oe),  which  exists  in  the 


GENERAL  STRUCTURE  AND  COMPOSITION.  13 

liver  in  small  quantities;  also  in  the  blood  and  lymph.  It  is 
largely  derived  from  glycogen,  which  is  very  readily  converted 
into  it. 

Lactose,  or  sugar  of  milk  (011H„011  +  H20),  found  in 
considerable  quantity  in  milk. 

Inosit  (C6H,„06  +  2H20),  also  called  muscle  sugar  and 
formerly  classed  in  this  group,  is  now  known  to  be  chemi- 
cally not  a  real  sugar  or  true  carbohydrate.  It  exists  in 
muscles,  liver,  spleen,  kidneys,  etc. 

Organic  Non-Nitrogenous  Acids.  Of  these  the  most  im- 
portant is  carbon  dioxide  (COJ,  which  is  the  form  in  which 
by  far  the  greater  part  of  the  carbon  taken  into  the  Body 
ultimately  leaves  it.  United  with  calcium  it  is  found  in  the 
bones  and  teeth  in  large  proportion. 

Formic,  acetic  and  butyric  acids  are  also  found  in  the 
Body;  stearic,  palmitic,  and  oleic  have  been  above  mentioned 
as  obtainable  from  fats.  Lactic  acid  is  sometimes  found  in 
the  stomach,  and  when  milk  turns  sour  is  formed  from  lactose. 
A  body  of  the  same  percentage  composition,  C,HeO,  (sarco- 
lactic  acid),  is  formed  in  muscles  when  they  work  or  die. 

Glycerin  phosphoric  acid  (C3H9P06)  is  obtained  on  the  de- 
composition of  lecithin,  a  complex  nitrogenous  fat  found  in 
nervous  tissue. 

Inorganic  Constituents.  Of  the  simpler  substances  en- 
tering into  the  structure  of  the  Body  the  following  are  the 
most  important  : 

Water;  in  all  the  tissues  in  greater  or  less  proportion  and 
forming  about  two  thirds  of  the  weight  of  the  whole  Body. 
A  man  weighing  75  kilos  (1G5  lbs.),  if  completely  dried 
would  therefore  lose  about  50  kilos  (110  lbs.)  from  the  evapo- 
ration of  water.  Of  the  constituents  of  the  Body  the  enamel 
of  the  teeth  contains  least  water  (about  2  per  cent),  and  the 
saliva  most  (about  99.5  per  cent);  between  these  extremes 
are  all  intermediate  steps — bones  containing  about  22  per  cent, 
muscles  75,  blood  79. 

Cdmmon  salt—Sodium  chloride — (NaCl);  found  in  all  the 
tissues  and  liquids,  and  in  many  cases  playing  an  important 
part  in  keeping  other  substances  in  .solution  in  water. 

Petassium  chloride  (KC1);  in  the  blood,  muscles,  nerves 
and  most  liquids. 

Calcium  phosphate  (Ca,2P04);  in  the  bones  and  teeth  in 
large  quantity,     in  less  proportion  in  all  the  other  tissues. 


14  THE  III  MA. \  noDY. 

Besides  the  above,  ammonium  chloride,  sodium  and  potas- 
sium phosphates,  magnesium  phosphate,  .-odium  sulphate, 
potassium  sulphate  and  calcium  fluoride  have  beeD  obtained 
from  the  body. 

Uncombined  hydrochloric  acid  (HC1)  is,  found  in  the 
gastric  juice. 


CHAPTER  II. 

THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS. 

The  Properties  of  the  Living  Body.  When  we  turn 
from  the  structure  and  composition  of  the  living  Body  to 
consider  its  powers  and  properties  we  meet  again  with  great 
variety  and  complexity,  the  most  superficial  examination  be- 
ing sufficient  to  show  that  its  parts  are  endowed  with  very 
different  faculties.  Light  falling  on  the  eye  arouses  in  us  a 
sensation  of  sight,  but  falling  on  the  skin  has  no  such  effect; 
pinching  the  skin  causes  pain,  but  pinching  a  hair  or  a  nail 
does  not;  when  the  ears  are  stopped,  sounds  arouse  in  us  no 
sensation;  we  readily  recognize,  too,  hard  parts  formed  for 
support,  joints  to  admit  of  movements,  apertures  to  receive 
food  and  others  to  get  rid  of  waste.  AVe  thus  perceive  that 
different  organs  of  our  Bodies  have  very  different  endow- 
ments and  serve  for  very  distinct  purposes;  and  here  also 
the  study  of  internal  organs  shows  us  that  the  varieties  of 
quality  observed  on  the  exterior  are  but  slight  indications  of 
differences  of  property  which  pervade  the  whole,  being  some- 
times dependent  on  the  specific  characters  of  the  tissues  con- 
cerned and  sometimes  upon  the  manner  in  which  these  are 
combined  to  form  various  organs.  Some  tissues  are  solid, 
rigid  and  of  constant  shape,  as  those  composing  the  bones 
and  teeth;  others,  as  the  muscles,  are  soft  and  capable  of 
changing  their  forms;  and  still  others  are  capable  of  working 
chemical  changes  by  which  such  peculiar  fluids  as  the  bile 
and  the  saliva  are  produced.  We  find  elsewhere  a  number  of 
tissues  combined  to  form  a  tube  adapted  to  receive  food  and 
carry  it  through  the  Body  for  digestion,  and  again  similar 
tissues  differently  arranged  to  receive  the  air  which  we  breathe- 
in,  and  expel  it  after  abstracting  part  of  its  oxygen  and 
adding  to  it  certain  other  things;  and  in  the  heart  and  hlood- 
Ifl  we  find  almost,  the  same  tissues  arranged  to  propel 
and   carry  the  blood  over  the  whole  Body.      The  working  of 

15 


16  THE  II UMAX   BODY. 

the  Body  offers  clearly  even  a  more  complex  subject  of  study 

than  its  structure. 

Physiological  Properties.  In  common  with  inanimate 
objects  the  Body  possesses  many  merely  physical  properties, 

as  weight,  rigidity,  elasticity,  color,  and  so  on;  but  in  addi- 
tion to  these  we  find  in  it  while  alive  many  others  which  it 
ceases  to  manifest  at  death.  Of  these  perhaps  the  power  of 
executing  spontaneous  movements  and  of  maintaining  a  high 
bodily  temperature  are  the  most  marked.  As  long  as  the 
Body  is  alive  it  is  warm  and,  since  the  surrounding  air  is 
nearly  always  cooler,  must  be  losing  heat  all  day  long  to 
neighboring  objects;  nevertheless  we  are  at  the  end  of  the 
day  as  warm  as  at  the  beginning,  the  temperature  of  the 
Body  in  health  not  varying  much  from  37°  0.  (98.4°  F.),  so 
that  clearly  our  Bodies  must  be  making  heat  somehow  all 
the  time.  After  death  this  production  of  heat  ceases  and  the 
Body  cools  down  to  the  temperature  in  its  neighborhood;  but 
so  closely  do  we  associate  with  it  the  idea  of  warmth  that 
the  sensation  experienced  on  touching  a  corpse  produces  so 
powerful  an  impression  as  commonly  to  be  described  as  icy 
cold.  The  other  great  characteristic  of  the  living  Body  is  its 
power  of  executing  movements;  so  long  as  life  lasts  it  is 
never  at  rest;  even  in  the  deepest  slumber  the  regular  breath- 
ing, the  tap  of  the  heart  against  the  chest-wall,  and  the  beat 
of  the  pulse  tell  us  that  we  are  watching  sleep  and  not  death. 
If  to  this  we  add  the  possession  of  consciousness  by  the  living 
Body,  whether  aroused  or  not  by  forces  immediately  acting 
upon  sense-organs,  we  might  describe  it  as  a  heat-producing, 
moving,  conscious  organism. 

The  production  of  heat  in  the  Body  needs  fuel  of  some 
kind  as  much  as  its  production  in  a  fire;  and  every  time  we 
move  ourselves  or  external  objects  some  of  the  Body  is  used 
up  to  supply  the  necessary  working  power,,  just  as  some  coals 
are  burnt  in  the  furnace  of  an  engine  for  every  bit  of  work  it 
does;  in  the  same  way  every  thought  that  arises  in  us  is  ac- 
companied with  the  destruction  of  some  part  of  the  Body. 
Hence  these  primary  actions  of  keeping  warm,  moving,  and 
being  conscious,  necessitate  many  others  for  the  supply  of 
new  materials  to  the  tissues  concerned  and  for  the  removal  of 
their  wastes;  still  others  are  necessary  to  regulate  the  pro- 
duction and  loss  of  heat  in  accordance  with  changes  in  the 
exterior  temperature,  to  bring  the  moving  tissues  into  rela- 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS.      17 

tion  with  the  thinking,  and  so  on.  By  such  subsidiary  ar- 
rangements the  working  of  the  whole  Body  becomes  so  com- 
plex that  it  would  fill  many  pages  merely  to  enumerate  what 
is  known  of  the  duties  of  its  various  parts.  However,  all  the 
proper  physiological  properties  depend  in  ultimate  analysis 
on  a  small  number  of  faculties  which  are  possessed  by  all 
living  things,  their  great  variety  in  the  human  Body  depend- 
ing upon  special  development  and  combination  in  different 
tissues  and  organs;  and  before  attempting  to  study  them  in 
their  most  complex  forms  it  is  advantageous  to 
examine  them  in  their  simplest  and  most  gen- 
eralized manifestations,  as  exhibited  by  some  of 
the  lowest  living  things  or  by  the  simplest  con- 
stituents of  our  own  Bodies. 

Cells.  Among  the  anatomical  elements 
which  the  histologist  meets  with  as  entering  into 
the  composition  of  the  human  Body  are  minute 
granular  masses  of  a  soft  consistence,  about 
0.012  millimeter  (^gVrr  °^  an  incn)  in  diameter 
(Fig.  5,  b).  Imbedded  in  each  lies  a  central 
portion,  not  so  granular  and  therefore  different 
in  appearance  from  the  rest.  These  anatomical 
units  are  known  as  cells,  the  granular  substance  fig.5— Forms 
being  the  cell-body  and  the  imbedded  clearer  por-  gf0^y ls  from  the 
tion  the  cell-nucleus.  Inside  the  nucleus  may 
often  be  distinguished  a  still  smaller  body — the  nucleolus. 
Cells  of  this  kind  exist  in  abundance  in  the  blood,  where  they 
are  known  as  the  white  blood-corpuscles,  and  each  exhibits  of 
itself  certain  properties  which  are  distinctive,  of  all  living 
things  as  compared  with  inanimate  objects. 

Cell  Growth.  In  the  first  place,  each  such  cell  can  take 
up  materials  from  its  outside  and  build  them  up  into  its  own 
peculiar  substance;  and  this  does  not  occur  by  the  deposit  of 
new  layers  of  material  like  its  own  on  the  surface  of  the  cell 
(as  a  crystal  might  increase  in  an  evaporating  solution  of  the 
same  salt),  but  in  an  entirely  different  way.  The  cell  takes 
up  chemical  elements,  either  free  or  combined  in  a  manner 
different  from  that  in  which  they  exist  in  its  own  living  sub- 
stance, and  works  chemical  changes  in  them  by  which  they 
are  made  into  pari  and  parcel  of  itself.  Moreover,  the  new 
material  thus  formed  is  not  deposited,  al  any  rah;  necessarily 
or  always,  on  the  surface  of  the  old,  but  is  laid  down  in  the 


18  THE  HUMAN  BODY. 

substance  of  the  already  existing  cell  among  its  constituent 
molecules.  The  new-formed  molecules  therefore  contribute 
to  the  growth  of  the  cell  not  by  superficial  accretion,  but  by 
interstitial  deposit  or  intussusception. 

Cell  Division.  The  increase  of  size,  which  may  be  brought 
about  in  t he  above  manner,  is  not-  indefinite,  but  is  limited  in 
two  ways.  Alongside  of  the  formation  and  deposit  of  new 
material  there  occurs  always  in  the  living  cell  a  breaking 
down   and   elimination  of  the  old;  and   when  this  process 


Fig.  6.— Diagrams  illustrating  direct  cell  division,    a,  cell,  body;  b,  nucleus; 

c,  nucleolus. 

equals  the  accumulation  of  new  material,  as  it  does  in  all  the 
cells  of  the  Body  when  they  attain  a  certain  size,  growth 
of  course  ceases.  In  fact  the  work  of  the  cell  increases 
as  its  mass,  and  therefore  as  the  cube  of  its  diameter; 
while  the  receptive  powers,  dependent  primarily  upon  the 
superficial  area,  only  increase  as  the  square  of  the  diameter. 
The  breaking  down  in  the  cell  increases  when  its  work 
does,  and  so  comes  at  last  to  equal  the  reception  and  con- 
struction. The  second  limitation  to  indefinite  growth  is 
connected  with  the  power  of  the  cell  to  give  rise  to  new  cells 
by  division. 

Until  recently  it  was  believed  that  cell  division  was  in  all 
cases  a  comparatively  simple  process  (Fig.  G).  It  was  thought 
that  the  nucleus,  without  any  important  structural  clump'. 
enlarged  somewhat,  became  elongated,  and  then  divided  by 
simple  constriction  into  two  equal  parts,  forming  two  smaller 
daughter  nuclei;  and  that  the  rest  of  the  cell  then  divided, 
its  halves  arranging  themselves  around  the  new  nuclei.  The 
nucleolus  when  present  was  supposed  to  divide  before  the 
nucleus.  Such  a  mode  of  cell  multiplication  is  known  as 
direct  division  :  it  possibly  occurs  in  some  cases,  but  in  the 
great  majority  of  cells  division  is  preceded  by  marked  changes 
in  the  structure  of  the  nucleus  and  by  a  rearrangement  of  its 
material:  such  cell  division  is  named  indirect,  and  the  attend- 
ant nuclear  changes  are  known  as  the  phenomena  of  kanjoki- 
nesis  or  mitosi" 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS.      19 


Fig.  V.— Diagram  of  an  animal 
cell,  a,  hyaloplasm;  b,  reticu- 
lum ;  c.  nucleus,  a  and  6  together 
form  the  cell-body. 


Indirect,  Karyokinetic  or  Mitotic  Cell  Division.  Before 
attempting  to  describe  the  phenomena  of  indirect  cell  divi- 
sions it  is  necessary  to  give  some 
account  of  the  structure  of  a  typical 
primitive  cell  as  made  out  in  speci- 
mens carefully  prepared  and  studied 
with  the  highest  powers  of  the 
microscope.  The  main  bulk  of  the 
cell,  surrounding  the  nucleus,  is  the 
cell-body,  and  in  some  cases  is  en- 
closed in  an  envelope  or  sac,  which, 
however,  when  present,  plays  but  a 
secondary  or  passive  part  in  cell  divi- 
sion. The  cell-body,  known  also  as 
the  cell-protoplasm  (Fig.  7),  consists  of  a  network  of  extremely 
fine  threads,  the  reticulum  or  9pongioplas?n,the  meshes  of  which 
are  occupied  by  a  different  substance,  the  hyaloplasm :  the 
proportions  of  hyaloplasm  and  spongioplasm  vary  in  different 
cells  and  often  in  different  parts  of  the  same  cell;  in  fact  a 
layer  of  hyaloplasm  unmixed  with  spongioplasm  frequently 
exists  on  the  exterior  of  the  cell,  and  the  hyaloplasm  appears 
to  be  the  more  immediately  concerned  in  the  activities  of  the 
living  cell.  In  addition  there  is  to  be  found,  imbedded  in  the 
cell-body  and  near  the  nucleus  or  attached  to  it,  an  extremely 
minute  particle,  the  attraction-particle  or  centrosome,  near 

which  a  radial  arrangement  of  the 
cell-substance  may  often  be  ob- 
served . 

The  nucleus  (Fig.  8)  of  a 
resting  cell,  that  is  of  a  cell  not 
in  process  of  division,  consists  of 
an  amorphous  material  {nucleo- 
plasm) which  is  perhaps  similar 
in  composition  to  the  hyaloplasm, 
and  a  filamentous  material,  dif- 
ferent from  spongioplasm,  and 
named  chrornoplasm  <>r  haryo- 
plasm.  As  proved  by  its  behavior 
with  staining  fluids  and  other 
reagents  karyoplasm  is  quite  different  chemically  from  the 
spongioplasm  of  the  cell-body.  One  or  more  granules  {nu- 
cleoli) which   may  b<-  found  within   most  nuclei  are  probably 


Fio.  H.— Diagram  of  a  resting 
nucleus,  it.  nuclear  membrane;  i>, 
nucleoplasm;  c,  nucleolus;  </.  enro- 
moplasm;  e,  some  of  the  surround- 
ing protoplasm  of  the  cell,  the 
structure  "f  which  is  not  Indicated. 


2o 


THE  HUMAN  BODY. 


local  accumulations  of  chromoplasm;  a  membrane  (nuclear 
membrane)  which  surrounds  the  nucleus  of  cells  not  in  process 
of  division  is  also  probably  composed  of  chromoplasm. 

The  first  observed  step  in  cell  division  is  binary  division  of 
the  attraction-particle:  its  halves  evolve  a  set  of  very  fine 
achromatin  filaments  uniting  them,  so  thai  each  half  is  one  of 
the  poles  of  a  spindle-shaped  collection  of  fibres,  the  nuclear 
spindle.  Meanwhile  the  nucleolus  and  nuclear  membrane 
disappear,  being  probably  taken  up  into  the  rest  of  the  chro- 
moplasm, which  now,  instead  of  its  original  reticular  arrange- 


Fig.  '■'  —  Diagrams  of  a  nucleus  in  an  early  stage  of  karyokinesis,  A  showing  the 
polar,  B  the  ant.ipolar  region;  o,  nuclear  or  achromatin  spindle;  i>,  part  of  treneral 
cell-protoplasm  around  the  nucleus;  c,  looped  chromatic  filament;  d,  nucleoplasm. 

ment,  takes  the  form  of  a  single  long  chromatic  filament 
coiled  in  the  nucleoplasm.  At  one  portion  of  the  nucleus 
(pole)  the  loops  of  the  chromatic  filament  leave  a  space  free 
from  them  (Fig.  0,  A),  and  in  the  neighborhood  of  this  space 
the  nuclear  spindle  is  first  seen  within  the  nucleus.  At  the 
opposite  side  of  the  nucleus  or  antipole  (Fig.  9,  B)  the  loops 
of  the  chromatic  filament  leave  no  clear  space,  but  cross  ir- 
regularly. In  the  next  stage  the  loops  at  the  antipolar  end 
break  through,  and  in  this  way  the  filament  is  divided  into  a 
number  of  irregular  elongated  Vs,  each  with  its  closed  angle 
near  the  pole  and  its  open  end  near  the  antipole.  The  spindle 
meanwhile  passes  to  the  centre  of  the  nucleus  and  takes  a  posi- 
tion in  which  its  long  axis  coincides  with  that  joining  pole  and 
antipole,  and  then  the  Vs  of  chromoplasm  become  shorter  and 
their  limbs  thicker,  and  they  also  shift  position  so  as  to  group 
themselves  radially  around  the  equator  of  the  spindle  (A,  Fig. 
10)  with  their  angles  directed  centrally.  Each  V  then  di- 
vides along  its  whole  length,  and  one  half  passes  towards  the 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS.      21 

pole,  the  other  towards  the  antipole.  The  whole  nucleus 
elongates  in  the  direction  of  the  long  axis  of  the  spindle;  the 
achromatin  filaments  disappear,  and  the  nucleus  dividing  in 


*ig.  10.—  Diagrams  representing  more  advanced  stages  of  karvokinesis  thai)  those 
illustrated  in  F  \z.  9.  a,  polar,  and  e,  anripolar  end  of  nuclear  spindle;  b  and  c,  por- 
tions of  the  chromatic  filament;  <Y.  nucleoplasm;  f,  cell  protoplasm  with  indications 
of  a  radial  arrangement  in  the  neighborhood  of  the  pole  and  antipole. 

The  nuclear  spiudle  is  seen  to  have  lengthened  and  become  placed  in  the  centre 
of  the  nucleus,  the  pole  and  antipole  of  which  its  ends  reach.  In  A  the  Vs  which 
resulted  from  divisions  of  the  chromatic  filament  at  its  antipolar  loops  are  seen  to 
have  become  much  shorter  and  thicker  and  to  have  changed  position,  so  that  in- 
stead of  lying  lengthwise  in  the  nucleus,  with  their  points  towards  the  pole,  they  lie 
equatonally,  with  their  points  towards  the  spindle  and  their  open  ends  towards  the 
periphery  of  the  nucleus.  For  the  sake  of  clearness  only  two  are  represented  out 
or  the  set  of  them  which  surrounds  the  spindle;  b  is  still  uncleft;  c  has  nearly  com- 
pleted its  longitudinal  division  into  two  Vs,  the  angle  of  one  of  which  is  commencing 
to  travel  towards  the  pole  and  of  the  other  towards  the  antipole.  In  B  the  splitting 
Of  the  Vs  and  the  progress  of  their  halves  towards  the  ends  of  the  nucleus  is  more 
advanced. 


the  equatorial  plane,  two  nuclei  are  formed,  each  with  nucleo- 
plasm and  chromoplasm  :  the  chromoplasm  of  each  is  derived, 
as  follows  from  the  preceding  description,  from  both  polar  and 
antipolar  regions  of  the  parent  nucleus.  The  chromoplasm  in 
each  daughter  nucleus  unites  into  a  .single  convoluted  chro- 
matic filament  like  that  represented  for  the  parent  nucleus  in 
Pig.  9,  and  this  filament  breaks  up  and  becomes  arranged 
into  reticulum,  nucleolus  and  nuclear  membrane  as  in  the 
resting  cell  (Figs.  7  and  8).  Around  the  new  nuclei  the  cell- 
protoplasm  rearranges  itself  and  divides  to  form  anew  cell-body 
enveloping  each;  during  its  rearrangement  its  material  fre- 
quently presents  a  radial  structure,  the  radii  converging  to- 
wards the  ends  of  the  nuclear  spindle.  The  poles  of  the  nuclear 
Bpindle,  which  it  will  be  remembered  represent  the  halves  of 


22  THE  HUMAN  BODY. 

the  original  centrosome,  probably  pass  out  of  tlie  new  nuclei 
and  become  the  attraction  particles  of  the  new  cells. 

The  phenomena  of  karyokinesis  show  clearly  that  in  spite  of 
its  small  size  the  animal  cell  is  a  complicated  structure,  made 
up  of  very  distinct  parts  possessing  very  distinct  properties 
and  no  doubt  very  different  functions. 

Assimilation :  Reproduction.  The  two  powers,  that  of 
working  up  into  their  own  substance  materials  derived  from 
outside,  known  as  assimilation,  and  that  of,  in  one  way  or  an- 
other, giving  rise  to  new  beings  like  themselves,  known  as  re- 
production, are  possessed  by  all  kinds  of  living  beings,  whether 
animals  or  plants.  There  is,  however,  this  important  differ- 
ence between  the  two :  the  power  of  assimilation  is  necessary 
for  the  maintenance  of  each  individual  cell,  plant  or  animal, 
since  the  already  existing  living  material  is  constantly  break- 
ing down  and  being  removed  as  long  as  life  lasts,  and  the  loss 
must  be  made  good  if  any  of  them  is  to  continue  its  existence. 
The  power  of  reproduction,  on  the  other  hand,  is  necessary 
only  for  the  continuance  of  the  kind  or  race,  and  need  be,  and 
often  is, possessed  only  by  some  of  the  individuals  composing  it. 
Working  bees,  for  example,  cannot  reproduce  their  kind,  that 
duty  being  left  to  the  queen-bee  and  the  drones  of  each  hive. 

The  breaking  down  of  already  existing  chemical  compounds 
into  simpler  ones,  sometimes  called  dissimilation,  is  as  inva- 
riable in  living  beings  as  the  building  up  of  new  complex  mole- 
cules referred  to  above.  It  is  associated  with  the  assumption 
of  uncombined  oxygen  from  the  exterior,  which  is  then  com- 
bined directly  or  indirectly  with  other  elements  in  the  cell,  as, 
for  example,  carbon,  giving  rise  to  carbon  dioxide,  or  hydro- 
gen, producing  water.  In  this  way  the  molecule  in  which  the 
carbon  and  hydrogen  previously  existed  is  broken  down  and 
at  the  same  time  energy  is  liberated,  which  in  all  cases  seems 
to  take  in  part  the  form  of  heat  just  as  when  coal  is  burnt  in 
a  fire,  but  may  be  used  in  part  for  other  purposes,  such  as  pro- 
ducing movements.  The  carbon  dioxide  is  usually  got  rid 
of  by  the  same  mechanism  as  that  which  serves  to  take  up  the 
oxygen,  and  these  two  processes  constitute  the  function  of 
respiration  which  occurs  in  all  living  things.  Assimilation 
and  dissimilation,  going  on  side  by  side  and  being  to  a  certain 
extent  correlative,  are  often  spoken  of  together  as  the  process 
of  nutrition  :  the  assimilative  or  chemically  constructive  pro- 
cesses are  also  named  anabolic,  and  the  dissimilative  hatabolic. 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS.      23 

Contractility.  Nutrition  and  (with  the  above-mentioned 
partial  exception)  reproduction  characterize  all  living  creat- 
ures; and  both  faculties  are  possessed  by  the  simple  nucleated 
ceils  already  referred  to  as  fouud  in  our  blood.  But  these 
cells  possess  also  certain  other  properties  which,  although  not 
so  absolutely  diagnostic,  are  yet  very  characteristic  of  living 
things.  Examined  carefully  with  a  microscope  in  a  fresh- 
drawn  drop  of  blood,  they  exhibit  changes  of  form  independent 
of  any  pressure  which  might  distort  them  or  otherwise  mechani- 
cally alter  their  shape.  These  changes  may  sometimes  show 
themselves  as  constrictions  ultimately  leading  to  the  division 
of  the  cell  ;  but  more  commonly  (Fig.  15*)  they  have  no  such 
result,  the  cell  simply  altering  its  form  by  drawing  in  its  sub- 
stance at  one  point  and  thrusting  it  out  at  another.  The 
portion  thus  protruded  may  in  turn  be  drawn  in  and  a  pro- 
cess be  thrown  out  elsewhere  ;  or  the  rest  of  the  cell  may  col- 
lect around  it,  and  a  fresh  protrusion  be  then  made  on  the 
same  side  ;  and  by  repeating  this  manoeuvre  these  cells  may 
change  their  place  and  creep  across  the  field  of  the  micro- 
scope. Such  changes  of  form  from  their  close  resemblance  to 
those  exhibited  by  the  microscopic  animal  known  as  the 
Aiixeba  (see  Zoology)  are  called  ammboid,  and  the  faculty  in 
the  living  cell  upon  which  they  depend  is  known  in  physiol- 
ogy as  contractility.  It  must  be  borne  in  mind  that  physiol- 
ogical contractility  in  this  sense  is  quite  different  from  the 
so-called  contractility  of  a  stretched  india-rubber  band, 
which  merely  tends  to  reassume  a  form  from  which  it  has 
previously  been  forcibly  removed. 

Irritability.  Another  property  exhibited  by  these  blood- 
cells  is  known  as  irritability.  An  Amoeba  coming  into  con- 
tact with  a  solid  particle  calculated  to  serve  it  as  food  will 
throw  around  it  processes  of  its  substance,  and  gradually 
carry  the  foreign  mass  into  its  own  body.  The  amount  of 
energy  expended  by  the  animal  under  these  circumstances  is 
altogether  disproportionate  to  the  force  of  the  external  contact. 
It  is  not  that  the  swallowed  mass  pushes-in  mechanically  the 
surface  of  the  Amoeba,  or  burrows  into  it.  but  the  mere  touch 
arouses  in  the  animal  an  activity  quite  disproportionate  to  the 
exciting  force,  and  comparable  to  that  set  free  by  a  spark 
falling  into  gunpowder  or  by  a  slight  tap  on  ;i  piece  of  gun- 
cotton,     it  is  this  disproportion  between  the  excitant  (known 

*  1\  4M. 


24  THE  HUMAN  BOD  Y. 

in  Physiology  as  a  stimulus)  and  the  result,  which  is  the  i  -• 
sential  characteristic  of  irritability  when  the  term  is  used  in 
a  physiological  connection.  The  granular  cells  of  the  blood 
can  take  foreign  matters  into  themselves  in  exactly  the  same 
manner  as  an  Amu-ba  does;  and  in  this  and  in  other  ways,  as 
by  contracting  into  rigid  spheres  under  the  influence  of  elec- 
trical shocks,  they  show  that  they  also  are  endowed  with  irri- 
tability. 

Conductivity.  Further,  when  an  Amoeba  or  one  of  these 
blood-cells  comes  into  contact  with  a  foreign  body  and  pro- 
ceeds to  draw  it  into  its  own  substance,  the  activity  excited 
is  not  merely  displayed  by  the  parts  actually  touched.  Dis- 
tant parts  of  the  cell  also  co-operate,  so  that  the  influence  of 
the  stimulus  is  not  local  only,  but  in  consequence  of  it  a  change 
is  brought  about  in  other  parts,  arousing  them.  This  prop- 
erty of  transmitting  disturbances  is  known  as  conductivity. 

Finally,  the  movements  excited  are  not,  as  a  rule,  random. 
They  are  not  irregular  convulsions,  but  are  adapted  to  attain 
a  certain  end,  being  so  combined  as  to  bring  the  external  par- 
ticle into  the  interior  of  the  cell.  This  capiacity  of  all  the 
parts  to  work  together  in  definite  strength  and  sequence  to 
fulfil  some  purpose,  is  known  as  co-ordination. 

These  Properties  Characteristic  but  not  Diagnostic. 
These  four  faculties,  irritability,  conductivity,  contractility 
and  co-ordination,  are  possessed  in  a  high  degree  by  our 
Bodies  as  a  whole.  If  the  inside  of  the  nose  be  tickled  with 
a  feather,  a  sneeze  will  result.  Here  the  feather-touch  (stim- 
ulus) has  called  forth  movements  which  are  mechanically 
altogether  disproportionate  to  the  energy  of  the  contact,  so 
that  the  living  Body  is  clearly  irritable.  The  movements, 
which  are  themselves  a  manifestation  of  contractility,  are  not 
exhibited  at  the  point  touched,  but  at  more  or  less  distant 
parts,  among  which  those  of  abdomen,  chest  and  face  are 
visible  from  the  exterior  ;  our  Bodies  therefore  possess  physio- 
logical conductivity.  And  finally  these  movements  are  not 
random,  but  combined  so  as  to  produce  a  violent  current  of 
air  through  the  nose  tending  to  remove  the  irritating  object: 
and  in  this  we  have  a  manifestation  of  co-ordination.  Speak- 
ing broadly,  these  properties  are  more  manifest  in  animals 
than  in  plants,  though  they  are  by  no  means  absolutely  con- 
fined to  the  former.  In  the  sensitive  plant  touching  one  leaflet 
will  excite  regular  movements  of  the  whole  leaf,  and  many  of 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS.      25 

the  lower  aquatic  plants  exhibit  movements  as  active  as  those 
of  animals.  On  the  other  hand,  no  one  of  these  four  faculties 
is  absolutely  distinctive  of  living  things  in  the  way  that  growth 
by  intussusception  and  reproduction  are.  Irritability  is  but 
a  name  for  unstable  molecular  equilibrium,  and  is  as  marked 
in  nitroglycerin  as  in  any  living  cells;  in  the  telephone  the 
influence  of  the  voice  is  conducted  as  a  molecular  change 
along  a  wire,  and  produces  results  at  a  distance;  and  many 
inanimate  machines  afford  examples  of  the  co-ordination  of 
movements  for  the  attainment  of  definite  ends. 

Spontaneity.  There  is,  however,  one  character  belonging 
to  many  of  the  movements  exhibited  by  amoeboid  cells,  in 
which  they  appear  at  first  sight  to  differ  fundamentally  from 
the  movements  of  inanimate  objects.  This  character  is  their 
apparent  spontaneity  or  automat  icity.  The  cells  frequently 
change  their  form  independently  of  any  recognizable  external 
cause,  while  a  dead  mass  at  rest  and  unacted  on  from  outside 
remains  at  rest.  This  difference  is,  however,  only  apparent 
and  depends  not  upon  any  faculty  of  spontaneous  action  pe- 
culiar to  the  living  cell,  but  upou  its  nutritive  powers.  It 
can  be  proved  that  any  system  of  material  particles  in  equi- 
librium and  at  rest  will  forever  remain  so  if  not  acted  upon 
by  an  external  force.  Such  a  system  can  ?.arry  on,  under  cer- 
tain conditions,  a  series  of  changes  when  once  a  start  has 
been  given;  but  it  cannot  initiate  them.  Each  living  cell 
in  the  long-run  is  but  a  complex  aggregate  of  molecules, 
composed  in  their  turn  of  chemical  elements,  and  if  we  sup- 
pose this  whole  set  of  atoms  at  rest  in  equilibrium  at  any 
moment,  no  change  can  be  started  in  the  cell  from  inside;  in 
other  words,  it  will  possess  no  real  spontaneity.  When,  how- 
ever, we  consider  the  irritability  of  amoeboid  cells,  or,  ex- 
pressed  in  mechanical  terms,  the  unstable  equilibrium  of  their 
particles,  it  becomes  obvious  that  a  very  slight  external  cause, 
such  as  may  entirely  elude  our  observation,  may  serve  to  set 
going  in  them  a  very  marked  series  of  changes,  just  as  pressing 
the  trigger  will  fire  oif  a  gun.  Once  the  equilibrium  of  the  cell 
lias  been  disturbed,  movements  either  of  some  of  its  constitu- 
ent molecules  or  of  its  whole  mass  will  continue  until  all  the 
molecules  have  again  settled  down  into  a  stable  state.  Hut 
in  living  cells  the  reattainment  of  this  state  is  commonly  in- 
definitely postponed  by  the  reception  of  new  particles,  food 
in  cm*"  form  or  another,  from  the  exterior.    The  nearest  ap- 


26  THE  III  MAS   BODY. 

proach  to  it  is  probably  exhibited  by  the  resting  Btate  into 
which  sour'  of  the  lower  ;ui i trials,  as  the  wheel-animalcules, 
pass  when  dried  slowly  at  a  low  temperature;  the  drying  act- 
ing by  checking  the  nutritive  processes,  which  would  other- 
wise have  prevented  the  reattainment  of  molecular  equilib- 
rium. All  signs  of  movement  or  other  change  disappear 
under  these  circumstances,  bul  as  soon  as  water  again  soaks 
into  their  substance  and  disturbs  the  existing  condition,  then 
the  so-called  "spontaneous"  movements  recommence.  If, 
therefore,  we  use  the  term  spontaneity  to  express  a  power  in 
u  resting  system  of  particles  of  initiating  changes  in  itself,  it 
is  possessed  neither  hy  living  nor  not-living  things.  But  if 
we  simply  employ  it  to  designate  changes  whose  primary 
cause  we  do  not  recognize,  and  whose  cause  was  in  many 
cases  long  antecedent  to  the  changes  which  we  see,  then  the 
term  is  unobjectionable  and  convenient,  as  it  serves  to  ex- 
press briefly  a  phenomenon  presented  by  many  living  things 
and  finding  its  highest  manifestation  in  many  human  actions. 
It  then,  however,  no  longer  designates  a  property  peculiar  to 
them.  A  steam-engine  with  its  furnace  lighted  and  water  in 
its  boiler  may  be  set  in  motion  by  opening  a  valve,  and  the 
movements  thus  started  will  continue  spontaneously,  in  the 
above  sense,  until  the  coals  or  water  are  used  up.  The  differ- 
ence between  it  and  the  living  cell  lies  not  in  any  spontaneity 
of  the  latter,  but  in  its  nutritive  powers,  which  enable  it  to 
replace  continually  what  answers  to  the  coals  and  water  of 
the  engine. 

Protoplasm.  The  cell-body  was  formerly  regarded  as  es- 
sentially made  up  of  a  single  substance,  which  was  named 
protoplasm  :  and  now  that  its  structure  is  known  to  be  com- 
plex the  term  is  retained  as  a  convenient  one  for  that  mixture 
of  spongioplasm  and  hyaloplasm  which  constitutes  the  main 
bulk  of  the  bodies  of  most  cells.  With  the  protoplasm  other 
things  are  frequently  present,  the  most  important  of  which 
are  either  materials  undergoing  anabolic  changes  but  not  yet 
completely  built  up  into  protoplasm,  or  katabolic  materials 
resulting  from  the  chemical  degradation  of  protoplasm  : 
these  secondary  matters,  mingled  with  the  completed  proto- 
plasm, are  conveniently  spoken  of  as  Hie  cell  deutoplasm  or 
paraplasm.  As  between  the  spongioplasm  and  hyaloplasm 
there  are  still  some  differences  of  opinion  as  to  which  is  the 
more  immediate  agent  in  the  manifestation  of  the  vital  activ- 


THE  FUNDAMENTAL  PHYSIOLOGICAL  ACTIONS.      27 

ities  of  the  cell.  So  far  as  the  manifestation  of  the  power  of 
movement  is  concerned  the  evidence  seems  in  favor  of  the 
hyaloplasm:  the  outermost  parts  of  a  white  blood-corpuscle, 
for  example,  exhibit  active  contractile  power,  yet  they  con- 
tain no  spongioplastic  filaments;  and  many  unicellular  living 
things  are  known  in  which  no  reticular  structure  can  be  dis- 
covered and  which  nevertheless  nourish  themselves  and  are 
reproductive,  irritable,  contractile,  conductive,  co  ordinative 
and  automatic.  It  is  therefore  possible  that  the  filaments 
when  present  are  to  be  regarded  as  secondary  in  importance 
to  the  hyaloplasm,  partly  serving  as  a  mechanical  support; 
but  in  addition  they  may  play  an  important  part  in  the  inter- 
nal economy  of  the  cell.  The  study  of  the  physiology  of  in- 
dividual cells  presents  very  great  difficulties  and  is  yet  in  its 
beginnings,  so  that  we  can  do  little  more  than  speak  of  the 
properties  of  the  cell  as  a  whole,  though  from  the  frequent 
radial  arrangement  of  the  cell-protoplasm  in  its  neighborhood 
and  from  the  part  it  plays  in  the  initiation  of  cell  division, 
the  attraction-particle  appears  to  have  a  very  important  role. 

Of  the  actual  chemical  composition  of  living  matter  we 
know  only  that  its  molecule  is  one  of  great  complexity:  all 
methods  of  chemical  analysis  break  it  up  and  alter  it  funda- 
mentally, so  that  what  is  really  analyzed  is  not  living  matter 
but  a  mixture  of  the  products  of  its  decomposition,  among 
which  proteid  substances  are  always  prominent. 

Cell-protoplasm  no  doubt  varies  a  little  in  different  cells, 
so  that  the  name  is  to  be  regarded  as  a  general  term  designat- 
ing a  number  of  closely-allied  substances  agreeing  with  one 
another  chemically  in  main  points,  as  the  proteids  do,  but 
differing  in  minor  details,  in  consequence  of  which  one  cell 
differs  from  another  in  faculty.  On  proximate  analysis  every 
mass  of  protoplasm  is  found  to  contain  much  water  and  a 
certain  amount  of  mineral  salts;  the  water  being  in  part  con- 
stituent or  entering  into  the  structure  of  the  particles  of  pro- 
toplasm, and  in  part  probably  deposited  in  layers  between 
them.  Of  organic  constituents  protoplasm  always  yields  one 
or  more  proteids,  some  fats,  and  some  starchy  or  saccharine 
body.  So  that  the  original  protoplasm  is  probably  to  be  re- 
garded as  containing  chemical  "residues"  of  proteids,  fats 
an  1  carbohydrates,  combined  with  salts  and  water. 

The  name  nuclein  has  been  given  to  a  substance  or  mix- 
ture of  substances  which  are  left  behind  when  the  cell-proto- 


28  THE  HUMAN  BODY. 

plasm  has  been  dissolved  away  by  various  reagents:  it  con- 
tains a  considerable  quantity  of  phosphorus.  In  the  living 
nucleus  nuclein  seems  to  be  combined  with  various  proteids 
to  form  nucleo-albu tnius. 

The  Fundamental  Physiological  Properties.  All  living 
animals  possess  in  greater  or  less  degree  the  properties  con- 
sidered in  this  chapter;  and  since  the  science  of  physiology 
is  virtually  concerned  with  considering  how  these  properties 
are  acquired,  maintained  and  manifested,  and  for  what  ends 
they  are  employed,  we  may  call  them  the  fundamental  physi- 
ological properties. 


CHAPTER   III. 

THE  DIFFERENTIATION   OF   THE   TISSUES   AND   THE 
PHYSIOLOGICAL   DIVISION   OF  EMPLOYMENTS. 

Development.  Every  Human  Body  commences  its  indi- 
vidual existence  as  a  single  nucleated  cell.  This  cell,  known 
as  the  ovum,  divides  or  segments  and  gives  rise  to  a  mass  con- 


Fig.  l\x.-A,  an  ovum;  B  to  E,  successive  stages  in  its  segmentation  until  the 
morula,  F,  is  produced;  a,  cell-sac;  b,  cell  contents;  c,  nucleus. 

sisting  of  a  number  of  similar  units  and  called  the  mulberry 
mass  or  the  morula.  At  this  stage,  long  before  birth,  there 
are  no  distinguishable  tissues  entering  into  the  structure  of 
the  Body,  nor  are  any  organs  recognizable. 

For  a  short  time  the  morula  increases  in  size  by  the 
growth  and  division  of  its  cells,  but  very  soon  new  processes 
occur  which  ultimately  give  rise  to  the  complex  adult  body 
with  its  many  tissues  and  organs.  Groups  of  cells  ceasing  to 
grow  and  multiply  like  their  parents  begin  to  grow  in  ways 
peculiar  to  themselves,  and  so  come  to  differ  both  from  the 
original  cells  of  the  morula  and  from  the  cells  of  other  groups, 
and  this  unlikenese  becoming  more  and  more  marked,  a 
varied  whole  is  finally  built  up  from  one  originally  alike  in 

29 


30  THE  HUMAN  BODY. 

all  its  parts.  Peculiar  growth  of  this  kind,  forming  a  com- 
plex from  a  simple  whole,  Is  called  development;  and  the  pro- 
cess itself  in  this  case  is  known  as  the  differentiation  of  the 
I  issues,  since  by  it  they  are,  so  to  speak,  separated  or  special- 
ized from  the  general  mass  of  mother-cells  forming  the 
morula. 

As  the  differences  in  the  form  and  structure  of  the  con- 
stituent cells  of  the  morula  become  marked,  differences  in 
property  arise,  and  it  becomes  obvious  that  the  whole  cell- 
aggregate  is  not  destined  to  give  rise  to  a  collection  of  inde- 
pendent living  things,  but  to  form  a  single  human  being,  in 
whom  each  part,  while  maintaining  its  own  life,  shall  have 
duties  to  perform  for  the  good  of  the  whole.  In  other  words, 
a  single  compound  individual  is  to  be  built  up  by  the  union 
and  co-operation  of  a  number  of  simple  ones  represented  by 
the  various  cells,  each  of  which  thenceforth,  while  primarily 
looking  after  its  own  interests  and  having  its  own  peculiar 
faculties,  has  at  the  same  time  its  activities  subordinated  to 
the  good  of  the  entire  community. 

The  Physiological  Division  of  Labor.  The  fundamental 
physiological  properties,  originally  exhibited  by  all  the  cells, 
become  ultimately  distributed  between  the  different  modified 
cells  which  form  the  tissues  of  the  fully  developed  Body 
much  in  the  same  way  as  different  employments  are  dis- 
tributed in  a  civilized  state;  for  the  difference  between  the 
fully  developed  Human  Body  and  the  collection  of  amoeboid 
cells  from  which  it  started  is  essentially  the  same  as  that 
between  a  number  of  wandering  savages  and  a  civilized  nation. 
In  the  former,  apart  from  differences  dependent  on  sex,  each 
individual  has  no  one  special  occupation  different  from  that 
of  the  rest,  but  has  all  his  own  needs  to  look  after:  he  must 
collect  his  own  food  and  prepare  it  for  eating,  make  his  own 
clothes,  if  he  wear  any,  provide  his  own  shelter,  and  defend 
himself  from  wild  beasts  or  his  fellow  men.  In  the  civilized 
country,  on  the  other  hand,  we  find  agriculturists  to  raise 
food  and  cooks  to  prepare  it,  tailors  to  make  clothes,  and 
policemen  and  soldiers  to  provide  protection.  And  just  as 
we  find  that  when  distribution  of  employments  in  it  is  more 
minute  a  nation  is  more  advanced  in  civilization,  so  is  an 
animal  higher  or  lower  in  the  scale  according  to  the  degree 
in  which  it  exhibits  a  division  of  physiological  duties  between 
its  different  tissues. 


THE  DIFFERENTIATION  OF  TISSUES.  31 

From  the  subdivision  of  labor  in  advanced  communities 
several  important  consequences  arise.  In  the  first  place,  each 
man  devoting  himself  to  one  kind  of  work  mainly  and  relying 
upon  others  for  the  supply  of  his  other  needs,  every  sort  of 
work  is  better  done.  The  man  who  is  constantly  making 
boots  becomes  more  expert  than  one  whose  attention  is  con- 
stantly distracted  by  other  duties,  and  he  can  not  only  make 
more  boots  in  a  given  time,  but  better  ones;  and  so  with  the 
performance  of  all  other  kinds  of  work.  In  the  second  place, 
a  necessity  arises  for  a  new  sort  of  industry,  in  order  to  cou- 
vey  the  produce  of  one  individual  in  excess  of  the  needs  of 
himself  and  his  family  to  those  at  a  distance  who  may  want 
it,  and  to  convey  back  in  return  the  excess  of  their  produce 
which  he  needs.  The  carriage  of  food  from  the  country  to 
cities,  and  of  city  produce  to  country  districts,  and  the  occu- 
pation of  shopkeeping,  are  instances  of  these  new  kinds  of 
labor  which  arise  in  civilized  communities.  In  addition  there 
is  developed  a  need  for  arrangements  by  which  the  work  of 
individuals  shall  be  regulated  in  proportion  to  the  wants  of 
the  whole  community,  such  as  is  in  part  effected  by  the 
agency  of  large  employers  of  labor  who  regulate  the  activities 
of  a  number  of  individuals  for  the  production  of  various 
articles  in  the  different  quantities  required  at  different  times. 

Exactly  similar  phenomena  result  from  the  subdivision  of 
labor  in  the  Human  Body.  By  the  distribution  of  employ- 
ments between  its  different  tissues,  each  one  specially  doing 
one  work  for  the  general  community  and  relying  on  the 
others  for  their  aid  in  turn,  every  necessary  work  is  better 
performed.  And  a  need  arises  for  a  distributive  mechanism 
by  which  the  excess  products,  if  any,  of  various  tissues  shall 
be  carried  to  others  which  require  them,  and  for  a  regulative 
mechanism  by  which  the  activities  of  the  various  tissues  shall 
Ik:  rendered  proportionate  to  the  needs  of  the  whole  Body  at 
different  times  and   under  different  circumstances. 

Classification  of  the  Tissues. — As  we  might  separate  the 
inhabitants  of  tin;  United  Slates  into  groups,  such  as  lawyers, 
doctors,  clergymen,  merchants,  farmers,  and  so  forth,  so  we 
may  classify  the  tissues  by  selecting  the  most  distinctive 
properties  of  each  of  those  entering  into  the  construction  of 
the  adult  Body  and  arranging  them  into  physiological  groups; 
those  of  each  group  being  characterized  i>y  some  one  promi- 
nent employment.     No  such   classification,  however,  can  be 


32  THE  HUMAN   BODY. 

more  than  approximately  accurate,  since  the  same  tissue  has 
often  more  than  one  well-marked  physiological  property. 
The  following  arrangement,  however,  is  practically  convenient. 

1.  D  sin  PFERENTIATED  TISSUES.  These  are  composed  of 
cells  which  have  developed  along  no  one  special  line,  but 
retain  very  much  the  form  and  properties  of  the  cells  forming 
the  very  young  Body  before  different  tissues  were  recognizable 
in  it.  The  lymph-corpuscles  and  the  colorless  corpuscles  of 
the  blood  belong  to  this  class. 

•.'.  Supporting  Tissues.  Including  curtilage  (gristle), 
bone  and  conned  ire  tissue.  Of  the  latter  there  are  several 
subsidiary  varieties,  the  two  more  important  being  white 
fibrous  connect  ice  tissue,  composed  mainly  of  colorless  inex- 
tensible  fibres,  and  yellow  fibrous  tissue,  composed  mainly  of 
yellow  elastic  fibres.  All  the  supporting  tissues  are  used  in 
the  Body  for  mechanical  purposes  :  the  bones  and  cartilages 
form  the  hard  framework  by  which  softer  tissues  are  supported 
and  protected ;  and  the  connective  tissues  unite  the  various 
bones  and  cartilages,  form  investing  membranes  around  dif- 
ferent organs,  and  in  the  form  of  fine  networks  penetrate  their 
substance  and  support  their  constituent  cells.  The  functions 
of  these  tissues  being  for  the  most  part  to  passively  resist 
strain  or  pressure,  none  of  them  has  any  very  marked  phy- 
siological property;  they  are  not,  for  example,  irritable  or 
contractile,  and  their  mass  is  chiefly  made  up  of  an  intercell- 
ular substance  which  has  been  formed  by  the  actively  living 
cells  sparsely  scattered  through  them,  as  for  instance  in 
cartilage,  Fig.  45,  where  the  cells  are  seen  imbedded  in  cavi- 
ties in  a  matrix  which  they  have  formed  around  them;  and 
this  matrix  by  its  firmness  and  elasticity  forms  the  func- 
tionally important  part  of  the  tissue. 

3.  Nutritive  Tissues.  These  form  a  large  group,  the 
members  of  which  fall  into  three  main  divisions,  viz. : 

Assi m Unlive  tissues,  concerned  in  receiving  and  preparing 
food  materials,  and  including — (a)  Secretory  tissues,  com- 
posed of  cells  which  make  the  digestive  liquids  poured  into  the 
alimentary  canal  and  used  to  bring  about  chemical  or  other 
changes  in  the  food.  (f>)  Receptive  tissues,  represented  by 
cells  which  line  parts  of  the  alimentary  canal  and  take  up  the 
digested  food. 

Eliminative  or  excretory  tissues,  represented  by  cells  in  the 


THE  DIFFERENTIATION  OF  TISSUES,  33 

kidneys,  skin,  and  elsewhere,  whose  main  business  it  is  to  get 
rid  of  the  waste  products  of  the  various  parts  of  the  Body. 

Respiratory  tissues.  These  are  concerned  in  the  gaseous 
interchanges  between  the  Body  and  the  surrounding  air. 
They  are  constituted  by  the  cells  lining  the  lungs  and  by  the 
colored  corpuscles  of  the  blood. 

As  regards  the  nutritive  tissues  it  requires  especialiy  to  be 
borne  in  mind  that  although  such  a  classification  as  is  here 
given  is  useful,  as  helping  to  show  the  method  pursued  in  the 
domestic  economy  of  the  Body,  it  is  only  imperfect  and 
largely  artificial.  Every  cell  of  the  Body  is  in  itself  assimi- 
lative, respiratory,  and  excretory,  and  the  tissues  in  this  class 
are  only  those  concerned  in  the  first  and  last  interchanges 
of  material  between  it  and  the  external  world.  They  provide 
or  get  rid  of  substances  for  the  whole  Body,  leaving  the  feed- 
ing and  breathing  and  excretion  of  its  individual  tissues  to  be 
ultimately  looked  after  by  themselves,  just  as  even  the  mandarin 
described  b}"  Bobinson  Crusoe  who  found  his  dignity  promoted 
by  having  servants  to  put  the  food  into  his  mouth,  had  finally 
to  swallow  and  digest  it  for  himself.  Moreover,  there  is  no 
logical  distinction  between  a  secretory  and  an  excretory  cell: 
each  of  them  is  characterized  by  the  separation  of  certain  sub- 
stances which  are  poured  out  on  a  free  surface  on  the  exterior 
or  interior  of  the  Body.  Many  secretory  cells  too  have  no 
concern  with  the  digestion  of  food,  as  for  example  those 
which  form  the  tears  and  sweat. 

4.  Storage  Tissues.  The  Body  does  not  live  from  hand 
to  mouth:  it  has  always  in  health  a  supply  of  food-materials 
accumulated  in  it  beyond  its  immediate  needs.  This  lies  in 
part  in  the  individual  cells  themselves,  just  as  in  a  prosperous 
community  nearly  every  one  will  have  some  little  pocket- 
money.  But  apart  from  this  reserve  there  are  certain  cells, 
;i  sort  of  capitalists,  which  store  up  considerable  quantities  of 
material  and  constitute  what  we  will  call  the  storage  tissues. 
Thee  are  especially  represented  by  the  liver-cells  and  fat- 
cells,  which  contain  in  health  a  reserve  fund  for  the  rest  of 
the  Body.  Since  both  of  these,  together  with  secretory  and 
excretory  cells,  are  the  seats  of  great  chemical  changes,  they 

are  all  often  called   metabolic  tissiii's. 

:>.  [suitable  Tissues,  The  maintenance,  or  at  any  rate 
the  best  prosperity,  of  a  nation  is  nut  fully  secured  when  a 
division  of  labor  lias  taken  place  in  food-supply  and  I'ood-dis- 


34  THE  HUMAN  BODY. 

tribution  employments.  It  is  extremely  desirable  that  means 
shall  be  provided  by  which  it  may  receive  information  of  ex 
ternal  changes  which  may  affect  it  as  a  whole,  such  as  the 
policy  of  foreign  countries;  or  which  shall  enable  the  inhabi- 
tants of  one  part  to  know  the  needs  of  another,  and  direct 
their  activity  accordingly.  Foreign  ministers  and  consuls  and 
newspaper  correspondents  are  employed  to  place  it  in  com- 
munication with  other  states  and  keep  it  informed  as  to  its 
interests;  and  we  find  also  organizations,  such  as  the  meteor- 
ological department,  to  warn  distant  parts  of  approaching 
storms  or  other  climatic  changes  which  may  seriously  affect 
the  pursuits  carried  on  in  them.  In  the  Human  Body  we 
have  a  comparable  class  of  intelligence-gaining  tissues  lying 
in  the  sense-organs,  whose  business  it  is  to  obtain  and  com- 
municate to  the  whole  information  of  external  changes  which 
occur  around  it.  Since  the  usefulness  of  these  tissues 
depends  upon  the  readiness  with  which  slight  causes  excite 
them  to  activity,  we  may  call  them  the  irritable  tissues. 

6.  Co-ordinating  and  Automatic  Tissues.  Such  in- 
formation as  that  collected  by  ministers  in  foreign  parts  or  by 
meteorological  observers  is  usually  sent  direct  to  some  central 
office  from  which  it  is  redistributed;  this  mere  redistribution 
is,  however,  in  many  cases  but  a  small  part  of  the  work  carried 
on  in  such  offices.  Let  us  suppose  information  to  be  obtained 
that  an  Indian  chief  is  collecting  his  men  for  an  attack  on 
some  point.  The  news  is  probably  first  transmitted  to  Wash- 
ington, and  it  becomes  the  duty  of  the  executive  officers  there 
to  employ  certain  of  the  constituent  units  of  the  nation  in 
such  definite  work  as  is  needed  for  its  protection.  Troops 
have  to  be  sent  to  the  place  threatened  perhaps;  recruits  en- 
listed; food  and  clothes,  weapons  and  ammunition,  must  be 
provided  for  the  army;  and  so  on.  In  other  words,  the  work 
of  the  various  classes  composing  the  society  has  to  be  organ- 
ized for  the  common  good:  the  mere  spreading  the  news  of 
the  danger  would  be  of  little  avail.  So  in  the  Body:  the 
information  forwarded  to  certain  centres  from  the  irritable 
tissues  is  used  in  such  a  way  as  to  arouse  to  orderly  activ- 
ity other  tissues  whose  services  are  required  ;  we  find 
thus  in  these  centres  a  group  of  co-ordinating  tissues, 
represented  by  nerve-cells  and  possibly  by  certain  other 
constituents  of  the  nerve-centres.  Certain  nerve-cells  are 
also  automatic  in  the  physiological    sense   already  pointed 


THE  DIFFERENTIATION  OF  TISSUES.  35 

out.  The  highest  manifestation  of  this  latter  faculty,  shown 
objectively  by  muscular  movements,  is  subjectively  known  as 
the  "  will,"  a  state  of  consciousness ;  and  other  mental  phe- 
nomena, as  sensations  and  emotions,  are  also  associated  with 
the  activity  of  nerve-cells  lying  in  the  brain.  How  it  is  that 
any  one  state  of  a  material  cell  should  give  rise  to  a  particular 
state  of  consciousness  is  a  matter  quite  beyond  our  powers 
of  conception;  but  not  really  more  so  than  how  it  is  that 
every  portion  of  matter  attracts  every  other  portion  according 
to  the  law  of  gravitation.  In  the  living  Body,  as  elsewhere 
in  the  universe,  we  can  study  phenomena  and  make  out  their 
relations  of  sequence  or  coexistence;  but  why  one  phenom- 
enon is  accompanied  by  another,  why  in  fact  any  cause  pro- 
duces an  effect,  is  a  matter  quite  beyond  our  reach  in  every 
case;  whether  it  be  a  sensation  accompanying  a  molecular 
change  in  a  nerve-cell,  or  the  fall  of  a  stone  to  the  ground  in 
obedience  to  the  force  of  gravity. 

7.  Motor  Tissues.  These  have  the  contractility  of  the 
original  protoplasmic  masses  highly  developed.  The  more 
important  are  ciliated  cells  and  muscular  tissue.  The  former 
line  certain  surfaces  of  the  body,  and  possess  on  their  free 
surfaces  fine  threads  which  are  in  constant  movement.  One 
finds  such  cells,  for  example  (Fig.  50),  lining  the  inside  of 
the  windpipe,  where  their  threads  or  cilia  serve,  by  their 
motion,  to  sweep  any  fluid  formed  there  towards  the  throat, 
where  it  can  be  coughed  up  and  got  rid  of.  Muscular  tissue 
occurs  in  two  main  varieties.  One  kind  is  found  in  the  mus- 
cles attached  to  the  bones,  and  is  that  used  in  the  ordinary  vol- 
untary movements  of  the  Body.  It  is  composed  of  fibres  which 
present  cross-stripes  when  viewed  under  the  microscope  (Fig. 
50),  and  is  hence  known  as  striped  or  striated  muscular  tis- 
sue. The  other  kind  of  muscular  tissue  is  found  in  the  walls 
of  the  alimentary  canal  and  some  other  hollow  organs,  and  con- 
sists of  elongated  cells  (Fig.  60)  which  present  no  cross-stria- 
tion.     It  is  known  as  plain  or  un striated  muscular  tissue. 

The  cells  enumerated  under  the  heading  of  "undiffer- 
entiated tissues"  might  also  be  included  among  the  motor 
tissues,  since  they  arc  capable  of  changing  their  form. 

8.  The  Conductive  Tissues.  These  are  represented  by 
the  nerve-fibres,  slender  threads,  each  of  which  has  as  its  essen- 
tial part  a  branch  of  anerve-cell  having  the  property  of  physio- 
logical   conductivity   highly  developed;    Ihe    fibres    therefore 


36  THE  HUMAN  BODY. 

readily  transmit  molecular  disturbances.  When  its  equilib- 
rium is  upset  at  one  end,  a  nerve-fibre  transmits  to  its  other 
end  a  molecular  movement  known  as  &"  nervous  impulse" 
and  so  can  excite  parts  distant  from  the  original  exciting 
force.  Nerve-fibrea  place,  on  the  one  hand,  the  irritable 
tissues  in  connection  with  the  automatic,  co-ordinating,  and 
sory;  and  on  the  other  put  the  three  latter  in  communica- 
tion with  the  muscular,  secretory  and  other  tissues. 

!•.  Pbotecttve  Tissues.  These  consist  of  certain  cells  lin- 
ing cavities  inside  the  body  and  called  epithelial  cells,  and  cells 
covering  the  whole  exterior  of  the  Body  and  forming  epider- 
mis, hairs  and  nails.  The  enamel  which 
covers  the  teeth  belongs  also  to  this 
group. 

The  class  of  protective  tissues  is,  how- 
ever, even  more  artificial  than  that  of  the 
b     nutritive  tissues,  and  cannot  be  defined  by 
'(.-;•;•>  positive  character.-.     Many  epithelial  cells 
are  secretory,  excretory  or  receptive;  and 
'^'/C--!^-J  ciliated  cells  have  already  been  included 

_     \/s  among  the  motor  tissues,  although  from 

Fm.    11b. —Flat    epithe-  c  & 

Bum-cells  from  the  sur-  the  fact  that  the  movements  of  their  cilia 

face  of  the  peritoneum.  .  . 

•i.  ceii-botiy;  c,  nucleus;  go  on  in  separated  cells  and  independent 'v 

b,  nucleoli.  -ii  i         ,  -     ■    t        .  i 

ot  recognizable  external  stimuli,  they 
might  well  have  been  put  among  the  automatic.  The  protec- 
tive tissues  may  be  best  defined  as  including  cells  which  cover 
free  surfaces,  and  whose  functions  are  mainly  mechanical  or 
physical.  In  their  simplest  form  epithelial  cells  are  flat 
scales,  as,  for  example,  those  represented  in  Fig.  11b,  from 
the  lining  membrane  of  the  abdominal  cavity. 

10.  The  Reproductive  Tissues.  These  are  concerned  in 
the  production  of  new  individuals,  and  in  the  Human  Body 
are  of  two  kinds,  located  in  different  sexes.  The  conjunction 
of  the  products  of  each  sex  is  necessary  for  the  origination 
of  offspring,  since  the  female  product,  egg-cell  or  ovum, 
which  directly  develops  into  the  new  human  being,  remains 
dormant  until  it  has  been  fertilized,  and  fertilization  consists 
itially  in  the  fusion  of  its  nucleus  with  the  nucleus  of  a 
cell  produced  by  the  male. 

The  Combination  of  Tissues  to  Form  Organs.  The  va- 
rious tissues  above  enumerated  forming  the  building  materials 
of  the  Body,  anatomy  is  primarily  concerned  with  their  struc- 


TUB  DIFFERENTIATION  OF  TISSUES.  37 

tare,  and  physiology  with  their  properties.  If  this,  however, 
were  the  whole  matter,  the  problems  of  anatomy  and  physi- 
ology would  be  much  simpler  than  they  actually  are.  The 
knowledge  about  the  living  Body  obtained  by  studying  only 
the  forms  and  functions  of  the  individual  tissues  would  be  com- 
parable to  that  attained  about  a  great  factory  by  studying 
separately  the  boilers,  jDistons,  levers,  wheels,  etc.,  found  in 
it,  and  leaving  out  of  account  altogether  the  way  in  which 
these  are  combined  to  form  various  machines;  for  in  the 
Body  the  various  tissues  are  for  the  most  part  associated  to 
form  organs,  each  organ  answering  to  a  complex  machine 
like  a  steam-engine  with  its  numerous  constituent  parts. 
And  just  as  in  different  machines  a  cogged  wheel  may  per- 
form very  different  duties,  dependent  upon  the  way  in  which 
it  is  connected  with  other  parts,  so  in  the  Body  any  one  tissue, 
although  its  essential  properties  are  everywhere  the  same, 
may  by  its  activity  subserve  very  various  uses  according  to 
the  manner  in  which  it  is  combined  with  others.  For  ex- 
ample: A  nerve-fibre  uniting  the  eye  with  one  part  of  the 
brain  will,  by  means  of  its  conductivity,  when  its  end  in  the 
eye  is  excited  by  the  irritable  tissue  attached  to  it  on  which 
light  acts,  cause  changes  in  the  sensory  nerve-cells  connected 
with  its  other  end  and  so  arouse  a  sight  sensation;  but  an  ex- 
actly similar  nerve-fibre  running  from  the  brain  to  the  mus- 
cles will,  also  by  virtue  of  its  conductivity,  when  its  ending 
in  the  brain  is  excited  by  a  change  in  a  nerve-cell  connected 
with  it,  stir  up  the  muscle  to  contract  under  the  control  of 
the  will.  The  different  results  depend  on  the  different  parts 
connected  with  the  ends  of  the  nerve-fibres  in  each  case,  and 
not  on  differences  in  the  properties  of  the  nerve-fibres  them- 
selves. 

It  becomes  necessary  then  to  study  the  arrangement  and 
uses  of  the  tissues  as  combined  to  form  various  organs,  and 
this  is  frequently  far  more  difficult  than  to  make  out  the 
structure  and  properties  of  the  individual  tissues.  An  ordi- 
nary muscle,  such  as  one  sees  in  the  lean  of  meat,  is  a  very 
complex  organ,  containing  not  only  contractile  muscular  tis- 
sue, but  supportintr  and  uniting  connective  tissue  and  con- 
ductive nerve-fibres,  and  in  addition  a  complex  commissariat 
arrangement,  composed  in  its  turn  of  several  tissues,  con- 
cerned in  the  food-supply  and  waste  removal  of  the  whole 
muscle.     The  anatomical  study  of  a  muscle  has  to  take  into 


38  77/ A'  11  UMAX  BODY. 

account  the  arrangement  of  all  these  parts  within  it.  and  also 
its  connections  with  other  organs  of  the  Body.  The  physi- 
ology of  any  muscle  must  take  into  account  the  actions  of  all 
these  parts  working  together  and  not  merely  the  functions 
of  the  muscular  fibres  themselves,  and  has  also  to  make  out 
under  what  conditions  the  muscle  is  excited  to  activity  by 
changes  in  other  organs,  and  what  changes  in  these  it  brings 
about  when  it  works. 

Physiological  Mechanisms.  Even  the  study  of  organs 
added  to  that  of  the  separate  tissues  does  not  exhaust  the 
matter.  In  a  factory  we  frequently  find  machines  arranged 
so  that  two  or  more  shall  work  together  for  the  perform- 
ance of  some  one  work:  a  steam-engine  and  a  loom  may, 
for  example,  be  connected  and  used  together  to  weave  carpets. 
Similarly  in  the  Body  several  organs  are  often  arranged  to 
work  together  so  as  to  attain  some  one  end  by  their  united 
actions.  Such  combinations  are  known  as  physiological  ap- 
paratuses. The  circulatory  apparatus,  for  example,  consists 
of  various  organs  (each  in  turn  composed  of  several  tissues) 
known  as  heart,  arteries,  capillaries  and  veins.  The  heart 
forms  a  force-pump  by  which  the  blood  is  kept  flowing 
through  the  whole  mechanism,  and  the  rest,  known  together 
as  the  blood-vessels,  distribute  the  blood  to  the  various  organs 
and  regulate  the  supply  according  to  their  needs.  Again,  in 
the  visual  apparatus  we  find  the  co-operation  of  (a)  a  set  of 
optical  instruments  which  bring  the  light  proceeding  from 
external  objects  to  a  focus  upon  (l>)  the  retina,  which  con- 
tains highly  irritable  parts;  these,  changed  by  the  light, 
stimulate  (c)  the  optic  nerve,  which  is  conductive  and  trans- 
mits a  disturbance  which  arouses  in  turn  (d)  sensory  parts  in 
the  brain.  In  the  production  of  ordinary  sight  sensations  all 
these  parts  are  concerned  and  work  together  as  a  visual  appa- 
ratus. So,  too,  we  find  a  respiratory  apparatus,  consisting 
primarily  of  two  hollow  organs,  the  In  mis,  which  lie  in  the 
chesl  and  commnnicate  by  the  windpipe  with  the  back  of  the 
throat,  from  which  air  enters  them.  But  to  complete  the 
respiratory  apparatus  are  many  other  organs,  bones,  muscles, 
nerves  and  nerve-centres,  which  work  together  to  renew  the 
air  in  the  lungs  from  time  to  time;  and  the  act  of  breathing 
is  the  final  result  of  tin;  activity  of  the  whole  apparatus. 

Many  similar  instances,  as  the  alimentary  apparatus,  the 


THE  DIFFERENTIATION  OF  TISSUES.  39 

auditory  apparatus,  and  so  on,  will  readily  be  thought  of. 
The  study  of  the  working  of  such  complicated  mechanisms 
forms  a  very  important  part  of  physiology. 

Anatomical  Systems.  From  the  anatomical  side  a  whole 
collection  of  bodily  organs  agreeing  in  structure  with  one 
another  is  often  spoken  of  as  a  system ;  all  the  muscles,  for 
example,  are  grouped  together  as  the  mnscirfar  system,  and 
all  the  bones  as  the  osseous  system,  and  so  on,  without  any 
reference  to  the  different  uses  of  different  muscles  or  bones. 
The  term  system  is,  however,  often  used  as  equivalent  to 
"apparatus":  one  reads  indifferently  of  the  "circulatory  sys- 
tem "  or  the  "  circulatory  apparatus."  It  is  better,  however, 
to  reserve  the  term  system  for  a  collection  of  organs  classed 
together  on  account  of  similarity  of  structure;  and  "appa- 
ratus "  for  a  collection  of  organs  considered  together  on  ac- 
count of  their  co-operation  to  execute  one  function.  The 
former  term  will  then  have  an  anatomical,  the  latter  a  phy- 
siological, significance. 

The  Body  as  a  Working  Whole.  Finally  it  must  all 
through  be  borne  in  mind  that  not  even  the  most  complex 
system  or  apparatus  can  be  considered  altogether  alone  as  an 
independently  living  part.  All  are  united  to  make  one  living 
Body,  in  which  there  is  throughout  a  mutual  interdepend- 
ence, so  that  the  whole  forms  one  human  being,  in  whom  the 
circulatory,  respiratory,  digestive,  sensory  and  other  appara- 
tuses are  constantly  influencing  one  another,  each  modifying 
the  activities  of  the  rest.  This  interaction  is  mainly  brought 
about  through  the  conductive  and  co-ordinating  tissues  of 
the  nervous  system,  which  place  all  parts  of  the  Body  in  com- 
munication. But  in  addition  to  this  another  bond  of  union 
is  formed  by  the  blood,  which  by  the  circulatory  apparatus  is 
carried  from  tissue  to  tissue  and  organ  to  organ  and  so,  bring- 
ing materials  derived  in  one  region  to  distant  parts,  enables 
each  organ  to  influence  all  the  rest  for  good  or  ill. 

Besides  the  blood  another  liquid,  called  lymph,  exists  in 
the  Body.  It  is  contained  in  vessels  distinct  from  those 
which  carry  the  blood,  but  emptying  into  the  blood-vessels  at 
certain  points.  This  liquid  being  also  in  constant  movement 
forms  another  agency  by  which  products  are  carried  from 
part  to  part,  and  the  welfare  or  ill-fare  of  one  member  en- 
abled to  influence  all. 


CHAPTER    IV. 
THE   INTERNAL  MEDIUM. 

The  External  Medium.  During  the  whole  of  life  inter- 
changes of  material  go  on  between  every  living  heing  and  the 
external  world;  by  these  exchanges  material  particles  that 
one  time  constitute  parts  of  inanimate  objects  come  at  an- 
other to  form  part  of  a  living  being;  and  later  on  these 
same  atoms,  after  having  been  a  part  of  a  living  thing,  are 
passed  out  from  it  in  the  form  of  lifeless  compounds.  As 
the  foods  and  wastes  of  various  organisms  differ  more  or 
less,  so  are  more  or  less  different  environments  suited  for 
their  existence;  and  there  is  accordingly  a  relationshiji  be- 
tween the  plants  and  animals  living  in  any  one  place  and  the 
conditions  of  air,  earth  and  water  prevailing  there.  Even 
such  simple  unicellular  animals  as  the  amoeba?  live  only  in 
water  or  mud  containing  in  solution  certain  gases,  and  in  sus- 
pension solid  food-particles;  and  they  soon  die  if  the  water 
be  changed  either  by  essentially  altering  its  gases  or  by  taking 
out  of  it  the  solid  food.  So  in  yeast  we  find  a  unicellular 
plant  which  thrives  and  multiplies  only  in  liquids  of  certain 
composition,  and  which  in  the  absence  of  organic  compounds 
of  carbon  in  solution  will  not  grow  at  all.  Each  of  these 
simple  living  things,  which  corresponds  to  one  only  of  the  in- 
numerable cells  composing  the  full-grown  Human  Bod}7,  thus 
requires  for  the  manifestation  of  its  vital  properties  the  pres- 
ence of  a  surrounding  medium  suited  to  itself:  the  yeast 
would  die,  or  at  the  best  lie  dormant,  in  a  liquid  containing 
only  the  solid  organic  particles  on  which  the  amoplia  lives; 
and  the  amoeba  would  die  in  such  solutions  as  those  in  which 
yeast  thrives  best. 

The  Internal  Medium.  A  similar  close  relationship  be- 
tween the  living  being  and  its  environment,  and  an  inter- 
change between  the  two  like  that  which  we  find  in  the  amoeba 
and  the  yeast-cell,  Ave  find  also  in  even  the  most  complex 
living  beings.     When,  however,  an  animal  comes  to  be  com- 


THE  INTERNAL  MEDIUM.  41 

po3ed  of  many  cells,  some  of  which  are  placed  far  away  from 
the  surface  of  its  body  and  from  immediate  contact  with 
the  environment,  there  arises  a  new  need — a  necessity  for  an 
internal  medium  or  plasma  which  shall  play  the  same  part 
toward  the  individual  cells  as  the  surrounding  air,  water  and 
food  to  the  whole  animal.  This  internal  medium  kept  in 
movement  and  receiving  at  some  regions  of  the  bodily  sur- 
faces materials. from  the  exterior,  while  losing  substances  to 
the  exterior  at  the  same  or  other  surfaces,  forms  a  sort  of 
middleman  between  the  individual  tissues  and  the  surround- 
ing world,  and  stands  in  the  same  relationship  to  each  of  the 
cells  of  the  Body  as  the  water  in  which  an  amoeba  lives  does 
to  that  animal,  or  beer-wort  does  to  a  yeast-cell.  We  find 
accordingly  the  Human  Body  pervaded  by  a  liquid  plasma, 
containing  gases  and  food -material  in  solution,  the  presence 
of  which  is  necessary  for  the  maintenance  of  the  life  of  the 
tissues.  Any  great  change  in  this  medium  will  affect  in- 
juriously few  or  many  of  the  groups  of  cells  in  the  Body,  or, 
may  even  cause  their  death;  just  as  altering  the  media  in 
which  they  live  will  kill  an  amoeba  or  a  yeast-cell. 

The  Blood.  In  the  Human  Body  the  internal  medium  is 
primarily  furnished  by  the  blood,  which,  as  every  one  knows, 
is  a  red  liquid  very  widely  distributed  over  the  frame,  since 
it  flows  from  any  part  when  the  skin  is  cut  through.  There 
are  in  fact  very  few  portions  of  the  Body  into  which  the 
blood  is  not  carried.  One  of  the  exceptions  is  the  epidermis 
or  outer  layer  of  the  skin:  if  a  cut  be  made  through  it  only, 
leaving  the  deeper  skin-layers  intact,  no  blood  will  flow  from 
the  wound.  Hairs  and  nails  also  contain  no  blood,  hi  the 
interior  of  the  Body  the  epithelial  layers  lining  free  surfaces, 
such  as  the  inside  of  the  alimentary  canal,  contain  no  blood, 
nor  do  the  hard  parts  of  the  teeth,  the  cartilages,  and  the 
refracting  media  of  the  eye  (see  Chap.  XXXII),  but  these 
interior  parts  are  moistened  with  liquid  of  some  kind,  and 
unlike  the  epidermis  are  protected  from  rapid  evaporation. 
All  these  bloodless  parts  together  form  a  group  of  non-vas- 
cular  tissues;  they  alone  excepted,  a  wound  of  any  part  of 
the  Body  will  cause  bleeding. 

In  many  of  the  lower  animals  there  is  no  need  that  the 
liquid  representing  their  blood  should  be  renewed  very  rapidly 
in  different  parts.  Their  cells  live  slowly,  and  so  require  but 
little  food  and  produce  but  little  waste.     In  a  sea-anemone, 


42  THE  HUMAN  BODY. 

for  example,  there  is  no  special  arrangemenl  to  keep  the 
blood  moving;  it  is  jusl  pushed  aboul  from  part,  to  part  by 
the  general  movements  of  the  body  of  the  animal.  Bui  in 
higher  animals,  especially  those  with  an  elevated  temperature, 
such  an  arrangement,  or  rather  absence  of  arrangement,  as 
this  would  not  Buffice.  In  them  the  constituent  cells  live 
very  Ea8t,  making  much  waste  and  using  much  food,  and 
altering  the  blood  in  their  neighborhood  very  rapidly.  lie- 
sides,  we  have  seen  that  in  complex  animals  certain  cells  are 
Bel  apart  to  get  food  for  the  whole  organism  and  certain 
others  to  finally  remove  its  wastes,  and  there  must  be  a  sure 
and  rapid  interchange  of  material  between  the  feeding  and 
excreting  tissues  and  all  the  others.  This  can  only  be  brought 
about  by  a  rapid  movement  of  the  blood  in  a  definite  course, 
and  that  is  accomplished  by  shutting  it  up  in  a  closed  set  of 
tubes,  and  placing  somewhere  a  pump,  which  constantly  takes 
in  blood  from  one  end  of  the  system  of  tubes  and  forces  it 
out  again  into  the  other.  Sent  by  this  pump,  the  heart, 
through  all  parts  of  the  Body  and  back  to  the  heart  again, 
the  blood  gets  food  from  the  receptive  cells,  takes  it  to  the 
working  cells,  carries  off  the  waste  of  these  latter  to  the  ex- 
creting  cells;  and  so  the  round  goes  on. 

The  Lymph.  The  blood,  however,  lies  everywhere  in 
closed  tubes  formed  by  the  vascular  system,  and  does  not 
come  into  direct  contact  with  any  cells  of  the  Body  except 
those  which  float  in  it  and  those  which  line  the  interior  of  the 
blood-vessels.  At  one  part  of  its  course,  how- 
ever, the  vessels  through  which  it  passes  have 
extremely  thin  coats,  and  through  the  walls  of 
these  capillaries  liquid  transudes  from  the  blood 
Sug«r  |  and  bathes  the  various  tissues.  The  transuded 
liquid   is  the  lymph,  and  it  is  this  which  forms 


Fie  10— Adia-  tho  immediate  nutrient  plasma  of  the  tissues 

pam of  adiaiyz-  except  the  few  which  the  blood   moistens  di- 
ms   apparatus,  i 

containing     two  ivetlv. 
liquids,  h  anil  C,  .     , 

separated    by  a        Dialysis.     \\  hen  two  liquids  containing  dif- 

mofst      animal  *  ° 

membrane.  ferent  matters  in  solution  are  separated  from 

one  another  by  a  moisl  animal  membrane,  an  interchange  of 

material  will  take  place  under  certain   renditions.      If  A  be  a 

1  (Fig.  12)  completely  divided  vertically  by  such  a  mem- 

brane,  and  a  solution  of  common  salt  in  water  be  placed  on 
the  side  //,  ami  a  solution  of  sugar  in  water  on  the  side  c,  it 


THE  INTERNAL  MEDIUM.  43 

will  be  found  after  a  time  that  some  salt  has  got  into  c  and 
some  sugar  into  b,  although  there  are  no  visible  pores  in  the 
partition.  .Such  au  interchange  is  said  to  be  due  to  dialysis 
or  osmosis,  and  if  the  process  were  allowed  to  go  on  for  some 
hours  the  same  proportions  of  salt  and  sugar  would  be  found 
in  the  solution  on  either  side  of  the  dividing  membrane. 

The  Renewal  of  the  Lymph.  Osmotic  phenomena  play 
a  great  part  in  the  nutritive  processes  of  the  Body.  The 
lymph  present  in  any  organ  gives  up  things  to  the  cells  there 
and  gets  things  from  them ;  and  thus,  although  it  may  have 
originally  been  tolerably  like  the  liquid  part  of  the  blood,  it 
soon  acquires  a  different  chemical  composition.  Diffusion 
or  dialysis  then  commences  between  the  lymph  outside  and 
the  blood  inside  the  capillaries,  and  the  latter  gives  up  to  the 
lymph  new  materials  in  place  of  those  which  it  has  lost  and 
takes  from  it  the  waste  products  it  has  received  from  the  tis- 
sues. When  this  blood,  altered  by  exchanges  with  the  lymph, 
gets  again  to  the  neighborhood  of  the  receptive  cells,  having 
lost  some  food-materials  it  is  poorer  in  these  than  the  richly 
supplied  lymph  around  those  cells,  and  takes  up  a  supplv  by 
dialysis  from  it.  When  it  reaches  the  excretory  organs  it  has 
previously  picked  up  a  quantity  of  waste  matters  and  loses 
these  by  dialysis  to  the  lymph  there  present,  which  is  special- 
ly poor  in  such  matters,  since  the  excretory  cells  constantly 
deprive  it  of  them.  In  consequence  of  the  different  wants 
and  wastes  of  various  cells,  and  of  the  same  cells  at  different 
times,  the  lymph  must  vary  considerably  in  composition  in 
various  organs  of  the  Body,  and  the  blood  flowing  through 
them  will  gain  or  lose  different  things  in  different  places. 
But  renewing  during  its  circuit  in  one  what  it  loses  in 
another,  its  average  composition  is  kept  pretty  constant,  and, 
through  interchange  with  it,  the  average  composition  of  the 
lymph  also. 

The  Lymphatic  Vessels.  The  blood,  on  the  whole,  loses 
more  liquid  to  the  lymph  through  the  capillary  walls  than  it 
receives  back  the  same  way.  This  depends  mainly  on  the 
fact  that  the  pressure  on  the  blood  inside  the  vessels  is  greater 
than  that  on  the  lymph  outside,  and  so  a  certain  amount  of 
filtration  of  liquid  from  within  out  occurs  through  the  vas- 
cular wall  in  addition  to  the  dialysis  proper.  The  excess  is 
collected  from  the  various  organs  of  the  Body  into  a  set  of 
lymphatic  vessels  which  carry  it  directly  back  into  some  of 


44  THE  III  MAX  HODY. 

the  larger  blood-vessels  near  where  these  empty  into  tin: 
heart ;  by  this  flow  of  the  lymph,  under  pressure  Erom  behind, 
it  is  renewed  in  various  organs,  fresh  liquid  filtering  through 
the  capillaries  to  take  its  place  as  East  as  the  old  is  carried  off. 

The  Lactcals.  In  the  walls  of  the  alimentary  canal  cer- 
tain food-materials  after  passing  through  the  receptive  cells 
into  the  lymph  are  not  transferred  locally,  like  the  rest,  by 
dialysis  into  t  he  blood,  but  are  carried  off  bodily  in  the  lymph- 
vessels  and  poured  into  the  veins  of  a  distant  part  of  the 
Body.  The  lymphatic  vessels  concerned  in  this  work,  being 
frequently  tilled  with  a  white  liquid  during  digestion,  are 
called  the  milky  or  lacteal  vessels. 

Summary.  To  sum  up:  the  blood  and  lymph  form  the 
internal  medium  in  which  the  tissues  of  the  Body  live;  the 
lymph  is  primarily  derived  Erom  the  blood  and  forms  the  im- 
mediate plasma  for  the  great  majority  of  the  living  cells  of 
the  Body;  and  the  excess  of  it  is  finally  returned  to  the 
blood.  The  lymph  moves  but  slowly,  but  is  constantly  reno- 
vated by  the  blood,  which  is  kept  in  rapid  movement,  and 
which,  besides  containing  a  store  of  new  food-matters  for  the 
lymph,  carries  off  the  wastes  which  the  various  cells  have 
poured  into  the  latter,  and  thus  is  also  a  sort  of  sewage  stream 
into  which  the  wastes  of  the  whole  Body  are  primarily  col- 
lected. 

Microscopic  Characters  of  Blood.  If  a  finger  be  pricked, 
and  the  drop  of  blood  flowing  out  be  spread  on  a  glass  slide, 
covered,  protected  from  evaporation,  and  examined  with  a 
microscope  magnifying  about  400  diameters,  it  will  be  seen 
to  consist  of  innumerable  solid  bodies  floating  in  a  liquid. 
The  solid  bodies  are  the  blood-corpuscles,  and  the  liquid  is 
the  blood-plasma  or  liquor  sanguinis. 

The  corpuscles  are  not  all  alike.  While  currents  still  exist 
in  the  freshly-spread  drop  of  blood,  the  great  majority  of 
them  are  readily  carried  to  and  fro;  but  a  certain  number 
more  commonly  stick  to  the  glass  and  remain  in  one  place. 
The  former  are  the  red,  the  latter  the  pale  or  colorless  blood- 
corpuscles. 

Red  Corpuscles.  Form  ami  Size.  The  red  corpuscles 
as  they  float  about  frequently  seem  to  vary  in  form,  but  by  a 
little  attention  it  can  be  made  out  that  this  appearance  is  due 
to  their  turning  round  as  they  float,  and  so  presenting  differ- 
ent aspects  to  view;  just  as  a  silver  dollar  presents  a  different 


THE  INTERNAL   MEDIUM. 


45 


outline  according  us  it  is  looked  at  from  the  front  or  edge- 
wise or  in  three-quarter  profile. 

Sometimes  the  corpuscle  (Fig.  13,  B)  appears  circular; 
then  it  is  seen  in  full  face;  sometimes  linear  (C),  and  slightly 
narrowed  in  the  middle;  sometimes  oval,  as  the  dollar  when 
half-way  between  a  full  and  a  side  view.  These  appearances 
show  that  each  red  corpuscle  is  a  circular  disk,  slightly  hol- 
lowed in  the  middle  (or  biconcave)  and  about  four  times  as 
wide  as  it  is  thick.  The  average  transverse  diameter  is  0.008 
milimeter   (g-^   inch).     Shortly  after  blood   is  drawn  the 


Fig.  13.— Blood-corpuscles.  A,  magnified  about  400  diameters.  The  red  corpus- 
cles have  arranged  themselves  in  rouleaux  :  a,  a,  colorless  corpuscles  ;  B,  red  cor- 
Suscles  more  magnified  and  seen  In  focus  :  E.  a  red  corpuscle  slightly  out  of  focus, 
ear  the  right-hand  top  corner  is  a  red  corpuscle  seen  in  three-quarter  face,  and  at 
Coue  seen  edgewise.    F,  Gr,  H,  I,  white  corpuscles  highly  magnified. 


corpuscles  arrange  themselves  in  rows,  or  rouleaux,  adhering 
to  one  another  by  their  broader  surfaces. 

Color. — Seen  singly  each  red  corpuscle  is  of  a  pale  yellow 
color;  it  is  only  when  collected  in  masses  that  they  appear 
red.  The  blood  owes  its  red  color  to  the  great  numbers  of 
these  bodies  in  it;  if  it  is  spread  out  in  a  very  thin  layer  it, 
too,  is  yellow.  In  a  cubic  millimeter  (^s  inch)  of  blood  there 
are  about  five  million  red  corpuscles. 

Structure.  Seen  from  tin;  front  the  central  part  of 
each  red  corpuscle  in  a  certain  focus  of  the  microscope 
appears   dimmer    or  darker   than    the   rest  (Fig.  13,  />),  ex- 


46  THE  III  MAX  BODY. 

oepl  a  narrow  band  near  the  outer  rim.  If  the  lens  of  the 
microscope  lie  raised,  however,  this  previously  dimmer  central 
pari  becomes  brighter,  and  the  previously  brighter  part  ob- 
scure (/•,').  This  difference  in  appearance  does  not  indicate 
the  presence  of  a  central  pari  or  nucleus  different  from  the 
rest,  but  is  an  optical  phenomenon  due  to  the  shape  of  the 
corpuscle,  in  consequence  of  which  it  acts  like  a  little  bicon- 
cave lens.  Kays  of  lighl  passing  through  near  the  centre  of 
the  corpuscles  are  refracted  differently  from  those  passing 
through  elsewhere;  and  when  the  microscope  is  so  focussed 
that  the  latter  reach  the  eye,  the  former  do  not,  and  vice 
versa;  thus  when  the  central  parts  look  bright, those  around 
them  look  obscure,  and  the  contrary. 

There  is  no  satisfactory  evidence  that  these  corpuscles 
have  any  enveloping  sac  or  cell-wall.  All  the  methods  used 
to  bring  one  into  view  under  the  microscope  are  such  as 
would  coagulate  the  outer  layers  of  the  substance  composing 
the  corpuscle  and  so  make  an  artificial  envelope.  >So  far  as 
optical  analysis  goes,  then,  each  corpuscle  is  homogeneous 
throughout.  By  other  means  we  can,  however,  show  that  at 
least  two  materials  enter  into  the  structure  of  each  red  cor- 
puscle. If  the  blood  be  diluted  with  several  times  its  own 
l)ii Ik  of  water  and  examined  with  the  microscope,  it  will  be 
found  that  the  formerly  red  corpuscles  are  now  colorless  and 
the  plasma  colored.  The  dilution  has  caused  the  coloring 
matter  to  pass  out  of  the  corpuscles  and  dissolve  in  the  liquid. 
This  coloring  constituent  of  the  corpuscle  is  haemoglobin,  and 
the  colorless  residue  which  it  leaves  behind  and  which  swells 
up  into  a  sphere  in  the  diluted  plasma  is  the  struma.  In  the 
living  corpuscle  the  two  are  intimately  mingled  throughout 
it,  and  so  long  as  this  is  the  case  the  blood  is  opaque;  but 
when  the  coloring  matter  dissolves  in  the  plasma,  then  the 
blood  becomes  transparent,  or,  as  it  is  called,  laky.  The 
difference  may  be  very  well  seen  by  comparing  a  thin  layer  of 
fresh  blood  diluted  with  ten  times  its  volume  of  ten-per-cent 
salt  solution  with  a  similar  layer  of  blood  diluted  with  ten 
volumes  of  water.  The  watery  mixture  is  a  dark  transparent 
red;  the  other,  in  which  the  coloring  matter  still  lies  in  the 
corpuscles,  is  a  brighter  opaque  red. 

Consistency. — Each  red  corpuscle  is  a  soft  jelly-like  mass 
which  can  be  readily  crushed  out  of  shape.  Unless  the  pres- 
sure be  such  as  to  rupture  it,  the  corpuscle  immediately  reas- 


THE  INTERNAL  MEDIUM.  47 

sumes  its  proper  form  when  the  external  force  is  removed. 
The  corpuscles  are,  then,  highly  elastic;  they  frequently  can 
be  seen  much  dragged  out  of  shape  inside  the  vessels  when 
the  circulation  of  the  blood  is  watched  in  a  living  animal 
(Chap.  XV),  but  immediately  springing  back  to  their  normal 
form  when  they  get  a  chance. 

Blood-crystals.  Haemoglobin  is,  as  above  shown,  readily 
soluble  in  water.  In  this  it  soon  decomposes  if  kept  in  a 
warm  room,  breaking  up  into  a  colorless  proteid  substance 
called  globulin  and  a  red  body,  Immatiu.  By  keeping  the 
haemoglobin  solution  very  cold,  however,  this  decomposition 
can  be  greatly  retarded,  and  at  the  same  time  the  solubility 
of  the  haemoglobin  in  the  water  much  diminished.  In  dilute 
alcohol  haemoglobin  is  still  less  soluble,  and  so  if  its  ice-cold 

watery  solution  have  one 
fourth  of  its  volume  of 
cold  alcohol  added  to  it 
and  the  mixture  be  put  in 
a  refrigerator  for  twenty- 
four  hours,  a  part  of  the 
haemoglobin  will  often 
crystallize  out  and  sink  to 
the  bottom  of  the  vessel.. 
a  where  it  can  be  collected  for 
crystals.  examination.     The  haemo- 

globin of  the  rat  is  less  soluble  than  that  of  man,  and  there- 
fore crystallizes  out  especially  easily;  but  these  haemoglobin 
crystals,  or,  as  they  are  often  called,  blood-crystals,  can  also 
be  obtained  from  human  blood.  In  100  parts  of  dry  human 
red  blood-corpuscles  there  arc  of  90  haemoglobin.  The  haemo- 
globin is  the  essential  constituent  of  the  red  blood-corpuscles, 
enabling  them  to  pick  up  large  quantities  of  oxygen  in  the 
lungs  and  carry  it  to  other  parts.     (See  Eespiration.) 

Haemoglobin  contains  a  considerable  quantity  of  iron,  much 
more  than  any  other  proximate  constituent  of  the  Body. 

The  Colorless  Blood-corpuscles  (Fig.  13,  F,  H,  67).  The 
colorless,  pale  or  white  corpuscles  of  the  blood  are  far  less 
numerous  fchar.  the  red;  in  health  there  is  on  the  average 
about  one  white  to  three  hundred  red,  but  the  proportion 
may  vary  considerably  Each  is  finely  granular  and  consists 
of  a  soft  mass  of  protoplasm  enveloped  in  no  definite  cell-wall, 
but  containing  a  nucleus.     The  granules  in  the  protoplasm 


m 


48  THE  111  MAS   BODY. 

commonly  hide  the  nucleus  in  a  fresli  corpuscle,  but  dilute 
acetic  acid  dissolves  mosl  of  them  and  brings  the  nucleus  into 
view.  These  pale  corpuscles  belong  to  t  he  group  of  undiffer- 
entiated tissues,  and  differ  in  do  important  recognizable 
character  from  the  cells  which  make  np  the  whole  very  young 
Human  Body,  oor  indeed  from  such  a  unicellular  animal  as 
an  Amoeba.  They  have  the  power  of  slowly  changing  their 
form  spontaneously.  At  one  moment  a  pale  corpuscle  will 
be  seen  as  a  spheroidal  muss;  a  few  seconds  later  (Fig.  15) 
processes  will  he  seen  radiating  from  this,  and  soon  after 
these  processes  may  be  retracted  and 
ot  hers  thrust  out;  and  so  the  corpuscle 
goes  on  changing  its  shape.  These  slow 
amoeboid  movements  are  greatly  promoted 
/?$P  ky  keeping  the  specimen  of  hlood  at  the 
temperature  of  the  Body.  By  thrusting 
out  a  process  oil  one  side,  then  drawing 
fig.  is. -A  white  blood-  the  rest  of  its  body  up  to  it,  and  then 
^sSveClfntervaiseof  t\  few  seuding  out  a  process  again  on  the  same 
StdftSutetoiS  side>  *he  corpuscle  can  slowly  change  its 

amoeboid  movements.  p]ace    an(j   creep  across   the    field    of    the 

microscope.  Inside  the  hlood-vessels  these  corpuscles  often 
execute  similar  movements;  and  they  sometimes  bore  right 
through  the  capillary  walls  and,  getting  out  into  the  lymph- 
spaces,  creep  about  among  the  other  tissues.  This  migration 
is  especially  frequent  in  inflamed  parts,  and  the  pus  or 
" matter "  which  collects  in  abscesses  is  largely  made  up  of 
white  blood-corpuscles  which  have  in  this  way  got  out  of  the 
blood-vessels.  The  average  diameter  of  the  white  corpuscles 
is  one  third  greater  than  that  of  the  red. 

The  colorless  corpuscles,  or  some  of  them,  are  capable  of 
taking  into  themselves  foreign  particles  present  in  the  blood; 
this  they  do  in  a  manner  similar  to  that  in  which  an  amoeba 
feeds:  the  process  is  known  as  phagocytosis  and  the  cells  ex- 
hibiting it  as  phagocytes.  Among  the  substances  observed  to 
be  taken  np  by  white  corpuscles  are  the  minute  organisms 
known  as  Bacteria,  certain  species  of  which  have  been  proved 
to '  be  the  causes  of  some  diseases  (zymotic  diseases).  The 
white  corpuscles  may  in  this  way  play  an  important  part  in 
the  cure  of  such  diseases,  or  in  their  prevention  in  persons 
exposed  to  infection.  The  accumulation  of  white  corpus- 
cles in  inflamed   or  injured  parts  is  probably  primarily  as- 


THE  INTERNAL  MEDIUM.  49 

sociated  with  the  removal  of  dead  and  broken-down  tissues, 
though  it  may  be  carried  to  excess  as  in  the  case  of  purulent 
accumulations. 

The  Blood  Platelets  or  Plaques  are  a  third  kind  of  blood- 
corpuscle,  considerably  smaller  than  the  red,  but  somewhat 
resembling  them  in  form.  They  adhere  together,  break  down 
and  form  sticky  clumps  with  great  rapidity  in  drawn  blood 
unless  special  precautions  are  taken. 

Blood  of  Other  Animals.  In  all  animals  with  blood  the 
pale  corpuscles  are  pretty  much  alike,  but  the  red  corpuscles, 
which  with  rare  exceptions  are  found  only  in  Vertebrates, 
vary  considerably.  In  all  the  classes  of  the  mammalia  they 
are  circular  biconcave  disks,  with  the  exception  of  the  camel 
tribe,  in  which  they  are  oval.  They  vary  in  diameter  from  0.02 
mni-  (13V0  incl1)  (musk  deer)  to  .011  mm.  (^^  inch)  (ele- 
phant). In  the  dog  they  are  nearly  the  same  size  as  those  of 
man.  In  no  mammals  do  the  fully-developed  red  corpuscles 
possess  a  nucleus.  In  all  other  vertebrate  classes  the  red  cor- 
puscles possess  a  central  nucleus,  and  are  oval  slightly  bi- 
convex disks,  except  in  a  few  fishes  in  which  they  are  cir- 
cular. They  are  largest  of  all  in  the  amphibia.  Those  of 
the  frog  are  0.02  mm.  (r/(TD  inch)  long  and  .007  mm.  (jfo 
inch)  broad. 

Histology  of  Lymph.  Pure  lymph  is  a  colorless  watery- 
looking  liquid;  examined  with  a  microscope  it  is  seen  to  con- 
tain numerous  pale  corpuscles  closely  resembling  those  of  the 
blood,  and  no  doubt  many  are  pale  blood-corpuscles  which 
have  migrated.  These  lymph-corpuscles  or  leucocytes  have, 
however,  another  more  important  origin.  In  many  parts  of  the 
Body  there  are  collections  of  a  peculiar  lymphoid  or  adenoid 
tissue,  sometimes  in  nodular  masses  (lymphatic  glands). 
This  tissue  consists  essentially  of  a  fine  network,  the  meshes 
of  which  are  occupied  with  leucocytes  which  frequently  show 
Bigns  of  division.  The  meshes  of  the  network  communicate 
with  lymphatic  vessels  and  the  lymph  flowing  through  picks 
up  and  cui lies  off  the  new-formed  leucocytes.  The  lymph 
being  ultimately  poured  into  the  blood,  the  leucocytes  be- 
come  the  colorless  corpuscles  of  the  latter;  and  the  migrating 
cells  of  the  blood  are  therefore  but  lymph-corpuscles  restored 
to  the  lymph,  perhaps  somewhat  changed  during  their  life  in 
the  blood-plasma. 

The  lymph  flowing  from  the  intestines  during  digestion 


50  THE  HUMAN  BODY. 

is,  as  already  mentioned,  not  colorless,  but  white  and  milky. 
It  is  known  as  chyle,  and  will  he  considered  with  the  process 
of  digestion.  During  fasting  the  lymph  from  the  intestines 
is  colorless,  like  that  from  other  parts  of  the  Body. 


CHAPTEE  V. 
THE   CLOTTING   OF   BLOOD. 

The  Coagtilation  of  the  Blood.  When  blood  is  first 
drawn  from  the  living  Body  it  is  perfectly  liquid,  flowing  in 
any  direction  as  readily  as  water.  This  condition  is,  however, 
only  temporary;  in  a  few  minutes  the  blood  becomes  viscid 
and  sticky,  and  the  viscidity  becomes  more  and  more  marked 
until,  after  the  lapse  of  five  or  six  minutes,  the  whole  mass 
sets  into  a  jelly  which  adheres  to  the  vessel  containing  it,  so 
that  this  may  be  inverted  without  any  blood  whatever  being 
spilled.  This  stage  is  known  as  that  of  gelatinization  and  is 
also  not  permanent.  In  a  few  minutes  the  top  of  the  jelly- 
like mass  will  be  seen  to  be  hollowed  or  "  cupped  "  and  in  the 
concavity  will  be  seen  a  small  quantity  of  nearly  colorless 
liquid,  the  blood-serum.  The  jelly  next  shrinks  so  as  to  pull 
itself  loose  from  the  sides  and  bottom  of  the  vessel  containing 
it,  and  as  it  shrinks  squeezes  out  more  and  more  serum.  Ulti- 
mately we  get  a  solid  clot,  colored  red  and  smaller  in  size 
than  the  vessel  in  which  the  blood  coagulated  though  retain- 
ing its  form,  floating  in  a  quantity  of  pale  yellow  serum.  If, 
however,  the  blood  be  not  allowed  to  coagulate  in  perfect  rest, 
a  certain  number  of  red  corpuscles  will  be  rubbed  out  of  the 
clot  into  the  serum  and  the  latter  will  be  more  or  less  reddish. 
The  longer  the  clot  is  kept  the  more  serum  will  be  obtained: 
if  the  first  quantity  exuded  be  decanted  off  and  the  clot  put 
aside  and  protected  from  evaporation,  it  will  in  a  short  time 
be  found  to  have  shrunk  to  a  smaller  size  and  to  have  pressed 
out  more  serum;  and  this  goes  on  until  putrefactive  changes 
commence. 

Cause  of  Coagulation.  If  a  drop  of  fresh-drawn  blood 
be  spread  out  very  thin  and  watched  for  a  few  minutes  with  a 
microscope  magnifying  GOO  or  )<><>  diameters,  it  will  be  seen 
that  the  coagulation  is  due  to  the  separation  of  very  line  solid 
threads  which  run  in  every  direction  through  the  plasma  and 
form  a  close  network  entangling  all  the  corpuscles.     These 

51 


52  THE  IIC MAS   BODY. 

threads  are  composed  <>f  the  proteid  substance  fibrin,  When 
they  first  form,  the  whole  drop  is  much  like  a  sponge  soaked 
full  of  water  (represented  by  the  serum)  mid  having  solid 
bodies  (the  corpuscles)  in  its  cavities.  After  the  fibrin  threads 
have  been  formed  they  tend  to  shorten;  hence  when  blood 
eluts  iii  mass  in  a  vessel,  the  fibrinous  network  tends  to  shrink 
in  every  direction  just  as  a  network  formed  of  stretched 
india-rubber  hands  would,  and  this  shrinkage  is  greater  the 
longer  the  clotted  blood  is  kept.  At  first  the  threads  stick 
too  firmly  to  the  hottom  and  sides  of  the  vessel  to  he  pulled 
away,  and  thus  the  first  sign  of  the  contraction  of  the  fibrin 
is  seen  in  the  cupping  of  the  surface  of  the  gelatinized  blood 
where  the  threads  have  no  solid  attachment,  and  there  the 
contracting  mass  presses  out  from  its  meshes  the  first  drops  of 
serum.  Finally  the  contraction  of  the  fibrin  overcomes  its  ad- 
hesion to  the  vessel  and  the  clot  pulls  itself  loose  on  all  sides, 
pressing  out  more  and  more  serum,  in  which  it  ultimately 
floats.  The  great  majority  of  the  red  corpuscles  are  held  hack 
in  the  meshes  of  the  fibrin,  but  a  good  many  pale  corpuscles, 
by  their  amoeboid  movements,  work  their  way  out  and  get 
into  the  serum. 

Whipped  Blood.  The  essential  point  in  coagulation 
being  the  formation  of  fibrin  in  the  plasma,  and  blood  only 
forming  a  certain  amount  of  fibrin,  if  this  be  removed  as  fast 
as  it  forms  the  remaining  blood  will  not  clot.  The  fibrin 
may  be  separated  by  what  is  known  as  "  whipping"  the  blood. 
For  this  purpose  fresh-drawn  blood  is  stirred  up  vigorously 
Avith  a  bunch  of  twigs,  and  to  these  the  sticky  fibrin  threads 
as  they  form,  adhere.  If  the  twigs  be  withdrawn  after  a  few 
minutes  a  quantity  of  stringy  material  will  be  found  attached 
to  them.  This  is  at  first  colored  red  by  adhering  blood-cor- 
puscles: but  by  washing  in  water  they  may  be  removed,  and 
the  pure  fibrin  thus  obtained  is  perfectly  Avhite  and  in  the 
form  of  highly  elastic  threads.  It  is  insoluble  in  water  and 
in  dilute  acids,  but  swells  up  to  a  transparent  jelly  in  the 
latter.  The  "  whipped  "  or  "  defibrinated  blood  "  from  which 
the  fibrin  has  been  in  this  way  removed,  looks  just  like  ordinary 
blood,  but  has  lost  the  power  of  coagulating  spontaneously. 

The  Buffy  Coat.  That  the  red  corpuscles  are  not  an 
essential  part  of  the  clot,  hut  are  merely  mechanically  caught 
up  in  it,  seems  clear  from  the  microscopic  observation  of 
the  process  of  coagulation;  and  from  the  fact  that  perfectly 


THE  CLOTTING    OF  BLOOD.  53 

formed  fibrin  can  be  obtained  free  from  corpuscles  by  whip- 
ping the  blood  and  washing  the  threads  which  adhere  to  the 
twigs.  Under  certain  conditions,  moreover,  one  gets  a  natu- 
rally formed  clot  containing  no  red  corpuscles  in  one  part  of 
it.  The  corpuscles  of  human  blood  are  a  little  heavier,  bulk 
for  bulk,  than  the  plasma  in  which  they  float;  hence,  when 
the  blood  is  drawn  and  left  at  rest  they  sink  slowly  in  it; 
and  if  for  any  reason  clotting  take  place  more  slowly  or  the 
corpuscles  sink  more  rapidly  than  usual,  a  colorless  top 
stratum  of  plasma,  with  no  red  corpuscles  in  it,  is  left 
before  gelatinization  occurs  and  stops  the  further  sinking  of 
the  corpuscles.  The  uppermost  part  of  the  clot  formed 
under  such  circumstances  is  colorless  or  pale  yellow,  and  is 
known  as  the  buffy  coat;  it  is  especially  apt  to  be  formed  in 
the  blood  drawn  from  febrile  patients,  and  was  therefore  a 
point  to  which  physicians  paid  much  attention  in  the  olden 
times  when  bloodletting  was  thought  to  be  almost  a  panacea. 
In  horse's  blood  the  difference  between  the  specific  gravity  of 
the  corpuscles  and  that  of  the  plasma  is  greater  than  in 
human  blood,  and  horse's  blood  also  coagulates  more  slowly, 
so  that  its  clot  has  nearly  always  a  buffy  coat.  The  colorless 
buffy  coat  seen  sometimes  on  the  top  of  the  clot  must,  how- 
ever, not  be  confounded  with  another  phenomenon.  When 
a  blood-clot  is  left  floating  exposed  to  the  air  its  top  becomea 
bright  scarlet,  while  the  part  immersed  in  the  serum  assumes 
a  dark  purple-red  color.  The  brightness  of  the  top  layer  is 
due  to  the  action  of  the  oxygen  of  the  air,  which  forms  a 
scarlet  compound  with  the  coloring  matter  of  the  red  cor- 
puscles. If  the  clot  be  turned  upside  down  and  left  for  a 
short  time,  the  previously  dark  red  bottom  layer,  now  exposed 
to  the  air,  becomes  bright;  and  the  previously  bright  top 
layer,  now  immersed  in  the  serum,  loses  iis  oxygen  and  be- 
comes dark. 

Uses  of  Coagulation.  The  clotting  of  the  blood  is  so  im- 
portant a  process  that  its  cause  has  been  frequently  investi- 
gated; but  it  is  not  yet  completely  understood.  The  living 
circulating  blood  in  the  healthy  blood-vessels  does  not  clot; 
it  contains  no  solid  fibrin,  but  this  forms  in  it,  sooner  or  later, 
when  the  blood  gets  by  any  means  out  of  the  vessels  or  when 
the  lining  of  these  is  injured.  In  this  way  the  mouths  of  the 
small  vessels  opened  in  a  cut  are  clogged  up,  and  the  bleed- 
ing, which   would  otherwise  go  on   indefinitely,  is   stopped. 


61  THE  ill  MA  \    BODY. 

So,  too,  when  u  .surgeon  ties  up  ;in  artery  before  dividing  it, 
the  tight  Ligature  crushes  or  tears  it-  delicate  inner  surface, 

and  the  blood  clots  where  that  is  injured,  and  from  there  a 
coagulum  is  formed  reaching  up  to  the  next  highest  branch  of 
the  vessel.  This  becomes  more  and  more  solid,  and  by  the  time 
the  ligature  is  removed  has  formed  a  firm  plug  in  the  cut  end 
of  the  artery,  which  greatly  diminishes  the  risk  of  bleeding. 

The  Sovirco  of  Blood-fibrin.  Since  fresh  blood-plasma 
contains  no  fibrin  but  does  contain  considerable  quantities  of 
other  proteids,  we  look  lirst  to  these  as  a  possible  source  of 
the  fibrin  formed  during  coagulation.  Blood  drawn  from  a 
living  animal  into  one  third  of  its  bulk  of  a  cold  saturated 
solution  of  magnesium  sulphate  and  kept  cold  will  not  clot 
for  a  long  time.  The  corpuscles  slowly  sink  in  the  mixture, 
and  after  a  time  considerable  quantities  of  colorless  "salted  " 
plasma  can  be  drawn  off  from  its  upper  part.  The  salted 
plasma  still  contains  something  which  can  form  fibrin,  for  if 
diluted  with  six  or  seven  times  its  volume  of  water  it  clots  in 
a  manner  quite  similar  to  pure  blood-plasma  (though  the  clot 
is  a  little  less  firm);  and  also,  fibrin  can  be  obtained  by 
whipping  it. 

If  salted  plasma  be  saturated  with  sodium  chloride  it 
yields  a  whitish  rather  sticky  precipitate,  called  plasmine. 
The  remaining  liquid  is  then  found  to  have  lost  the  power  of 
clotting,  but  if  the  plasmine  be  treated  with  a  little  dilute 
saline  solution  it  dissolves,  and  the  solution  soon  clots,  with 
the  formation  of  fibrin. 

The  plasmine  is  not  a  single  body.  If  its  solution  before 
it  clots  have  sodium  chloride  added  to  it  in  the  proportion 
of  about  15$,  a  white  sticky  precipitate  is  formed,  and  may 
be  collected  on  a  filter;  it  is  a  substance  named  fibrinogen. 
If  more  sodium  chloride  or  some  magnesium  sulphate  be 
added  to  the  filtrate  a  second  white  precipitate  is  obtained: 
this  is  paraglobuhn. 

Paraglobulin  dissolves  in  dilute  solutions  of  common  salt: 
such  solutions  cannot  be  made  to  yield  fibrin,  though  they 
are  coagulated  with  the  formation  of  coagulated  proteid 
(p.  10)  at  the  temperature  75°  C.  (1GT°  F).  Purified  fibrin- 
ogen also  dissolves  in  dilute  solution  of  common  salt,  and 
such  solution  is  coagulated  by  heat  (5G°  C.  or  133°  F.):  but 
under  certain  conditions  it  clots  with  the  formation  of  true 
fibrin.     During  the   clotting  the   fibrinogen   disappears,  but 


THE  CLOTTING    OF  BLOOD.  55 

the  quantity  of  fibrin  formed  never  is  quite  equal  in  weight 
to  the  fibrinogen  which  disappears,  so  the  process  is  not  a 
mere  direct  transformation  of  one  substance  into  the  other. 

We  are  thus  led  to  the  conclusion  that  the  natural  clot- 
ting of  fresh  blood  is  due  to  the  formation  of  fibrin  from 
fibrinogen  which  existed  in  solution  in  the  plasma  of  the 
circulating  blood  and  has  been  altered  in  the  clotted,  giving 
origin  to  fibrin.  But  as  normal  blood  circulating  in  healthy 
uninjured  blood-vessels  does  not  clot  nor  do  pure  solutions 
of  fibrinogen,  we  have  still  to  seek  the  exciting  cause  of  the 
change. 

If  to  a  solution  of  fibrinogen  there  be  added  a  few  drops 
of  blood  or  of  blood-serum,  or  of  the  washings  of  a  blood-clot, 
fibrin  will  be  formed;  therefore  drawn  blood  and  serum  and 
natural  clot  each  contain  something  which  can  effect  the  con- 
version of  fibrinogen  into  fibrin.  This  substance  is  the 
enzyme  named  fibrin-ferment. 

The  Fibrin-ferment.  When  blood-serum  is  treated  with 
several  times  its  volume  of  strong  alcohol  its  various  proteids 
and  most  of  its  baits  are  precipitated :  if  the  precipitate  be 
left  standing  in  alcohol  for  some  months  the  proteids  become 
almost  entirely  insoluble  in  water,  but  a  few  drops  of  the 
watery  extract  cause  clotting  in  a  saline  solution  of  fibrin- 
ogen, and  clearly  contain  some  of  the  ferment.  A  very 
minute  quantity  of  the  ferment  will  cause  the  conversion  of 
an  indefinite  quantity  of  fibrinogen  and  does  not  appear  to  be 
itself  used  up  in  the  process:  it  acts  somehow  by  its  mere 
presence,  and  the  clotting  of  blood  is  to  be  relegated  to  that 
obscure  group  of  physico-chemical  processes  known  as  cata- 
lytic. Solutions  containing  the  ferment  always  give  some 
proteid  reactions  and  it  may  be  a  proteid,  but  this  is  doubt- 
ful; for  the  proteid  present  may  be  only  an  impurity.  Watery 
solutions  of  ferment  completely  lose  their  activity  when 
boiled. 

If  fibrinogen  be  dissolved  in  the  least  possible  amount  of 
dilute  caustic  potash  and  a  few  drops  of  as  pure  as  possible  a 
solution  of  fibrin  ferment,  freed  from  its  salts  by  dialysis, 
be  added,  clotting  does  not  occur:  but  it  may  be  brought 
about  by  the  addition  of  a  very  small  quantity  of  a  calcium 
salt.  The  presence  of  some  calcium  seems  to  be  an  essential, 
but  the  part  it  plays  is  unknown.  Of  the  four  substances 
which  take  part    in   the  coagulation  of  blood,  the   fibrinogen 


56  THE  HUMAN  BODY. 

primarily  determines  the  quantity  of  fibrin  formed :  the  more 
fibrinogen  the  more  fibrin,  though  never  quite  .so  much  as  the 
fibrinogen  which  disappears.  The  fermenl  acting  on  fibrin- 
ogen in  the  presence  of  a  sail  of  calcium,  in  sonic  way  causes 
it  to  become  fibrin,  but  does  not  itself  enter  into  the  fibrin;  it 
is  nut  used  up  in  the  process,  and  the  amount  of  fibrin  ulti- 
mately formed  is  the  same  whether  much  or  little  ferment 
be  present;  but  the  more  ferment  the  quicker  the  clotting. 
The  presence  in  small  quantity  of  many  neutral  salts  seems  to 
favor  coagulation,  but  none  except  the  lime-salts  are  essential. 
The  part  they  play  is  obscure;  and  when  present  in  large  pro- 
portions they  prevent  coagulation  of  blood  or  plasma,  prob- 
ably by  hindering  the  formation  of  ferment.  If  fresh  blood 
be  mixed  with  an  equal  bulk  of  a  saturated  solution  of  mag- 
nesium sulphate  (Epsom  salts)  or  of  common  salt,  it  will  not 
clot;  but  if  this  mixture  be  largely  diluted  with  water,  then 
some  ferment  is  formed  and  clotting  takes  place. 

The  Proximate  Causes  of  Normal  Blood  Coagulation. 
As  all  the  phenomena  of  clotting,  with  the  formation  of  fibrin 
agreeing  in  all  respects  with  that  formed  during  the  natural 
coagulation  of  drawn  blood,  can  be  obtained  in  artificial  solu- 
tions of  fibrinogen,  it  is  obvious  that  the  process  is  not,  as  was 
once  supposed,  a  so-called  vital  but  a  purely  chemical  one: 
but  we  still  are  far  from  a  satisfactory  explanation  why  the 
fibrinogen  of  the  plasma  does  not  clot  in  normal  circulating 
blood  contained  in  healthy  blood-vessels.  It  is,  in  fact,  much 
easier  to  point  out  what  are  not  the  proximate  causes  of  the 
coagulation  of  drawn  blood  than  what  are. 

Blood  when  removed  from  the  Body  and  received  in  a 
vessel  comes  to  rest,  cools,  and  is  exposed  to  the  air,  from 
which  it  may  receive  or  to  which  it  may  give  off  gaseous 
bodies.  But  it  is  easy  to  prove  that  none  of  these  three 
things  is  the  cause  of  coagulation.  Stirring  the  drawn  blood 
and  so  keeping  it  in  movement  does  not  prevent  but  hastens 
its  coagulation:  atid  blood  carefully  imprisoned  in  a  living 
blood-vessel,  and  so  kept  at  rest,  will  not  clot  for  a  long  time; 
not  until  the  inner  coat  of  the  vessel  begins  to  change  from 
the  want  of  fresh  blood.  Secondly,  keeping  the  blood  at  the 
temperature  of  the  Body  hastens  coagulation,  and  cooling  re- 
tards it;  blood  received  into  an  ice-cold  vessel  and  kept  sur- 
rounded with  ice  will  clot  more  slowly  than  blood  drawn  and 
left  exposed  to  ordinary  temperatures.     Finally,  if  the  blood 


THE  CLOTTING   OF  BLOOD.  57 

be  collected  over  mercury  from  a  blood-vessel,  without  having 
been  exposed  to  the  air  even  for  an  instant,  it  will  clot  per- 
fectly. 

The  formation  of  fibrin  is  then  due  to  changes  taking 
place  in  the  blood  itself  when  it  is  removed  from  the  blood- 
vessels; the  clotting  depends  solely  upon  some  rearrangement 
of  the  blood-constituents,  and  the  primary  change  seems  to 
be  the  formation  of  fibrin-ferment.  That  healthy  circulating 
blood  contains  no  ferment  but  that  this  forms  in  drawn  blood 
may  be  shown  as  follows:  Blood  is  drawn  from  an  artery 
into  four  separate  vessels.  To  one  specimen  a  large  quantity 
of  alcohol  is  added  at  once;  to  a  second  after  five  minutes,  to 
a  third  after  ten,  to  the  fourth  after  fifteen.  The  precipitate 
in  each  is  collected  and  dried,  and  then  treated  with  water 
which  will  dissolve  any  ferment  present.  The  watery  extract 
from  the  first  specimen  will  not  cause  clotting  when  added  to 
a  fibrinogen  solution:  from  the  second  only  slowly;  the  third 
more  quickly,  and  the  fourth  quickest  of  all.  It  is  hence  con- 
cluded that  there  is  no  ferment  in  perfectly  fresh  blood,  but 
that  this  begins  to  form  as  soon  as  blood  is  drawn  and  for 
some  time  goes  on  increasing,  so  that  there  is  more  in  blood 
drawn  ten  minutes  than  in  blood  drawn  only  five.  The 
alcohol  in  each  sample  precipitates  all  the  ferment  already 
present  and  prevents  the  formation  of  more.  There  is  some 
evidence  that  a  good  many  pale  corpuscles  disintegrate  when 
blood  is  drawn,  and  it  has  been  maintained  that  they  then 
give  origin  to  the  fibrin-ferment  along  with  other  things:  but 
of  late  evidence  seems  rather  to  point  to  the  platelets  as 
the  main  source  of  the  ferment.  As  already  stated  they 
rapidly  break  down  when  blood  is  removed  from  the  body, 
part  of  their  substance  going  into  solution  in  the  plasma  and 
part  remaining  as  a  sticky  mass  which  tends  to  adhere  to  its 
fellows  to  form  little  clumps.  If  the  formation  of  fibrin  in 
clotting  blood  be  watched  with  the  aid  of  a  microscope  the 
fibrin  threads  are  seen  to  appear  first  in  the  neighborhood  of 
these  clumps,  and  in  many  cases  to  radiate  from  them.  More- 
over those  substances  which  check  or  retard  the  clotting  of 
blood  also  hinder  the  disintegration  of  the  platelets:  and  if  a 
fine  thread  be  passed  through  the  blood-vessel  of  a  living 
animal  fibrin  forms  around  it  after  a  time,  and  this  formation 
is  preceded  by  adhesion  to  the  thread  and  disintegration  of 
platelets.     Bnt  bo  the  source  of  the  ferment  platelets  01  pale 


58  THE  111  MAS  BODY. 

corpuscles  or  both,  we  have  still  the  problem  why,  under 
normal  conditions, do  not  these  break  down  in  the  circulating 
blood:  have  perchance  the  blood-vessels  some  part  in  the 
matter  ? 

Relation  of  the  Blood-vessels  to  Coagulation.  As  to 
the  role  of  the  blood-vessels  with  respect  to  coagulation,  two 
views  are  held,  between  which  the  facts  at  present  known  do 
not  permit  a  decisive  judgment  to  be  made;  and  there  may 
be  some  truth  in  both.  One  theory  is  that  the  vessels  actively 
prevent  coagulation  by  constantly  absorbing  from  the  blood 
some  substance,  as  the  fibrin-ferment,  the  presence  of  which 
is  a  necessary  condition  for  the  formation  of  fibrin  and  which 
is  supposed  to  be  constantly  forming  in  the  blood,  but  to  be 
as  steadily  removed  from  it  or  destroyed  by  the  lining  cells  of 
the  blood-vessels.  In  support  of  this  opinion  is  brought  for- 
ward  the  fact  that  it  is  possible  to  inject  considerable  quanti- 
ties of  a  solution  of  fibrin-ferment  into  the  blood  of  a  living 
animal  without  causing  intravascular  coagulation. 

The  other  view  is  that  the  blood-vessels  are  passive.  They 
simply  do  not  excite  those  changes  in  the  blood  constituents 
which  give  rise  to  the  formation  of  fibrin-ferment,  while 
foreign  bodies  in  contact  with  the  blood  do  excite  these 
changes  and  so  lead  to  coagulation.  In  support  of  this  view 
are  brought  forward  the  facts  that  drawn  blood  clots  faster  in 
vessels  of  such  shapes  that  a  large  surface  of  blood  is  exposed 
to  foreign  contact;  and  that  coagulation  takes  place  rapidly 
in  a  vessel  with  a  rough  interior,  while  in  a  chemically  clean 
glass  vessel  it  occurs  slowly.  The  experiment  already  men- 
tioned of  getting  a  clot  around  a  thread  passed  through  a  blood- 
vessel, and  also  that  of  getting  extensive  clotting  within  the 
blood-vessels  by  the  injection  into  a  vein  of  extract  of  the 
thymus  body,  may  be  cited  as  tending  to  show  that  the  linings 
of  the  blood-vessels  cannot  actively  prevent  coagulation;  but 
it  may  be  objected  that  in  the  one  case  locally^  and  in  the  other 
generallv,  the  ferment  is  set  free  in  the  blood  so  fast  that  the 
vessels  cannot  remove  it  in  time  to  prevent  the  formation  of 
fibrin.  Blood  poured  out  from  a  torn  vessel  among  other 
tissues  of  the  body  often  clots  very  slowly;  this  may  be  due 
either  to  the  tissues  in  general  possessing  the  power  of  de- 
st  loving  fibrin-ferment  or  to  their  being  merely  indifferent 
substances  not  exciting  the  changes  which  lead  to  fibrin 
formation. 


THE   CLOTTING   OF  BLOOD.  59 

Whatever  the  part  played  by  the  blood-vessels  in  reference 
to  coagulation  it  is  only  exhibited  when  their  inner  surfaces 
are  healthy  and  uninjured.  If  their  lining  be  ruptured  or 
diseased  the  blood  clots.  Accordingly,  after  death,  when 
post-mortem  changes  have  affected  the  blood-vessels,  the 
blood  clots  in  them;  but  often  \'ery  slowly,  since  the  vessels 
only  gradually  alter.  If  the  Body  be  left  in  one  position 
after  death  the  clots  formed  in  the  heart  have  often  a  marked 
buffy  coat,  because  the  corpuscles  have  had  a  long  time  to 
sink  in  the  plasma  before  coagulation  occurred.  In  medico- 
legal cases  it  is  thus  sometimes  possible  to  say  what  was  the 
position  of  a  corpse  for  some  hours  after  death,  although  it 
has  been  subsequently  moved. 

Lymph  clots  like  the  blood,  but  not  so  firmly.  The  clot 
formed  is  colorless. 

Composition  of  the  Blood.  The  average  specific  gravity 
of  human  blood  is  1055.  It  has  an  alkaline  reaction,  which 
becomes  less  marked  as  coagulation  occurs.  About  one  half 
of  its  mass  consists  of  moist  corpuscles  and  the  remainder  of 
plasma.  Exposed  in  a  vacuum,  100  volumes  of  blood  yield 
about  60  of  gas  consisting  of  a  mixture  of  oxygen,  carbon 
dioxide  and  nitrogen. 

Chemistry  of  Serum.  Blood -serum  is  plasma  which  has 
lost  its  fibrinogen  and  gained  fibrin-ferment  and  probably 
some  additional  paraglobulin;  from  an  analysis  of  it  we  can 
draw  conclusions  as  to  the  plasma.  In  100  parts  of  serum 
there  are  about  90  parts  of  water,  8.5  of  proteids,  and  1.5  of 
fats,  salts  and  other  less-known  solid  bodies.  Of  the  proteids 
present  the  most  abundant  are  serum-albumin  and  para- 
globulin. Serum-albumin  agrees  with  egg-albumin  in  coagu- 
lating when  heated:  for  this  reason  serum  when  boiled  sets 
into  an  opaque  white  mass,  just  as  the  white  of  an  egg  does. 
►Serum-albumin  differs  from  egg-albumin  in  not  being  coagu- 
lated by  ether;  and  in  the  fact  that  although  present  in  such 
large  quantities  in  the  blood,  it  is  not  excreted  by  the  kid- 
neys, as  egg-albumin  is,  if  injected  into  a  blood-vessel.  The 
paraglobulin  is  also  precipitated  by  heat,  but  may  be  pre- 
cipitated alone  by  saturation  of  the  serum  with  magnesium 
sulphate.  Fats  are  present  in  the  serum  in  small  quantity 
except  after  a  meal  at  which  fatty  substances  have  been 
eaten;  serum  obtained  from  the  blood  of  an  animal  soon 
after  such  a  meal  is  often  milky  in  appearance  from  the  large 


60  THE  nUMAN  BODY. 

amount  of  fats  present,  instead  of  being  colorless  or  pale  yel- 
low and  transparent  as  it  is  after  fasting.  The  salts  dissolved 
in  the  serum  are  mainly  sodium  chloride  and  carbonate; 
small  quantities  of  sodium,  calcium,  and  magnesium  phos- 
phates are  also  present. 

Chemistry  of  the  Red  Corpuscles.  In  these  in  the  fresh 
moist  state  there  are,  in  100  parts,  56  of  water  and  44  of 
solids.  Of  the  solids  about  one  per  cent  is  salts,  chiefly  potas- 
sium phosphate  and  chloride.  The  remaining  solids  contain, 
in  100  parts,  90  of  haemoglobin  and  about  8  of  other  proteids; 
the  residue  consists  of  less  well-known  bodies. 

Chemistry  of  the  White  Corpuscles.  Besides  much  water, 
these  yield  several  proteids,  some  fats,  glycogen  (see  Chap. 
XXIX)  and  salts;  and  smaller  quantities  of  other  bodies. 
The  predominant  salts,  like  those  of  the  red  corpuscles,  are 
potassium  phosphates. 

Variations  in  the  Composition  of  the  Blood.  The  above 
statements  refer  only  to  the  average  composition  of  the 
healthy  blood  and  to  its  better  known  constituents.  From 
what  was  said  in  the  last  chapter  it  is  clear  that  the  blood 
flowing  from  any  organ  will  have  lost  or  gained,  or  gained 
some  things  and  lost  others,  when  compared  with  the  blood 
which  entered  it.  But  the  losses  and  gains  in  particular  parts 
of  the  Body  are  in  such  small  amount  as,  with  the  exception 
of  the  blood-gases,  to  elude  analysis  for  the  most  part:  and 
the  blood  from  all  parts  being  mixed  in  the  heart,  they 
balance  one  another  and  produce  a  tolerably  constant  average. 
In  health,  however,  the  specific  gravity  of  the  blood  may  vary 
from  1045  to  1075;  the  red  corpuscles  also  are  present  in 
greater  proportion  to  the  plasma  after  a  meal  than  before  it. 
Healthy  sleep  in  proper  amount  leads  to  increase  in  the  pro- 
portion of  red  corpuscles,  and  want  of  it  tends  to  diminution 
of  their  number,  as  may  be  recognized  in  the  pallid  aspect  of 
a  person  who  has  lost  several  nights'  rest. 

The  proportion  of  the  red  corpuscles  has  a  great  impor- 
tance since,  as  we  shall  subsequently  see,  they  serve  to  carry 
oxygen,  which  is  necessary  for  the  performance  of  its  func- 
tions, all  over  the  Body.  Ancemia  is  a  diseased  condition 
characterized  by  pallor  due  to  deficiency  of  red  blood-corpus- 
cles, and  accompanied  by  languor  and  listlessness.  It  is  not 
unfrequent  in  girls  on  the  verge  of  womanhood,  and  in  per 


THE  CLOTTING   OF  BLOOD.  61 

sons  overworked  and  confined  within  doors.     In  snch  cases 
the  best  remedies  are  open-air  exercise  and  good  food. 

Summary.  Practically  the  composition  of  the  blood  may 
be  thus  stated:  It  consists  of  (1)  plasma,  consisting  of  watery 
solutions  of  serum-albumin,  paraglobulin,  fibrinogen,  sodi- 
um and  other  salts,  and  extractives  of  which  the  most  con- 
stant are  urea,  kreatin,  and  grape-sugar;  (2)  red  corjmscles, 
containing  rather  more  than  half  their  weight  of  water,  the 
remainder  being  mainly  haemoglobin,  other  proteids,  and  pot- 
ash salts;  (3)  white  corpuscles,  consisting  of  water,  various 
proteids,  glycogen,  and  potash  salts;  (4)  the  platelets;  (5) 
gases,  partly  dissolved  in  the  plasma  or  combined  with  its 
sodium  salts,  and  partly  combined  (oxygen)  with  the  haemo- 
globin of  the  red  corpuscles. 

Quantity  of  Blood.  The  total  amount  of  blood  in  the 
Body  is  difficult  of  accurate  determination.  It  is  about  -^ 
of  the  whole  weight  of  the  Body,  so  the  quantity  in  a  man 
weighing  75  kilos  (1G5  lbs.)  is  about  5.8  kilos  (12.7  lbs.).  Of 
this  at  any  given  moment  about  one  fourth  would  be  found  in 
the  heart,  lungs  and  larger  blood-vessels;  and  equal  quantities 
in  the  Vessels  of  the  liver,  and  in  those  of  the  muscles  which 
move  the  skeleton;  while  the  remaining  fourth  is  distributed 
among  the  remaining  parts  of  the  Body. 

The  Origin  and  Fate  of  the  Blood-corpuscles.  The  white 
blood-corpuscles  vary  so  rapidly  and  frequently  in  number  in 
the  blood  that  they  must  be  constantly  in  process  of  altera- 
tion or  removal,  and  formation ;  their  number  is  largely  in- 
creased after  taking  food,  even  more  than  that  of  the  red,  so 
that  their  proportion  to  the  red  rises,  from  1  to  1000  during 
fasting,  to  1  to  250  or  300  after  a  meal.  This  increase  is 
mainly  due  to  increased  flow  of  lymph  at  this  time  through 
the  lymphatics  of  the  alimentary  canal  which  have  much 
lymphoid  tissue  on  their  course;  and,  as  already  pointed  out, 
lymph-corpuscles  are  constantly  multiplying  in  this  tissue 
and  are  gathered  from  it  by  the  lymph,  to  be  poured  into  the 
blood  (see  also  Chap.  XXIII).  Migrated  pale  corpuscles  of 
the  blood  and  the  leucocytes  of  the  lymph  retain  many  of  the 
characters  of  undifferentiated  and  unspecialized  embryonic 
cells;  and  there  is  some  evidence  that  they  may  develop  new 
tissues  in  the  repair  of  injured  parts. 

Ampbioxus,  the  lowest  undoubted  vertebrate  animal  (see 
Zoology),  possesses  only  colorless   corpuscles   in   its   blood. 


62  THE  HUMAN  BODY. 

Higher  and  more  complex  animals  need  more  oxygen  and,  as 
blood-plasma  dissolves  very  little  of  that  gas,  they  develop  in 
addition  the  haemoglobin-containing  corpuscles  which  pick 
it  up  in  the  gills  or  lungs  and  carry  it  to  all  parts  of  the 
Body,  leaving  it  where  wanted  (see  Chap.  XXVI).  In  cold- 
blooded vertebrates  the  red  corpuscles  are  not  nearly  so  many 
in  proportion  as  in  the  warm-blooded,  which  use  far  more 
oxygen.  The  older  view  was  that  the  mammalian  red  cor- 
puscle represented  the  nucleus  of  one  of  the  white,  in  which 
haemoglobin  had  been  formed  and  from  about  which  the  rest 
of  the  corpuscle  had  disappeared.  This,  however,  does  not 
seem  to  be  the  case.  In  adults  new  red  blood-corpuscles  are 
formed  by  the  segregation  of  portions  of  the  protoplasm  of 
peculiar  cells  (luematoblasts)  found  in  various  parts  of  the 
Body,  but  especially  in  the  red  marrow  of  certain  bones  (p. 
95).  In  the  embryo  some  cells  of  the  liver,  and  in  new-born 
animals  (possibly  also  in  adult)  some  connective-tissue  cor- 
puscles (p.  112)  form  new  red  blood-corpuscles. 

How  long  an  individual  red  corpuscle  lasts  is  not  known, 
nor  with  certainty  how  or  where  it  disappears  :  there  is,  how- 
ever, some  reason  to  believe  that  many  are  finally  destroyed 
in  the  spleen  (see  Chap.  XXIII).  Their  average  rate  of  dis- 
appearance and  new  formation  is  unknown,  but  in  emergen- 
cies (as  after  severe  haemorrhages)  they  can  be  reproduced 
with  great  rapidity. 

Chemistry  of  Lymph.  Lymph  is  a  colorless  fluid  when 
pure,  feebly  alkaline,  and  with  a  specific  gravity  of  about 
1045.  It  may  be  described  as  blood  minus  its  red  corpuscles 
and  much  diluted,  but  of  course  in  various  parts  of  the  Body 
it  will  contain  minute  quantities  of  substances  derived  from 
neighboring  tissues.  It  contains  a  considerable  quantity  of 
carbon  dioxide  gas  which  it  gives  up  in  a  vacuum,  butiio  un- 
combined  oxygen,  since  any  of  that  gas  which  passes  into  it 
by  diffusion  from  the  blood  is  immediately  picked  up  by  the 
living  tissues  among  which  the  lymph  flows. 


CHAPTER  VI. 

THE   SKELETON. 

Exoskeleton  and  Endoskeleton.  The  skeleton  of  an 
animal  includes  all  its  hard  protecting  or  supporting  parts, 
and  is  met  with  in  two  main  forms.  One  is  an  exoskeleton 
developed  in  connection  with  either  the  superficial  or  deeper 
layer  of  the  skin,  and  represented  by  the  shell  of  a  clam, 
the  scales  of  fishes,  the  horny  plates  of  a  turtle,  the 
bony  plates  of  an  armadillo,  and  the  feathers  of  birds. 
In  man  the  exoskeleton  is  but  slightly  developed,  but  it 
is  represented  by  the  hairs,  nails  and  teeth;  for  although 
the  latter  lie  within  the  mouth,  the  study  of  development 
shows  that  they  are  developed  from  an  offshoot  of  the  skin 
which  grows  in  and  lines  the  mouth  long  before  birth.  Hard 
parts  formed  from  structures  deeper  than  the  skin  constitute 
the  endoskeleton,  which  in  man  is  highly  developed  and  con- 
sists of  a  great  many  bones  and  cartilages  or  gristles,  the 
bones  forming  the  mass  of  the  hard  framework  of  the  Body, 
while  the  cartilages  finish  it  off  at  various  parts.  This  frame- 
work is  what  is  commonly  meant  by  the  skeleton;  it  pri- 
marily supports  all  the  softer  parts  and  is  also  arranged  so  as 
to  surround  cavities  in  which  delicate  organs,  as  the  brain, 
heart  or  spinal  cord,  may  lie  with  safety.  The  gross  skeleton 
thus  formed  is  completed  and  supplemented  by  another  made 
of  the  connective  tissues,  which  not  only,  in  the  shape  of 
tough  bands  or  ligaments,  tie  the  bones  and  cartilages  to- 
gether, but  also  in  various  forms  pervade  the  whole  Body  as 
a  sort  of  subsidiary  skeleton  running  through  all  the  soft 
organs  and  forming  networks  of  fibres  around  their  other 
constituents;  they  make,  as  it  were,  a  microscopic  skeleton 
for  the  individual  modified  cells  of  which  the  Body  is  so 
largely  composed,  and  also  form  partitions  between  the  mus- 
cles, cases  for  such  organs  as  the  liver  and  kidneys,  and 
sheaths  around  the  blood  vessels.  The  bony  and  cartilagin- 
ous framework  with  its  ligaments  might  be  called  the  skele- 

63 


64  THE  HUMAN  BODY. 

ton  of  the  organs  of  the  Body,  and  this  finer  supporting 
meshwork  the  skeleton  of  the  tissues.  Besides  forming  ;i 
support  in  the  substance  of  various  organs,  the  connective 
tissues  are  often  laid  down  as  a  sort  of  packing  material  in  the 
crevices  between  them;  and  so  widely  are  they  distributed 
everywhere  from  the  skin  outside  to  the  lining  of  the  alimen- 
tary canal  inside,  that  if  souk;  solvent  could  be  employed 
which  would  corrode  away  all  the  rest  and  leave  only  these 
tissues,  a  very  perfect  model  of  the  whole  Body  would  be  left; 
something  like  a  "skeleton  leaf,"  but  far  more  minute  in  its 
tracery. 

The  Bony  Skeleton  (Fig.  16).  If  the  hard  framework 
of  the  Body  were  joined  together  like  the  joists  and  beams  of 
a  house,  the  whole  mass  would  be  rigid;  its  parts  could  not 
move  with  relation  to  one  another,  and  we  should  be  unable 
to  raise  a  hand  to  the  mouth  or  put  one  foot  before  another. 
To  allow  of  mobility  the  bony  skeleton  is  made  of  many  sepa- 
rate pieces  which  are  joined  together,  the  points  of  union  be- 
ing called  articulations,  and  at  many  places  the  bones  enter- 
ing into  an  articulation  are  movably  hinged  together,  forming 
what  are  known  as  joints.  The  total  number  of  bones  in  the 
Body  is  more  than  two  hundred  in  the  adult;  and  the  number 
in  children  is  still  greater,  for  various  bones  which  are  dis- 
tinct in  the  child  (and  remain  distinct  throughout  life  in 
many  lower  animals)  grow  together  so  as  to  form  one  bone  in 
the  full-grown  man.  The  adult  bony  skeleton  may  be  de- 
scribed as  consisting  of  an  axial  skeleton,  found  in  the  head, 
neck  and  trunk;  and  an  appendicular  skeleton,  consisting  of 
the  bones  in  the  limbs  and  in  the  arches  {u  and  s,  Fig.  16) 
by  which  these  are  carried  and  attached  to  the  trunk. 

Axial  Skeleton.  The  axial  skeleton  consists  primarily 
of  the  vertebral  column  or  spine,  a  side  view  of  which  is  rep- 
resented in  Fig.  17.  The  upper  part  of  this  column  is  com- 
posed of  twenty-four  separate  bones,  each  of  which  is  a  ver- 
tebra. At  the  posterior  part  of  the  trunk,  beneath  the 
movable  vertebrae,  comes  the  sacrum  (S  1),  made  up  of  five 
vertebrae,  which  in  the  adult  grow  together  to  form  one  bone, 
and  below  the  sacrum  is  the  coccyx  (Co  1—4),  consisting  of 
four  very  small  tail  vertebra?,  which  in  advanced  life  also 
unite  to  form  one  bone. 

On  the  top  of  the  vertebral  column  is  borne  the  skull, 
made  up  of  two  parts,  viz.,  a  great  box  above  which  iucloses 


THE  SKELETON.  65 

the  brain  and  is  called  the  cranium,  and  a  large  number  of 


Fig.  16.— The  bony  and  cartilaginous 
skeleton. 


6pj£J} 


Fig.  17.— Sifle  view  of  the 
spinal  column. 


bones  on  the  ventral  side  of  this  which  form  the  skeleton  of 


66  THE  HUMAN  BODY. 

the  face.  Attached  by  ligaments  to  the  under  side  of  the 
cranium  is  the  hyoid  bone,  to  which  the  root  of  the  tongue  is 
fixed. 

Of  the  twenty-four  separate  vertebra?  of  the  adult  the  seven 
nearest  the  skull  (Fig.  17,  C  1-7)  lie  in  the  neck  and  are 
known  as  the  cervical  vertebra.  These  are  followed  by 
twelve  others  which  have  ribs  attached  to  them  (see  Fig.  16) 
and  lie  at  the  back  of  the  chest;  they  are  the  thoracic  or  dorsal 
vertebra?  (D  1-12).  The  ribs  (Fig.  28)  are  slender  curved 
bones  attached  by  their  dorsal  ends,  called  their  heads,  to  the 
thoracic  vertebrae  and  running  thence  round  the  sides  of  the 
chest.  In  the  ventral  median  line  of  the  latter  is  the  breast- 
bone or  sternum  (d,  Fig.  10).  Each  rib  near  its  sternal  end 
ceases  to  be  bony  and  is  composed  of  cartilage. 

These  parts — skull,  hyoid  bone,  vertebral  column,  ribs, 
and  sternum — constitute  the  axial  skeleton. 

The  Thoracic  or  Dorsal  Vertebrae.  If  a  single  vertebra, 
say  the  eleventh  from  the  skull,  be  examined  carefully  it  will 
be  found  to  consist  of  the  following  parts  (Figs.  18  and  1!)): 

First  a  bony  mass,  C,  rounded  on  the  sides  and  flattened 
on  each  end  where  it  is  turned  towards  the  vertebra?  above  and 
below  it.  This  stout  bony  cylinder  is  the  "  body  "  or  centrum 
of  the  vertebra,  and  the  series  of  vertebral  bodies  (Fig.  17) 
forms  in  the  trunk  that  bony  partition  between  the  dorsal 
and  ventral  cavities  of  the  body  spoken  of  in  Chapter  I.  To 
the  dorsal  side  of  the  body  is  attached  an  arch — the  neural 
arch,  A,  which  with  the  back  of  the  body  incloses  a  space, 
Fv,  the  neural  ring.  In  the  tube  formed  by  the  rings  of  the 
successive  vertebras  lies  the  spinal  cord.  Projecting  from  the 
dorsal  side  of  the  neural  arch  is  a  long  bony  bar,  Ps,  the 
spinous  process  :  and  the  projections  of  these  processes  from 
the  various  vertebra?  can  be  felt  through  the  skin  all  down 
the  middle  of  the  back.  Hence  the  name  of  spinal  column 
often  given  to  the  whole  back-bone. 

Six  other  processes  arise  from  the  arch  of  the  vertebra: 
two  project  forwards,  i.e.,  towards  the  head;  these,  Pas,  are 
the  anterior  articular  processes  and  have  smooth  surfaces, 
covered  with  cartilage,  on  their  dorsal  sides.  A  pair  of  simi- 
lar posterior  articular  processes,  Pai,  runs  back  from  the 
neural  arch,  and  these  have  smooth  surfaces  on  their  ventral 
aspects.  In  the  natural  position  of  the  vertebra,  the  smooth 
surfaces  of  its  anterior  articular  processes  fit  upon  the  poste- 


THE  SKELETON. 


67 


rior  articular  processes  of  the  vertebra  next  in  front,  forming 
a  joint,  and  the  two  processes  are  united  by  ligaments.     >Sim- 


Fig.  18. 


Fig.  19. 


Fig.  18.— A  thoracic  vertebra  seen  from  behind,  i.e.,  the  end  turned  from  the  head 
Fig.  19.— Two  thoracic  vertebra?  viewed  from  the  left  side,  and  in  their  natural 
relative  positions.  C,  the  body  ;  A,  neural  aicti  ;  Fi\  the  neural  ring  ;  Ps,  spinous 
process;  Pas,  anterior  articular  process  ;  Pui,  posterior  articular  process ;  Pt 
transverse  process  :  Ft,  facet  for  articulation  with  the  tubercle  of  a  rib  ;  Fes,  Fci' 
articular  surfaces  on  the  centrum  for  articulation  with  a  rib. 

ilarly  its  posterior  articular  processes  form  joints  (Fig.  19) 
with  the  anterior  articular  processes  of  the  vertebra  next  be- 
hind. 

The  remaining  processes  are  the  transverse,  Pt,  which 
run  outwards  and  a  little  dorsally.  Each  of  these  has  a 
smooth  articular  surface,  Ft,  near  its  outer  end. 

On  the  "  body  "  are  seen  two  articular  surfaces  on  each 
side:  one,  Fes,  at  its  anterior,  the  other,  Fci,  at  its  posterior 
end,  and  both  close  to  the  attachment  of  the  neural  arch. 
Each  of  these  surfaces  forms  with  corresponding  areas  on 
the  vertebrae  in  front  and  behind  a  pit  into  which  the  end 
of  a  rib  fits  and  the  rib  attached  in  this  way  to  the  anterior 
part  of  the  "body"  is  also  fitted  on,  a  little  way  from  its 
dorsal  end,  to  the  articular  surface  at  the  end  of  the  transverse 
proce 

The  Segments  of  the  Axial  Skeleton.  If  a  thoracic  verte- 
bra, say  the  first  (Fig.  20),  be  detached  with  the  pair  of  ribs, 
Ov,  belonging  to  it  and  the  bit  of  the  sternum,  S,  to  which 
these  ribs  are  fixed  vcntndlv,  we  would  find  a  bony  parti- 
tion   formed   by   the   body  of  the   vertebra,   lying   between 


6S 


THE  HUMAN  BODY. 


two  arches  which  surround 
inclosed  by  the  "body"  and  ' 
nally  part  oi'  the  spinal  cord. 


cavil ies.  The  dorsal  cavity 
neural  arch  "  contained  origi- 
The  other  ring,  made  up  by 
the  body  of  the  vertebra  dor- 
sally,  the  sternum  vent  rally, 
and  the  ribs  on  the  sides,  sur- 
rounds the  chest-cavity  with 
its  contents.  All  of  these  parts 
together  form  a  typical  seg- 
ment of  the  axial  skeleton, 
which,  however,  only  attains 
this  completeness  in  the  tho- 
racic region  of  the  trunk.  In 
the  skull  it  is  greatly  modified; 
s  asrSapro?  and  in  the  neck  and  the  lower 


representa- 
axial  skeleton. 


Fig.    20.— Diagrammatic 
tion  of  a  segment  of  tli 


above  with  the  body  and  trail 
cess  of  the  vertebra;  S,  the  breast-b 
The  lighter-shaded  part  between  8  and  C 
is  the  rib-cartilage. 


part  of  the  trunk  the  ribs  arc 
either  absent  or  very  small, 
appearing  only  as  processes  of  the  vertebra?;  and  the  sternal 
portion  is  wanting  altogether. 

Nevertheless  Ave  may  regard  the  whole  axial  skeleton  as 
made  up  of  a  series  of  such  segments  placed  one  in  front  of 
another,  but  having  different  portions  of  the  complete  seg- 
ment much  modified  or  rudimentary  or  even  altogether 
wanting  in  some  regions.  Parts  which  in  this  way  really 
correspond  to  oue  another  though  they  differ  in  detail,  which 
are  so  to  speak  different  varieties  of  one  thing,  are  said  in 
anatomical  language  to  be  homologous  to  one  another;  and 
when  they  succeed  one  another  in  a  row,  as  the  trunk  seg- 
ments do,  the  homology  is  spoken  of  as  serial. 

The  Cervical  Vertebrae.  In  the  cervical  region  of  the 
vertebral  column  the  bodies  of  the  vertebra?  are  smaller  than 
in  the  dorsal,  but  the  arches 
are  larger;  the  spinous  pro- 
cesses are  short  and  often  bifid 
and  the  transverse  processes 
appear  to  be  perforated  by  a 
canal,  the  vertebral  foramen. 
The    bony  bar    bounding    this  IJT   \J       l 

aperture   on   the   ventral    side, 

,  .       .  •..,  Fig.  21.— A  cervical  vertebra.    Fit, 

hOWeVer,     IS     111     reality    a    Very  vertebral  foramen;  Pai,  anterior  ar- 

small  rib  which  has  grown  into  ticular  process- 

continuity  with   the   body   and  true   transverse   process  of 


THE  SKELETON. 


69 


the  vertebra,  although  separate  in  very  early  life:  the  trans- 
verse process  proper  bounds  the  vertebral  foramen  dorsally. 
In  this  latter  during  life  runs  an  artery,  which  ultimately 
enters  the  skull-cavity. 

The  Atlas  and  Axis.  The  first  and  second  cervical  verte- 
bra? differ  considerably  from  the  rest.  The  first,  or  atlas 
(Fig.  22),  which  carries  the  head,  has  a  very  small  body,  Aa, 
and  a  large  neural  ring.  This  ring  is  subdivided  by  a  cord, 
the  transverse  ligament,  L,  into  a  dorsal  moiety  in  -which  the 
spinal  cord  lies  and  a  ventral  into  which  the  bony  process  D 
projects.  This  is  the  odontoid  process,  and  arises  from  the 
front  of  the  axis  or  second  cervical  vertebra  (Fig.  23). 
Around  this  peg  the  atlas  rotates  when  the  head  is  turned 
from  side  to  side,  carrying  the  skull  (which  articulates  with 
the  large  hollow  surfaces  Fas)  with  it. 

The  odontoid  process  really  represents  a  large  piece  of  the 
body  of  the  atlas  which  in  early  life  separates  from  its  own 
vertebra  and  becomes  united  to  the  axis. 

The  Lumbar  Vertebrae  (Fig.  24)  are  the  largest  of  all  the 
movable  vertebra?  and  have  no  ribs  attached  to  them.  Their 
spines  are  short  and  stout  and  lie  in  a  more  horizontal  plane 


Aa    Fas 


Ma 


Frt 


Fig.  22. 


Fig.  23. 


Fig.  22.— The  atlas.  Fig.  33.—  The  axis.  Aa.  body  of  atlas;  D,  odontoid  process; 
Fas,  facet  on  front  of  atlas  with  which  the  stcull  articulates:  and  in  Fip.  23,  ante- 
rior articular  surface  of  axis;  L.  transverse  ligament;  Frt,  vertebral  foramen;  Ap, 
neural  arch;  Tp,  spinous  process. 

than  those  of  the  vertebra3  in  front.     The  articular  and  trans- 
verse processes  are  also  short  and  stout. 

The  Sacrum,  which  is  represented  along  with  the  last  lum- 
bar vertebra  in  Fig.  •.'."»,  consists  in  the  adult  of  a  single  bone; 
but  cross-ridges  on  its  ventral  surface  indicate  the  limits  of 
the  five  separate  vertebras  of  which  it  is  composed  in 
childhood.     It    is    somewhat     triangular    in    form,    its    base 


70 


THE  HUMAN   HobY. 


being  directed  upwards  and  articulating  with  the  under 
surface  of  the  body  of  the  fifth  lumbar  vertebra.  On  its 
sides  are  large  surfaces  to  which  the  arch  bearing  the  lower 


»'t        l 


Fig.  24.— A  lumbar  vertebra  seen  from  the  left  side.    Ps,  spinous  process;  Pus, 
anterior  articular  process;  Pai,  posterior  articular  process;  Pt, transverse  process. 


Fsa 


Fig.  25. — The  l.n«f  lumbar  vertebra  and  the  sacrum  seen  from  the  ventral  side. 
Fsa,  anterior  sacral  foramina. 


limbs  is  attached  (see  Fig.  16).     Its  ventral   surface  is  con- 
cave and   smooth  and   presents  four  pairs  of  anterior  sacral 


THE  SKELETON.  71 

foramina,  Fsa,  which  communicate  with  the  neural  canal. 
Its  dorsal  surface,  convex  and  roughened,  has  four  similar 
pairs  of  posterior  sacral  foramina. 

The  coccyx  (Fig.  20)  calls  for  no  special  description.  The 
four  bones  which  grow  together,  or  ankylose, 
to  form  it,  represent  only  the  bodies  of  vertebrae, 
and  even  those  incompletely.  It  is  in  reality 
a  short  tail,  although  not  visible  as  such  from 
the  exterior. 

The  Spinal  Column  as  a  Whole.  The  ver- 
tebral column  is  in  a  man  of  average  height 
about  twenty- eight  inches  long.  Viewed  from 
the  side  (Fig.  1?)  it  presents  four  curvatures; 
one  with  the  convexity  forwards  in  the  cervical 
region  is  followed,  in  the  thoracic,  by  a  curve  with  its  concavity 
towards  the  chest.  In  the  lumbar  region  the  curve  has  again 
its  convexity  turned  ventrally,  while  in  the  sacral  and  coccy- 
geal regions  the  reverse  is  the  case.  These  curvatures  give  the 
whole  column  a  good  deal  of  springiness  such  as  would  be 
absent  were  it  a  straight  rod,  and  this  is  farther  secured  by  the 
presence  of  compressible  elastic  pads,  the  intervertebral  disks, 
made  up  of  cartilage  and  connective  tissue,  which  lie  between 
!he  bodies  of  those  vertebras  which  are  not  ankylosed  together, 
and  fill  up  completely  the  empty  spaces  left  between  the 
bodies  of  the  vertebra?  in  Fig.  17.  By  means  of  these  pads, 
moreover,  a  certain  amount  of  movement  is  allowed  between 
each  pair  of  vertebras;  and  so  the  spinal  column  can  be  bent 
to  considerable  extent  in  any  direction;  while  the  movement 
between  any  two  vertebras  is  so  limited  that  no  sharp  bend 
can  take  place  at  any  one  point,  such  as  might  tear  or  other- 
wise injure  the  spinal  cord  contained  in  the  neural  canal. 
The  amount  of  movement  permitted  is  greatest  in  the  cervical 
region. 

In  the  case  of  the  movable  vertebrae,  the  arch  is  somewhat 
narrowed  where  it  joins  the  body  on  each  side  ;  this  nar- 
rowed stalk  is  the  pedicle  (It,  Fig.  19),  while  the  broader 
remaining  portion  of  the  arch  is  its  lamina.  Between 
the  pedicles  of  two  contiguous  vertebrae  there  are  in  this 
way  left  apertures,  the  intervertebral  holes  whicb  form  a 
series  on  each  side  of  the  vertebral  column,  and  one  of  which, 
/•'/,  i-  shown  between  the  two  dorsal  vertebra*  in  Fig.  19. 
Through  the.se  foramina  nerves  run  out  from  the  spinal  cord 


72 


THE  III  MAS   BODY, 


to  various  regions  of  the  Body.  The  sacral  foramina,  anterior 
and  posterior,  are  the  representatives  <»f  these  apertures,  but 
modified  in  arrangement,  on  account  of  the  fusion  of  the 
arches  and  bodies  of  the  vertebra  between  which  they  lie. 

Sternum.  The  sternum  or  breast-bone  (Fig.  '.;  and  d. 
Pig.  lti)  is  wiilci-  from  side  to  side  than  dorso-ventrally.  It 
(•.insists  in  the  adult  of  three  pieces,  and  seen  from  the  ven- 
tral side  has  somewhat  the  form  of  a  dagger.  The  piece  1/ 
nearest  the  head  is  called  the  handle  or  manubrium,  and  pre- 
sents anteriorly  a  notch,  Tel,  on  each  side,  with  which  the 
collar-bone  articulates  (m,  Fig.  16);  farther  hack  are  two 
other  notches.  A- 1  and  Tc2,  to  which  the  sternal  ends  of  the 
tiist  and  second  ribs  are  attached.  The  middle  piece,  r,  of 
the  sternum  is  called  the  body;  it  completes  the  notch  for 
the  second  rib  and  has  on  its  sides  others. 
ft;  3-7,  for  the  third,  fourth,  fifth,  sixth, 
and  seventh  ribs.  The  last  piece  of  the 
sternum,  P,  is  the  ensiform  or  xiphoid 
process;  it  is  composed  of  cartilage,  ami 
has  no  ribs  attached  to  it. 

The  Ribs.  (Fig.  28).  There  are  twelve 
pairs  of  ribs,  each  being  a  slender  curved 
bone  attached  dorsally  to  the  body  and 
transverse  process  of  a  vertebra  in  the 
manner  already  mentioned,  and  continued 
vent  rally  by  a  cosh!/  cartilage.  In  the 
case  of  the  anterior  seven  pairs,  the  costal 
cartilages  are  attached  directly  to  the  sides 
of  the  breast-hone;  the  next  three  carti- 
lages are  each  attached  to  the  cartilage  of 
the  preceding  rib,  while  the  cartilages  of 
the   eleventh   and    twelfth   ribs  are   <|uite 

Men  on  its  ventral  aspect!  unattached   ventrallv,  so  these  are  called 
M, manubrium;  r.  body;     ,  .  _»       ,  .        '     .,  m1 

p,  xiphoid  process:  id,  the  free  or  floating  ribs.      Lhe  convexity 

notch  for  tbe  collar-bone;      ,.     '    ,  -,      •,'  •  j  i 

/<•  l-r.  notches  tor  the  oi  each  curved  rib  is  turned  outwards  so 

as  to  give  roundness  to  the  sides  of  the 
chest  and  increase  its  cavity,  and  each  slopes  downwards  from 
its  vertebral  attachment,  so  that  its  sternal  end  is  consider- 
ably lower  than  its  dorsal. 

The  Skull.  (Fig.  29)  consists  of  twenty-two  hones  in  the 
adult,  of  which  eight,  forming  the  cranium,  are  arranged  so 
as  to  inclose  the  brain-case  and   protect  the  auditory  organ, 


Fig.  87.— The  sternum 
seen 


THE  SKELETON. 


73 


while    the    remaining    fourteen    support  the  face    and   sur- 
round the  mouth,  the  nose,  and  the  eye-sockets. 


Fto.  28.— The  ribs  of  the  left  sii'ie,  with  (lie  dorsal  and  two  h'tnbar  vertebrae,  the 
rib-cartilages  and  the  sternum:  a,  first  and,  b,  twelfth  thoracic  vertebra;  c,  first 
lumbar  vertebra. 


Cranium.  The  cranium  is  a  box  with  a  thick  floor  and 
thinner  walls  and  roof.  Its  floor  or  base  represents  in  the 
head  (as  is  depicted  in  the  diagram  Fig.  '.')  that  partition  be- 
tween   tin-  dorsal  and  ventral  cavities  which  in  the  trunk  is 

made  up  of  the  bodies  of  the  vertebrae     In  very  early  life  it 


74 


THE  HUMAN  BODY. 


presents  in  the  middle  line  ;i  series  of  four  bones,  the  bast- 
occipital,  basi-sphenoid,  pre8phenoid,a.nd  basi-ethmoid,  which 
answer  pretty  much  to  the  bodies  of  four  vertebra*,  and  have 
attached  to  them  the  thin  bones  which  inclose  the  skull-cavity 
(which  may  be  likened  to  an  enlarged  neural  canal)  on  the 
sides   and   top.     In  the   Human    Body,  however,  these  bones 


visp 


Fig.  29  —  A  side  view  of  the  skull.  O.  occipital  bone  ;  T,  temporal  ;  JFY,  parie- 
tal;  F,  frontal;  S,  sphenoid  ;  Z,  malar;  Mx,  maxilla;  A',  nasal;  E,  ethmoid;  L, 
lachrymal;  AW,  inferior  maxilla. 

very  soon  ankylosewith  others  or  with  one  another;  although 
they  remain  distinct  throughout  life  in  the  skulls  of  very 
many  lower  animals.  On  the  base  of  the  skull,  besides  many 
small  apertures  by  which  nerves  and  blood-vessels  pass  in  or 
out,  is  a  large  aperture,  the  foramen  magnum,  through  which 
the  spinal  cord  passes  in  to  join  the  brain. 


THE  SKELETON. 


75 


The  cranial  bones  are  the  following: 

1.  The  occipital  bone  (Fig.  29,  0),  impaired  and  having  in 
it  the  foramen  magnum.  It  is  made  up  by  the  fusion  of  the 
basi-occipital  with  other  flatter  bones. 
2.  The  frontal  bone  (Fig.  29,  F),  also 
unpaired  in  the  adult,  but  in  the 
child  each  half  is  a  separate  bone.  3. 
A  pair  of  thin  platelike  parietal  bones 
(Fig.  29,  Pr)  which  meet  one  another 
along  the  middle  line  in  the  top  of  the 
skull,  and  roof-in  a  great  part  of  the 
cranial  cavity.  4.  A  pair  of  temporal 
bones  (Fig.  29,  T),  one  on  each  side  of 
the  skull  below  the  parietal.  On 
each  temporal  bone  is  a  large  aperture 
leading  into  the  ear-cavity,  the  essen- 
tial parts  of  the  organs  of  hearing 
being  contained  in  these  bones.  5. 
The  sphenoid  bone,  made  up  by  the 
union  of  the  ba si- sphenoid  and  pre- 
sphenoid  (lying  on  the  base  of  skull  in 

front  of  the  basi-occipital)  with  one  ^ffiMB?«!f S?*£g 
another  and  with  flatter  bones,  is  seen  t^S^ffSiS^SS. 
partly  (Fig.  29,  S)  on  the  sides  of  the  J^SS^SSflg 
cranium  in  front  of  tho  temporals.  6.  atlas,  on  its  sides. 
The  ethmoid,  like  the  sphenoid,  single  in  the  adult,  is  really 
made  up  by  the  union  of  a  single  median  basi-ethmoid  with 
a  pair  of  lateral  bones.  It  closes  the  skull-cavity  in  front, 
and  lies  between  it  and  the  top  of  the  nasal  chambers,  being 
perforated  by  many  small  holes  through  which  the  nerves  of 
smell  pass.  A  little  bit  of  it  is  seen  on  the  inner  side  of  the 
eye-socket  at  E  in  Fig.  29. 

Facial  Skeleton.  The  majority  of  the  face-bones  are  in 
pairs;  two  only  being  single  and  median.  One  of  these  is 
the  lower  jaw-bone  or  inferior  maxilla  (Fig.  29,  Md) ;  the 
other  is  the  vomer,  which  fcrms  part  of  the  partition  between 
the  two  nostrils. 

The  paired  face-bones  are:  1.  The  maxillae,  or  upper  jaw- 
bones {Mx,  Fig.  29),  one  on  each  side,  carrying  the  upper 
row  of  teeth  and  forming  a  great  pu-t  of  the  hard  palate, 
which  separatee  the  mouth  from  the  nose.  2.  The  palatine 
bone*,  completing  the  skeleton  of  the  hard  palate,  and  behind 


Fig.  30—  The  base  of  the 
skull.  The  lower  jaw  has  been 
removed.  At  the  lower  part 
of  the  figure  is  the  hard  palate 
forming  the  roof  of  the  mouth 
and  surrounded  by  the  upper 
set  of  teeth.  Above  this  are 
the  paired  openings  of  the  pos- 


76  THE  HUMAN  BODY. 

which  the  nose  communicates  by  the  posterior  nares  (Fig.  30) 
with  the  throat-cavity,  so  thai  air  can  pass  in  or  out  in  breath- 
ing. :'>.  The  malar  bones,  or  cheek-bones,  (Z,  Fig.  29.)  lying 
beneath  and  on  the  outside  of  the  orbit  on  each  side.  4.  The 
nasal  bones  (V.  Fig.  29),  roofing  in  the  nose.  5.  The  lach- 
rymal  bones  (A.  Fig.  *-'!>),  very  small  and  thin  and  lying  be- 
tween tin'  nose  and  orbit.  G.  The  inferior  turbinate  bones, 
lying  inside  the  nose,  one  in  each  nostril-chamber. 

The  Hyoid.  Besides  the  cranial  and  facial  hones  there 
is,  as  already  pointed  out,  one  other,  the  hyoid  (Fig.  31), 
which  really  belongs  to  the  skull,  although  it  lies  in  the 
neck.  It  can  be  felt  in  the  front  of  the  throat,  just  above 
"Adam's  apple."  The  hyoid  bone  is  U-shaped,  with  its  con- 
vexity turned  ventrally,  and  consists  of  a 
body  and  two  pairs  of  processes  called  cor- 
nua.  The  smaller  cornua  (Fig.  31,  3)  are 
attached  to  the  base  of  the  skull  by  long 
fig. 3i^The  hyoi.i  ligaments.  These  ligaments  in  many  ani- 
K?eat  cornua5- ;  3  nu^s  are  represented  by  bones,  so  that  the 
small  cornua.  hyoid,    with    them,  forms   a  bony  arch  at- 

tached to  the  base  of  the  skull  much  as  the  ribs  are  attached 
to  the  bodies  of  the  vertebra?.  In  fishes  behind  this  hyoidean 
arch  come  several  others  which  bear  the  gills;  and  in  the 
very  young  Human  Body  these  also  are  represented,  though 
they  almost  entirely  disappear  long  before  birth.  The  hyoid, 
then,  with  its  cornua  and  ligaments  answers  pretty  much  to 
a  gill-arch,  or  really  to  parts  of  two  gill-arches,  since  the 
great  and  small  cornua  belong  to  originally  separate  arches 
present  at  an  early  stage  of  development.  It  is  a  remnant  of 
a  structure  which  has  no  longer  any  use  in  the  Unman  Body; 
but  in  the  young  frog-tad  pole  parts  answering  to  it  carry 
gills  and  have  clefts  between  them  which  extend  into  the 
throat  just  as  in  fishes.  The  gills  are  lost  afterwards  and  the 
clefts  closed  up  when  the  frog  gets  its  lungs  and  begins  to 
breathe  by  them.  In  the  embryonic  human  being  these  gill- 
clefts  are  also  present  and  several  more  behind  them,  but  the 
arches  between  them  do  not  bear  gills,  and  the  clefts  them- 
selves are  closed  long  before  birth.  As  they  have  no  use  their 
presence  is  hard  to  account  for;  those  who  accept  the  doc- 
trine of  evolution  regard  them  as  developmental  reminis- 
cences of  an  extremely  remote  ancestor  in  which  they  were 
of   functional    importance   somewhat  as  in  the  tadpole:    of 


THE  SKELETON.  11 

course  this  does  not  mean  that  men  were  developed  from 
tadpoles. 

The  Appendicular  Skeleton.  This  consists  of  the 
shoulder-girdle  and  the  bones  of  the  fore  limbs,  and  the 
pelvic  girdle  and  the  bones  of  the  posterior  limbs.  The  two 
supporting  girdles  in  their  natural  position  with  reference  to 
the  trunk  skeleton  are  represented  in  Fig.  32. 

The  Shoulder-girdle,  or  Pectoral  Arch.  This  is  made 
up,  on  each  side,  of  the  scapula  or  shoulder-blade,  and  the 
clavicle  or  collar-bone. 

The  scapula  (S,  Fig.  32)  is  a  flattish  triangular  bone 
which  can  readily  be  felt  on  the  back  of  the  thorax.  It  is 
not  directly  articulated  to  the  axial  skeleton,  but  lies  im- 
bedded in  the  muscles  and  other  parts  outside  the  ribs  on  each 
side  of  the  vertebral  column.  From  its  dorsal  side  arises  a 
crest  to  which  the  outer  end  of  the  collar-bone  is  fixed,  and 
on  its  outer  edge  is  a  shallow  cup  into  which  the  toj)  of  the 
arm-bone  fits:  this  hollow  is  known  as  the  glenoid  fossa. 

The  collar-bone  (C,  Fig.  32)  is  cylindrical  and  attached  at 
its  inner  end  to  the  sternum  as  shown  in  the  figure,  fitting 
into  the  notch  represented  at  Id  in  Fig.  27. 

The  Fore  Limb.  In  the  limb  itself  (Fig.  33)  are  thirty 
bones.  The  largest,  a,  lies  in  the  upper  arm,  and  is  called  the 
humerus.  At  the  elbow  the  humerus  is  succeeded  by  two 
bones,  the  radius  and  ulna,  c  and  b,  which  lie  side  by  side, 
the  radius  being  on  the  thumb  side.  At  the  distal  ends  of 
these  bones  come  eight  small  ones,  closely  packed  and  forming 
the  wrist,  or  carpus.  Then  come  five  cylindrical  bones 
which  can  be  felt  through  the  soft  parts  in  the  palm  of  the 
hand;  one  for  the  thumb,  and  one  for  each  of  the  fingers. 
These  are  the  metacarpal  bones,  and  are  distinguished  as  first, 
second,  third,  and  so  on,  the  first  being  that  of  the  thumb. 
In  the  thumb  itself  are  two  bones,  and  in  each  finger  three, 
arranged  in  rows  one  after  the  other;  these  bones  are  all  called 
phalanges. 

The  Pelvic  Girdle  (Fig.  32).  This  consists  of  a  large 
bone,  the  os  innominatum,  ()<■,  on  each  side,  which  is  firmly 
fixed  dorsally  to  the  sacrum  and  meets  its  fellow  in  the  mid- 
dle ventral  line,  in  the  child  each  os  innominatum  consists 
of  three  bones,  viz.,  the  ilium,  the  ischium,  and  pubis. 
Where  these  three  bones  meet  and  finally  ankylose  there  is  a 
deep  socket,  the  acetabulum,  into  which  the  head  of  the  thigh- 


78 


THE  UUMAN   BODY. 


bone  fits  (see  Pig.  L6).  Between  the  pubic  and  ischial  bones 
is  the  largest  foramen  in  the  whole  skeleton,  known  as  the 
doorlike   or  thyroid  foramen.     The   pubic   bone   lies  above 


Fig.  Z'i.—  The  skeleton  of  the  trunk  and  the  limb  arches  seen  from  the  front.  C. 
clavicle;  S,  scapula;  Oc,  innominate  hone  attached  to  the  side  of  the  sacrum  dor- 
sally  and  meeting  its  fellow  at  the  pubic  symphysis  in  the  ventral  median  line. 

and  the  ischial  below  it.  The  ilium  forms  the  upper  expanded 
portion  of  the  os  innominaturn  to  which  the  line  drawn  from 
Oc  in  Fig.  32  points. 

The  Hind  Limb.  In  this  there  are  thirty  bones,  as  in  the 
fore  limb,  but  not  quite  similarly  arranged;  there  being  one 
less  at  the  ankle  than  in  the  wrist,  and  one  at  the  knee  not 
present  at  the  elbow-joint.  The  thigh-bone  or  femur  (a, 
Fi<;.  34)  is  the  largest  bone  in  the  body  and  extends  from  the 
hip  to  the  knee-joint.  It  presents  above  a  large  rounded 
head  which  fits   into  the  acetabulum  and,  below,  it  is  also 


THE  SKELETON. 


79 


enlarged  and  presents  smooth  surfaces  which  meet  the  bones 
of  the  leg.  These  latter  are  two  in  number,  known  as  the 
tibia,  c,  or  shin-bone,  and  fibula,  d;  the  tibia  being  on  the 
great-toe  side.  In  front  of  the  knee-joint  is  the  knee-cap,  or 
patella,  b. 


Fig.  33.  Fig.  34. 

Fie.  88.— The  bones  of  the  arm.  a.  humerus;  b.  ulna;  c.  radius;  d,  the  carpus; 
e.  the  fifth  metacarpal;  /.  the  three  phalanges  of  the  fifth  digit  (little  finger);  g, 
the  phalanges  of  the  polfex  (thumb). 

Fig.  34.  Bones  of  the  leg.  a,  femur;  ft,  patella;  c,  tibia;  d,  fibula;  h,  calca- 
neum;  e,  remaining  tarsal  bones;  /,  metatarsal  bones;  g,  phalanges. 

At  the  distal  end  of  the  leg-bones  comes  the  foot,  consist- 
ing of  tarsus,  metatarsus,  and  phalanges.  The  tarsus,  which 
answers  to  the  carpus  of  the  fore  limb,  is  made  up  of  seven 
irregular  bones,  the  largest  being  the  heel-bone,  or  calcaneum,, 


80 


THE  1IC  MAN  1SODY. 


h.  The  metatarsus  consists  of  five  bones  lying  side  by  side, 
and  each  carries  a  fcoe  at  its  distal  cud.  In  the  great  toe  (or 
hallux)  there  arc  two  phalanges,  in  each  of  the  others  three, 
arranged  as  in  the  ringers,  but  smaller. 

Comparison  of  the  Anterior  and  Posterior  Limbs.     It  is 
clear  that  the  skeletons  of  the  arm  and  leg  correspond  pretty 


Trmj 


0         P 


Fig.  35.— The  skeleton  of  the  ami  and  leg.  H.  the  humerus;  Cd.  its  articular 
head  which  fits  into  the  glenoid  fossa  of  the  scapula;  U.  the  ulna:  R.  the  radius; 
O,  the  olecranon ;  Fe,  the  femur;   P,  the  patella;  Ft,  the  fibula;  T,  the  tibia. 

closely  to  one  another.  They  are  in  fact  quite  alike  in  very 
early  life,  and  their  differences  at  birth  depend  upon  their 
taking  different  ways  as  they  develop  from  their  primitive 


THE  SKELETON.  81 

simplicity;  as  both  may  be  regarded  as  modifications  of  the 
same  original  structure,  they  are  homologous.  The  pelvic 
girdle  clearly  corresponds  generally  to  the  pectoral  arch,  the 
tibia  and  fibula  to  the  radius  and  ulna;  the  five  metatarsal 
bones  to  the  five  metacarpal,  and  the  phalanges  of  the  toes  to 
those  of  the  thumb  and  fingers.  On  the  other  hand,  there  is 
in  the  arm  no  separate  bone  at  the  elbow-joint  corresponding 
to  the  patella  at  the  knee,  but  the  ulna  bears  above  a  bony 
process,  the  olecranon  (0,  Fig.  35),  which  at  first  is  a  separate 
bone  and  is  the  representative  of  the  patella.  There  are  in 
the  carpus  eight  bones  and  in  the  tarsus  but  seven.     The 


Fig.  36.— Diagram   showing    the  relation   of  the   pectoral  arch  to   the  axial 
skeleton. 

astragalus  of  the  tarsus  (Ta,  Fig.  38)  represents,  however,  two 
bones  which  have  grown  together.  The  elbow-joint  bends 
ventrally  and  the  knee-joint  dorsally. 

Comparing  the  limbs  as  a  whole,  greater  differences  come 
to  light,  differences  which  are 
mainly  correlated  with  the  dif- 
ferent uses  of  the  two  limbs. 
The  arms,  serving  as  prehensile  ^ 
organs,  have  all  their  parts  as 
movable  as  is  consistent  with 
the   requisite    strength,   while 

.,       ,  v      i       1         •  i  Fig.  37.— Diagram    showing  "the   at- 

the  lower  limbs,  having  to  bear    taehment  of  the  pelvic  arch  to  the  axial 

the  whole  weight  of  the  Body, 

require  to  have  their  parts  much  more  firmly  knit  together. 
Accordingly  we  find  the  shoulder-girdle,  represented  red  in 
the  diagram  (Fig.  30),  only  directly  attached  to  the  axial 
skeleton  by  the  union  of  the  inner  ends  of  the  clavicles  with 
the  sternum,  and  capable  of  considerable  independent  move- 
ment, as  seen,  for  instance,   in  "shrugging  the   shoulders." 


82 


Till-;  HUMAN  BODY. 


The  pelvic  arch,  on  the  contrary,  is  firmly  and  immovably 
fixed  to  the  sides  of  the  sacrum.     The  socket  of  the  scapula, 

into  which  the  head  of  the  humerus  fits,  is  very  shallow  and 
allows  a  far  greater  range  of  movement  than  is  permitted  by 
the  deeper  socket  on  the  pelvis,  into  which  the  head  of  the 
femur  fits.  Further,  if  we  hold  the  right  humerus  tightly 
in  the  left  hand  and  do  not  allow  it  to  move,  we  can  still 
move  the  forearm  bones  so  as  to  turn  the  palm  of  the  hand 
either  up  or  down:  no  such  movement  is  possible  between 
the  tibia  and  fibula.  Finally,  in  the  foot  the  hones  are  much 
less  movable  than  in  the  hand,  and  are  arranged  so  as  to  make 
a  springy  arch  (Fig.  38)  which  bears  behind  on  the  calcaneum, 
Ca,  and  in  front  on  the  distal  ends  of  the  metatarsal  bones, 
Os'}  over  the  crown  of  the  arch,  at  Ta,  is  the  surface  with 


Fig.  38.—  The  bones  of  the  foot.  Ca,  calcaneum,  or  on  calcis  :  Ta.  articular  sur- 
face tor  tibia  on  the  astragalus ;  N,  scaphoid  bone;  CI,  CII,  first  ami  second 
cuneiform  bones  ;  Cb,  cuboid  bone  ;  A/1,  metatarsal  bone  of  great  toe. 


which  the  leg-bones  articulate  and  on  which  the  weight  of 
the  Body  bears  in  standing. 

The  toes,  too,  are  far  less  movable  than  the  fingers,  and 
this  difference  is  especially  well  marked  between  the  great 
toe  and  the  thumb.  The  latter  can  be  made  to  meet  each  of 
the  finger-tips  and  so  the  hand  can  seize  and  manipulate  very 
small  objects,  while  this  power  of  opposing  the  first  digit  to 
the  rest  is  nearly  absent  in  the  foot  of  civilized  man.  In 
children,  however,  who  have  never  worn  boots,  and  in  savages, 
the  great  toe  is  far  more  movable,  though  it  never  forms  as 
complete  a  thumb  as  in  many  apes,  which  use  their  feet,  as 
well  as  their  hands,  for  prehension.  By  practice,  however, 
our  own  toes  can  be  made  much  more  mobile  than  they 
usually  are,  so  that  the  foot  can  to  a  certain  extent  replace 
the  hand;  as  has  been  illustrated  in  the  case  of  persons  born 


THE  SKELETON.  83 

■without  hands  Avho  liave  learned  to  write  and  paint  with 
their  toes. 

Peculiarities  of  the  Human  Skeleton.  These  are  largely 
connected  with  the  division  of  labor  between  the  fore  and 
hind  limbs  referred  to  above,  which  is  carried  farther  in  man 
than  in  any  other  creature.  Even  the  highest  apes  frequently 
use  their  fore  limbs  in  locomotion  and  their  hind  limbs  in 
prehension,  and  we  find  accordingly  that  anatomically  they 
present  less  differentiation  of  hand  and  foot.  The  other  more 
important  characteristics  of  the  human  skeleton  are  correlated 
for  the  most  part  with  the  maintenance  of  the  erect  posture, 
which  is  more  complete  and  habitual  in  man  than  in  the 
animals  most  closely  allied  to  him  anatomically.  These 
peculiarities,  however,  only  appear  fully  in  the  adult.  In  the 
infant  the  head  is  proportionately  larger,  which  gives  the 
centre  of  gravity  of  the  Body  a  comparatively  very  high  posi- 
tion and  renders  the  maintenance  of  the  erect  posture  difficult 
and  insecure.  The  curves  of  the  vertebral  column  are  nearly 
absent,  and  the  posterior  limbs  are  relatively  very  short.  In 
all  these  points  the  infant  approaches  more  closely  than  the 
adult  to  the  ape.  The  subsequent  great  relative  length  of  the 
posterior  limbs,  which  grow  disproportionately  fast  in  child- 
hood as  compared  with  the  anterior,  makes  progression  on 
them  more  rapid  by  giving  a  longer  stride  and  at  the  same 
time  makes  it  almost  impossible  to  go  on  "all  fours''  except 
by  crawling  on  the  hands  and  knees.  In  other  Primates  this 
disproportion  between  the  anterior  and  posterior  limbs  does 
not  occur  to  nearly  the  same  extent. 

In  man  the  skull  is  nearly  balanced  on  the  top  of  the 
vertebral  column,  the  occipital  condyles  which  articulate  with 
the  atlas  being  about  its  middle  (Fig.  30),  so  that  but  little 
effort  is  needed  to  keep  the  head  erect.  In  four-footed  beasts, 
on  the  contrary,  the  skull  is  carried  on  the  front  end  of  the 
horizontal  vertebral  column  and  needs  special  ligaments  to 
sustain  it.  For  instance,  in  the  ox  and  sheep  there  is  a  great 
elastic  cord  running  from  the  cervical  vertebra?  to  the  back 
of  the  skull  and  helping  to  hold  up  the  head.  Even  in  the 
highest  apes  the  skull  does  not  balance  on  the  top  of  the 
spinal  column;  the  face  part  is  much  heavier  than  the  back, 
while  in  man  the  face  parts  are  relatively  smaller  and  the  cra- 
nium larger,  so  that  the  two  nearly  equipoise.  To  keep  the 
head  erect  and  look  things  straight  in  the  face,  "like  a  man," 


8-i  THE  11 V MAX  BODY. 

is  for  the  apes  far  more  fatiguing,  and  so  they  cannot  long 
maintain  that  position. 

The  human  spinal  column,  gradually  widening  from  the 
neck  to  the  sacrum,  is  well  fitted  to  sustain  the  weight  of  the 
head,  upper  limbs,  etc.,  carried  by  it;  and  its  curvatures, 
which  are  peculiarly  human,  give  it  considerable  elasticity 
combined  with  strength.  The  pelvis,  to  the  sides  of  which 
the  lower  limbs  are  attached,  is  proportionately  very  broad  in 
man,  so  that  the  balance  can  be  more  readily  maintained 
during  lateral  bending  of  the  trunk.  The  arched  instep  and 
broad  sole  of  the  human  foot  are  also  very  characteristic. 
The  majority  of  four-footed  beasts,  as  horses,  walk  on  the 
tips  of  their  toes  and  fingers;  and  those  animals,  as  bears  and 
apes,  which  like  man  place  the  tarsus  also  on  the  ground,  or  in 
technical  language  are  plantigrade,  have  a  much  less  marked 
arch  there.  The  vaulted  human  tarsus,  composed  of  a  num- 
ber of  small  bones,  each  of  which  can  glide  a  little  over  its 
neighbors,  but  none  of  which  can  move  much,  is  admirably 
calculated  to  break  any  jar  which  might  be  transmitted  to 
the  spinal  column  by  the  contact  of  the  sole  with  the  ground 
at  each  step.  A  well-arched  instep  is  therefore  rightly  con- 
sidered a  beauty;;  it  makes  progression  easier,  and  by  its 
springiness  gives  elasticity  to  the  step.  In  London  flat-footed 
candidates  for  appointment  as  policemen  are  rejected,  as  they 
cannot  stand  the  fatigue  of  walking  the  daily  "  beat." 


CHAPTEE  VII. 
THE  STRUCTURE   AND   COMPOSITION   OF  BONE.     JOINTS. 

Gross  Structure  of  the  Bones.  The  bones  of  the  Body 
have  all  a  similar  structure  and  composition,  but  on  account 
of  differences  in  shape  they  are  divided  by  anatomists  into 
the  following  groups:  (1)  Long  bones,  more  or  less  cylindrical 
in  form,  like  the  bones  of  the  thigh  and  arm,  leg  and  forearm, 
metacarpus,  metatarsus,  fingers  and  toes.  (2)  Tubular  bones, 
in  the  form  of  expanded  plates,  like  the  bones  on  the  roof  and 
sides  of  the  skull,  and  the  shoulder-blades.  (3)  Short  bones, 
rounded  or  angular  in  form  a:id  not  much  greater  in  one 
diameter  than  in  another,  like  the  bones  of  the  tarsus  and 
carpus.  (4)  Irregular  bones,  including  all  which  do  not  fit 
well  into  any  of  the  preceding  groups,  and  commonly  lying 
in  the  middle  line  of  the  Body  and  divisible  into  similar 
halves,  as  the  vertebra?.  Living  bones  have  a  bluish-white 
color  and  possess  considerable  elasticity,  which  is  best  seen  in 
long  slender  bones  such  as  the  ribs. 

To  get  a  general  idea  of  the  structure  of  a  bone,  we  may 
select  the  humerus  for  examination.  Externally  in  the  fresh 
state  it  is  covered  by  a  dense  white  fibrous  membrane  very 
closely  adherent  to  it  and  containing  many  small  blood-vessels. 
This  membrane  is  the  periosteum  /  on  its  under  side  new 
osseous  tissue  is  formed  while  the  bone  is  still  growing,  and 
all  through  life  it  is  concerned  in  maintaining  the  nutrition 
of  the  bone,  Avhich  dies  if  it  be  stripped  off.  The  periosteum 
covers  the  whole  surface  of  the  bone  except  its  ends  in  the 
elbow  and  shoulder  joints;  the  surfaces  there  which  come  into 
contact  with  other  bones  and  glide  over  them  in  the  move- 
ments of  the  joint  have  no  periosteum,  but  are  covered  by 
a  thin  layer  of  gristle,  known  as  the  articular  cartilage.  Very 
early  in  the  development  of  the  Body  the  bone  in  fact  was 
represented  entirely  by  cartilage;  but  afterwards  nearly  all 
this  was  replaced  by  osseous  tissue,  leaving  only  a  thin  car- 
tilaginous layer  at  the  ends. 

85 


86 


THE  HUMAN  BODY. 


The  bone  itself.  Fig.  :>9, consists  of  a  central  nearly  cylin- 
drical portion  or  shaft,  extending  between  the  dotted  lines  x 
and  z  in  the  figure,  and  two  enlarged  articular  extremities. 


Tmj 


Jsm 


Fig.  39.— The  right  humerus,  seen 
from  the  front.  For  description,  see 
text. 


mk 


f*. 


\*(i 


ft  '*/»> 


Fig.  4d. — Tlic  humerus 
bisected  lengthwise,  a, 
marrow-cavity;  b,  hard 
bone;  c,  spongy  bone; 
d,  articular  cartilage. 


On  the  upper  articular  extremity  is  the  rounded  surface, 
Cp,  which   enters  into  the  shoulder-joint,  fittiug  against  the 


STRUCTURE  AND  COMPOSITION  OF  BONE.     JOINTS.     87 

glenoid  cavity  of  the  scapula;  and  on  the  lower  are  the  simi- 
lar surfaces,  Cpl  and  Tr,  which  articulate  with  the  radius  and 
ulna  respectively.  Besides  carrying  the  articular  surfaces, 
each  extremity  presents  several  prominences.  On  the  upper 
are  those  marked  Tmj  and  Tin  (the  greater  and  smaller  tro- 
chanters), which  give  attachment  to  muscles;  and  similar 
eminences,  the  external  and  internal  condyles,  El  and  Em, 
are  seen  on  the  lower  end.  Besides  these,  several  bony  ridges 
and  rough  patches  on  the  shaft  indicate  places  to  which  mus- 
cles of  the  arm  were  fixed. 

Internal  Structure.  If  the  bone  be  divided  longitudinally, 
it  will  be  seen  that  its  shaft  is  hollow,  the  space  being  known 
as  the  'medullar//  can't//,  and  in  the  fresh  bone  filled  with 
marrow.  Fig.  40  represents  such  a  longitudinal  section.  It 
will  be  seen  that  the  marrow-cavity  does  not  reach  into  the  ar- 
ticular extremities,  but  that  there  the  bone  has  a  loose  spongy 
texture,  except  a  thin  layer  on  the  surface.  In  the  shaft,  on 
the  other  hand,  the  outer  compact  layer  is  much  the  thicker, 
the  spongy  or  cancellated  bone  forming  only  a  thin  stratum 
immediately  around  the  medullary  cavity.  To  the  naked 
eye  the  cancellated  bone  appears  made  up  of  a  trellis-work  of 
thin  bony  plates  which  intersect  in  all  directions  and  sur- 
round cavities  rather  larger  than  the  head  of  an  ordinary 
pin;  the  compact  bone,  on  the  contrary,  appears  to  have  no 
cavities  in  it  until  it  is  examined  with  a  magnifying-glass. 
In  the  spaces  of  the  spongy  portion  lies,  during  life,  a  sub- 
stance known  as  the  red  marrow,  which  is  quite  different  from 
the  yellow  fatty  marrow  lying  in  the  central  cavity  of  the 
shaft. 

Microscopic  Structure  of  Bone.  The  microscope  shows 
that  the  compact  bone  contains  cavities  and  only  differs  from 
the  spongy  portion  in  the  fact  that  these  are  much  smaller, 
and  the  hard  true  bony  plates  surrounding  them  much  more 
numerous  in  proportion  than  in  the  spongy  parts.  If  a 
thin  transverse  section  of  the  shaft  of  the  humerus  be 
examined  (Fig.  41)  with  a  microscope  magnifying  twenty 
diameters,  it  will  be  seen  that  numerous  openings  exist  all 
over  the  compact  parts  of  the  section  and  gradually  become 
larger  as  this  passes  into  the  cancellated  part,  next  the  medul- 
lary cavity.  These  openings  are  the  cross-sections  of  tubes 
known  as  tin-  Haversian  canals,  which  ramify  all  through  the 
bone,  running  mainly  in  the  direction  of  its  long  axis,  but 


88 


Til K   IH  MAX   BODY. 


united  by  numerous  cross  or  oblique  branches  as  seen  in  the 
Longitudinal  section  (Fig.  4,i).  The  outermost  ones  open  on 
the  surface  of  the  bone  beneath  the  periosteum,  and  in  the 

living  hone  blood-vessels  run  from  this  through  the  Haversian 

canals  and  convey  materials  for  its  growth  and  nourishment. 


Fio.  i\.—A,  a  transverse  section  of  the  ulna,  natural  size;  showing:  the  medullary 
cavity.    B,  the  more  deeply  shaded  part  of  A  magnified  twenty  diameters. 


The  average  diameter  of  the  Haversian  canals  is  0.05  mm. 
iih  of  an  inch). 

Around  each  Haversian  canal  lies  a  set  of  plates,  or  lamella, 
of  hard  bony  substance  (see  the  transverse  section  Fig.  41), 
each  canal  with  its  lamellae  forming  an  Haversian  system  : 
and  the  whole  bone  is  made  up  of  a  number  of  such  systems, 
with  the  addition  of  a  few  lamellae  lying  in  the  corners  be- 
tween them,  and  a  certain  number  which  run  around  the 
whole  bone  on  its  outer  and   inner  surfaces.     In  the  spongy 


STRUCTURE  AND  COMPOSITION  OF  BONE.    JOINTS.     89 

parts  of  the  bone  the  Haversian  canals  are  very  large  and  the 
intervening  lamella?  few  in  number. 

Between  the  lamellae  lie  small  cavities,  the  lacunce,  each  of 
which  is  lenticular  in  form,  somewhat  like  the  space  which 
„.,«*,      .,,„,,  would  be  inclosed  by  two  watch- 

1  $*i*iU)  -J$??   vfy^m.   §^asses    joined     by    their    edges. 


/t-i;  jm&gp?  yjjmm  si,iooco  Jwmcu    ,jj   tiici1  cuse&- 

^/)WM  <&?■ -WWm.  Fl'0m  the  lacunai  many  extremely 
O/SrV^  rt^S^W  fine  bnuichin»  canals>  tlie  cawafo'- 
fti&W, ■',;/.'■  ,!^4^i>>*N^M  c"^'"'  rac^ate  an(i  penetrate  the 
i  fjfr.\£'if'    ffil-#^Vv  ^    bony   lamella?    in    all   directions. 

z.  Wt'r^i  \  ^'M':\%^%'^'    Tlje  innermost  canaliculi  of  each 

t'j  / ''^  Vh  "    system  open  into  the  central  Ha- 

Fig.42.-a  thin  k»nRitudinai  sec-  versian  canal;  and  those  of  various 

&^naarHfvSn^Il35°  lac"lia3  intercommunicating,  these 

fine  tubes  form  a  set  of  passages 
through  which  liquid  which  has  transuded  from  the  blood- 
vessels in  the  Haversian  canals  can  ooze  all  through  the 
bone.     The  lacunae  and  canaliculi  are  shown  in  Fig.  42. 

In  the  living  bone  a  granular  nucleated  cell  lies  in  each 
lacuna.  These  cells,  or  bone-corjmscles,  are  the  remnants  of 
those  which  built  up  the  bone,  the  hard  parts  of  the  latter 
being  really  an  intercellular  substance  or  skeleton  formed 
around  and  by  these  cells,  much  in  the  same  way  as  a  calca- 
reous skeleton  is  formed  around  a  Foraminifer  by  the  activity 
of  its  protoplasm.  By  the  co-operation  of  all  the  bone- 
corpuscles,  and  the  union  of  their  skeletons,  the  whole  bone 
is  built  up. 

In  other  bones  we  find  the  same  general  arrangement  of 
the  parts,  an  outer  dense  layer  and  an  inner  spongy  portion. 
In  the  flat  and  irregular  bones  there  is  no  medullary  cavity, 
and  the  whole  centre  is  filled  up  with  cancellated  tissue  with 
red  marrow  in  its  spaces.  For  example,  in  the  thin  bones 
roofing  in  the  skull  we  find  an  outer  and  an  inner  hard  layer  of 
compact  bone  known  as  the  outer  and  inner  table  respectively, 
the  inner  especially  being  very  dense.  Between  the  two  tables 
lies  the  spongy  bone,  red  in  color  to  the  naked  eye  from  the 
marrow  within  it,  and  called  the  diploe.  The  interior  of  the 
vertebra?  also  is  entirely  occupied  by  spongy  bone.  Every- 
where, except  where  a  bone  joins  some  other  part  of  the  skel- 
eton, it  is  covered  by  the  periosteum. 

Chemical  Composition  of  Bone.  Apart  from  the  bone- 
corpuscles  and  the  soft  contents  of  the  Eaversian  canals  and 


90  THE  HUMAN  BODY. 

of  the  spaces  of  the  cancellated  bone,  the  bony  substance 
proper,  as  found  in  the  lamellae,  is  comjiosed  of  earthy  and 
organic  portions  intimately  combined,  so  that  the  smallest 
distinguishable  portion  of  bone  contains  both.  The  earthy 
matters  form  about  two  thirds  of  the  total  weight  of  a  dried 
bone,  and  may  be  removed  by  soaking  the  bone  in  dilute 
hydrochloric  acid.  The  organic  portion  left  after  this  treat- 
ment constitutes  a  flexible  mass,  retaining  the  form  of  the 
original  bone;  it  consists  chiefly  of  an  albuminoid,  ossein, 
which  by  long  boiling,  especially  under  pressure  at  a  higher 
temperature  than  that  at  which  water  boils  when  exposed 
freely  to  the  air,  is  converted  into  gelatin,  which  dissolves 
in  the  hot  water.  .Much  of  the  gelatin  of  commerce  is  pre- 
pared in  this  manner  by  boiling  the  bones  of  slaughtered 
animals,  and  even  well-picked  bones  may  be  used  to  form  a 
good  thick  soup  if  boiled  under  pressure  in  a  Papin's  digester; 
much  nutritious  matter  being,  in  the  common  modes  of  do- 
mestic cooking,  thrown  away  in  the  bones. 

The  earthy  salts  of  bone  may  be  obtained  free  from  organic 
matter  by  calcining  a  bone  in  a  clear  fire,  which  burns  away 
the  organic  matter.  The  residue  forms  a  white  very  brittle 
mass,  retaining  perfectly  the  shape  and  structural  details  of 
the  original  bone.  It  consists  mainly  of  normal  calcium 
phosphate,  or  bone-earth  (Ca3, 2P04);  but  there  is  also  pres- 
ent a  considerable  proportion  of  calcium  carbonate  (CaCO,) 
and  smaller  quantities  of  other  salts. 

Hygiene  of  the  Bony  Skeleton.  In  early  life  the  bones 
are  less  rigid,  from  the  fact  that  the  earthy  matters  then  pres- 
ent in  them  bear  a  less  proportion  to  the  softer  organic  parts. 
Hence  the  bones  of  an  aged  person  are  more  brittle  and  easily 
broken  than  those  of  a  child.  The  bones  of  a  young  child 
are  in  fact  tolerably  flexible  and  may  be  distorted  by  any  con- 
tinued strain;  therefore  children  should  never  be  kept  sitting 
for  hours,  in  school  or  elsewhere,  on  a  bench  which  is  so  high 
that  the  feet  are  not  supported.  If  this  be  insisted  upon  (for 
no  child  will  continue  it  voluntarily)  the  thigh-bones  will  al- 
most certainly  be  bent  over  the  edge  of  the  seat  by  the  weight 
of  the  legs  and  feet,  and  a  permanent  distortion  may  be  pro- 
duced. For  the  same  reason  it  is  important  that  a  child 
be  made  to  sit  straight  while  writing,  to  avoid  the  risk  of 
producing  a  lateral  curvature  of  the  spinal  column.  The 
facility  with  which  the  bones  may  be  moulded  by  prolonged 


STRUCTURE  AND  COMPOSITION  OF  BONE.     JOINTS.     91 

pressure  in  early  life  is  well  seen  in  the  distortion  of  the 
feet  of  Chinese  ladies,  produced  by  keeping  them  in  tight 
shoes;  and  in  the  extraordinary  forms  which  some  races  of 
man  produce  in  their  skulls,  by  tying  boards  on  the  heads  of 
the  children. 

Throughout  the  whole  of  life,  moreover,  the  bones  remain 
among  the  most  easily  modified  parts  of  the  Body  ;  although 
judging  from  the  fact  that  dead  bones  are  the  most  permanent 
parts  of  fossil  animals  we  might  be  inclined  to  think  other- 
wise. The  living  bone,  however,  is  constantly  undergoing 
changes  under  the  influence  of  the  protoplasmic  cells  im- 
bedded in  it,  and  in  the  living  Body  is  constantly  being  ab- 
sorbed and  reconstructed.  The  experience  of  physicians 
shows  that  any  continued  pressure,  such  as  that  of  a  tumor, 
will  cause  the  absorption  and  disaj)pearance  of  bone  almost 
quicker  than  that  of  any  other  tissue;  and  the  same  is 
true  of  any  other  continued  pressure.  Moreover,  during  life 
the  bones  are  eminently  plastic;  under  abnormal  pressures 
they  are  found  to  quickly  assume  abnormal  shapes,  being 
absorbed  and  disappearing  at  points  where  the  pressure 
is  most  powerful,  and  increasing  at  other  points;  tight 
lacing  may  in  this  way  produce  a  permanent  distortion  of 
the  ribs. 

When  a  bone  is  fractured  a  surgeon  should  be  called  in 
as  soon  as  possible,  for  once  inflammation  has  set  in  and 
the  parts  have  become  swollen  it  is  much  more  difficult  to 
place  the  broken  ends  of  the  bone  together  in  their  proper 
position  than  before  this  has  occurred.  Once  the  bones  are 
replaced  they  must  be  held  in  position  by  splints  or  bandages, 
or  the  muscles  attached  to  them  will  soon  displace  them 
again.  With  rest,  in  young  and  healthy  persons  complete 
union  will  commonly  occur  in  three  or  four  weeks;  but  in 
old  persons  the  process  of  healing  is  slower  and  is  apt  to  be 
imperfect. 

Articulations.  The  bones  of  the  skeleton  are  joined 
together  in  very  various  ways;  sometimes  so  as  to  admit 
of  no  movement  at  all  between  them;  in  other  cases  so  as 
to  permit  only  a  limited  range  or  variety  of  movement;  and 
elsewhere  bo  as  to  allow  of  very  free  movement  in  many 
directions.  All  kinds  of  unions  between  bones  are  called  ar- 
ticulations. 

Of  articulations   permitting  no   movements,  those  which 


92  THE  HUMAN  BODY. 

unite  the  majority  of  the  cranial  bones  afford  a  good  example. 
Except  the  lower  jaw,  and  certain  tiny  bones  inside  the  tem- 
poral hone  belonging  to  the  organ  of  hearing,  all  the  skull- 
bones  are  immovably  joined  together.  This  union  in  mosl 
cases  occurs  by  means  of  toothed  edges  which  lit.  into  one 
another  and  form  jagged  lines  of  union  known  as  sutures. 
Some  of  these  can  he  well  seen  in  Fig.  29  between  the 
frontal  and  parietal  bones  {coronal  suture)  and  between  the 
parietal  and  occipital  bones  {lambdoidal  suture);  while  an- 
other lies  along  the  middle  line  in  the  top  of  the  crown 
between  the  two  parietal  bones,  and  is  known  as  the  sagittal 
snl ure.  In  new-born  'children  where  the  sagittal  meets  the 
coronal  and  lambdoidal  sutures  there  are  large  spaces  not  yet 
covered  in  by  the  neighboring  bones,  which  subsequently 
extend  over  them.  These  openings  are  known  a&fontanelles. 
At  them  a  pulsation  can  often  be  felt  synchronous  with  each 
beat  of  the  heart,  which,  driving  more  blood  into  the  brain, 
distends  it  and  causes  it  to  push  out  the  skin  where  bone  is 
absent.  Another  good  example  of  an  articulation  admitting 
of  no  movement  is  that  between  the  rough  surfaces  on  the 
sides  of  the  sacrum  and  the  innominate  bones. 

AVe  find  good  examples  of  the  second  class  of  articulations 
•  —those  admitting  of  a  slight  amount  of  movement — in  the 
vertebral  column.  Between  every  pair  of  vertebrae  from  the 
second  cervical  to  the  sacrum  is  an  elastic  pad,  the  interver- 
tebral disk,  which  adheres  by  its  surfaces  to  the  bodies  of  the 
vertebras  between  which  it  lies,  and  only  permits  so  much 
movement  between  them  as  can  be  brought  about  by  its  own 
compression  or  stretching.  When  the  back-bone  is  curved  to 
the  right,  for  instance,  each  of  the  intervertebral  disks  is 
compressed  on  its  right  side  and  stretched  a  little  on  its  left, 
and  this  combination  of  movements,  each  individually  but 
slight,  gives  considerable  flexibility  to  the  spinal  column  as  a 
whole. 

Joints.  Articulations  permitting  of  movement  by  the  glid- 
ing of  one  bone  over  another  are  known  as  joints,  and  all 
have  the  same  fundamental  structure,  although  the  amount 
of  movement  permitted  in  different  joints  is  very  different. 

Hip-joint.  We  may  take  this  as  a  good  example  of  a  true 
joint  permitting  a  great  amount  and  variety  of  movement. 
On  the  os  innominatum  is  the  cavity  of  the  acetabulum  (Fig. 
4:>),  which  is  lined  inside  by  a  thin  layer  of  articular  carti- 


STRUCTURE  AND  COMPOSITION  OF  BONE.     JOINTS.      93 

lage  which  has  an  extremely  smooth  surface.  The  bony  cup 
is  also  deepened  a  little  by  a  cartilaginous  rim.  The  proximal 
end  of  the  femur  consists  of  a  nearly  spherical  smooth  head, 
borne  on  a  somewhat  narrower  neck,  and  fitting  into  the  ace- 
tabulum. This  head  also  is  covered  with  articular  cartilage; 
and  it  rolls  in  the  acetabulum  like  a  ball  in  a  socket.  To 
keep  the  bones  together  and  limit  the  amount  of  movement, 
ligaments  pass  from  one  to  the  other.  These  are  composed 
of  white  fibrous  connective  tissue  (Chap.  VIII)  and  are  ex- 
tremely pliable,  but  quite  inextensible  and  very  strong  and 


Fig.  43. — Section  through  the  hip-joint. 


tough.  One  is  the  capsular  ligament,  which  forms  a  sort  of 
loose  bag  all  round  the  joint,  and  another  is  the  round  liga- 
ment, which  passes  f»*om  the  acetabulum  to  the  head  of  the 
femur.  Should  the  latter  rotate  above  a  certain  extent  in 
its  socket,  the  round  ligament  and  one  side  of  the  capsular 
ligament  are  put  on  the  stretch,  and  any  further  movement 
which  might  dislocate  the  femur  (that  is,  remove  the  head 
from  it.s  socket)  is  checked.  Covering  the  inside  of  the  cap- 
Bular  ligamenl  and  the  outside  of  the  round  ligament  is  a 
layer  of  flat  cells,  which  are  continued  in  a  modified  form 
over  the  articular  cartilages  and  form  the  synovial  meni.hra.ne.. 
This,  which   thus  forms  the   lining  of  the  joint,  is  always 


94  THE  II r MAN  BODY. 

moistened  in  health  by  a  small  quantity  of  glairy  synovial 
fluid,  something  like  the  white  of  ;i  raw  egg  in  consistency, 
and  playing  the  part  of  the  oil  with  which  the  contiguous 

moving  surfaces  of  a  machine  ;ire  moistened;  it  makes  all 
run  smoothly  with  very  little  friction. 

In  the  natural  state  <>t'  t  he  parts,  the  head  of  the  femur  and 
the  bottom  and  sides  of  the  acetabulum  lie  in  close  contact, 
the  two  synovial  membranes  rubbing  together.  This  contact 
is  Tiot  maintained  by  the  ligaments,  which  are  too  loose  and 
serve  only  to  check  excessive  movement,  but  by  the  numerous 
stout  muscles  which  pass  from  the  thigh  to  the  trunk  and 
bind  the  two  firmly  together.  Moreover,  the  atmospheric 
pressure  exerted  on  the  surface  of  the  Body  and  transmitted 
through  the  soft  parts  to  the  outside  of  the  air-tight  joint 
helps  also  to  keep  the  parts  in  contact.  If  all  the  muscles 
and  ligaments  around  the  joint  be  cut  away,  it  is  still  found  in 
the  dead  Body  that  the  head  of  the  femur  will  be  kept  in  its 
socket  by  this  pressure,  and  so  firmly  as  to  bear  the  weight  of 
the  whole  limb  without  dislocation,  just  as  the  pressure  of 
the  air  will  enable  a  boy's  "  sucker  "  to  lift  a  tolerably  heavy 
stone. 

Ball-and-socket  Joints.  Such  a  joint  as  that  at  the  hip  is 
called  a  ball-and-socket  joint  and  allows  of  more  free  move- 
ment than  any  other.  Through  movements  occurring  in  it 
the  thigh  can  be  flexed,  or  bent  so  that  the  knee  approaches 
the  chest;  or  extended,  that  is,  moved  in  the  opposite  direc- 
tion. It  can  be  abducted,  so  that  the  knee  moves  outwards; 
and  adducted,  or  moved  back  towards  the  other  knee  again. 
The  limb  can  also  by  movements  at  the  hip-joint  be  circum- 
ducted, that  is,  made  to  describe  a  cone  of  which  the  base  is 
at  the  foot  and  the  apex  at  the  hip.  Finally,  rotation  can 
occur  in  the  joint,  so  that  with  knee  and  foot  joints  held 
rigid  the  toes  can  be  turned  in  or  out,  to  a  certain  extent,  by 
a  rolling  around  of  the  femur  in  its  socket. 

At  the  junction  of  the  humerus  with  the  scapula  is  another 
ball-and-socket  joint  permitting  all  the  above  movements  to 
even  a  greater  extent.  This  greater  range  of  motion  at  the 
shoulder-joint  depends  mainly  on  the  shallowness  of  the 
glenoid  cavity  as  compared  with  the  acetabulum,  and  upon 
the  absence  of  any  ligament  answering  to  the  round  ligament 
of  the  hip-joint.  Another  ball-and-socket  joint  exists  between 
the  carpus  and  the  metacarpal  bone  of  the  thumb;  and  others 


STRUCTURE  AND  COMPOSITION  OF  BONE.     JOINTS.     95 

with  the  same  variety,  but  a  much  less  range,  of  movement 
between  each  of  the  remaining  metacarpal  bones  and  the 
proximal  phalanx  of  the  finger  which  articulates  with  it. 

Hinge-joints.  Another  form  of  synovial  joint  is  known  as 
a  hinge-joint.  In  it  the  articulating  bony  surfaces  are  of 
such  shape  as  to  permit  of  movement,  to  and  fro,  in  one  plane 
ouly,  like  a  door  on  its  hinges.  The  joints  between  the  pha- 
langes of  the  fingers  are  good  examples  of  hinge-joints.  If 
no  movement  be  allowed  where  the  finger  joins  the  palm  of 
the  hand  it  will  be  found  that  each  can  be  bent  and  straight- 
ened at  its  own  two  joints,  but  not  moved  in  any  other  way. 
The  knee  is  also  a  hinge-joint,  as  is  the  articulation  between 
the  lower  jaw  and  the  base  of  the  skull  which  allows  us  to 
open  and  close  our  mouths.  The  latter  is,  however,  not  a 
perfect  hinge-joint,  since  it  permits  of  a  small  amount  of 
lateral  movement  such  as  occurs  in  chewing,  and  also  of  a 
gliding  movement  by  which  the  lower  jaw  can  be  thrust  for- 
ward so  as  to  protrude  the  chin  and  bring  the  lower  row  of 
teeth  outside  the  upper. 

Pivot-joints.  In  this  form  one  bone  rotates  around 
another  which  remains  stationary.  We  have  a  good  example 
of  it  between  the  first  and  second  cervical  vertebras.  The 
first  cervical  vertebra  or  atlas  (Fig.  22)  has  a  very  small 
body  and  a  very  large  arch,  and  its  neural  canal  is  subdivided 
by  a  transverse  ligament  (L,  Fig.  22)  into  a  dorsal  and  a  ven- 
tral portion ;  in  the  former  the  spinal  cord  lies.  The  second 
vertebra  or  axis  (Fig.  23)  has  arising  from  its  body  the  stout 
bony  peg,  D,  called  the  odontoid  process.  This  projects  into 
the  ventral  portion  of  the  space  surrounded  by  the  atlas,  and, 
kept  in  place  there  by  the  transverse  ligament,  forms  a  pivot 
around  which  the  atlas,  carrying  the  skull  with  it,  rotates 
when  we  turn  the  head  from  side  to  side.  The  joints  on  each 
Bide  between  the  atlas  and  the  skull  are  hinge-joints  and  per- 
mit only  the  movements  of  nodding  and  raising  the  head. 
When  the  head  is  leaned  over  to  one  side,  the  cervical  part  of 
tin-  spinal  column  is  bent. 

Another  kind  of  pivot-joint  is  seen  in  the  forearm.  If 
the  limb  he  held  straight  out,  with  the  palm  up  and  the  elbow 
resting  on  the  table,  so  that,  the  shoulder-joint  be  kept  steady 
while  the  hand  is  rotated  until  its  back  is  turned  upwards,  it- 
will  be  found  that  the  radius  has  partly  rolled  round  the  ulna. 
When   the  palm  is   upwards  and   the   thumb   outwards,  the 


96 


THE  HUMAN  BODY. 


fr- 


it'--I 


\....u 


u 


n 


[■■V 


lower  end  of  the  radius  can  be  felt  on  the  outer  side  of  the 
forearm  just  above  t he  wrist,  and  if  t his  be  done  while  the  hand 
is  turning  over,  it  will  be  easily  discerned  that  during  the 
movement  this  end  of  the  radius,  carrying  the  hand  with  it, 
travels  around  the  lower  end  of  the  ulna  so  as  to  get  to  its 
inner  side.  The  relative  position  of  the  bones  when  the  palm 
is  upwards  is  shown  at  A  in  Fig.  44,  and  when  the  palm  is 

down  at  B.  The  former  position 
is  known  as  supination  ;  the  latter 
as  pronation.  The  elbow  end  of 
the  humerus  (Fig.  39)  bears  a 
large  articular  surface:  on  the 
inner  two  thirds  of  this,  Tr,  the 
ulna  fits,  and  the  ridges  and 
grooves  of  both  bones  interlock- 
ing form  a  hinge-joint,  allowing 
only  of  bending  or  straightening 
the  forearm  on  the  arm.  The 
radius  fits  on  the  rounded  outer 
third,  C])l,  and  forms  there  a  ball- 
and-socket  joint  at  which  the 
movement  takes  place  when  the 
hand  is  turned  from  the  supine 
to  the  prone  position ;  the  ulna 
forming  a  fixed  bar  around  which 
the  lower  end  of  the  radius  is 
moved. 

Gliding  Joints.  These  per- 
mit as  a  rule  but  little  movement: 
examples  are  found  between  the  closely  packed  bones  of  the 
tarsus  (Fig.  38)  and  carpus,  which  slide  a  little  over  one 
another  when  subjected  to  pressure. 

Hygiene  of  the  Joints.  When  a  bone  is  displaced  or 
dislocated  the  ligaments  around  the  joint  are  more  or  less 
torn  and  other  soft  parts  injured.  This  soon  leads  to  inflam- 
mation and  swelling  which  make  not  only  the  recognition  of 
the  injury  but,  after  diagnosis,  the  replacement  of  the  bone, 
or  the  reduction  of  the  dislocation,  difficult.  Moreover  the 
muscles  attached  to  it  constantly  pull  on  the  displaced  bone 
and  drag  it  still  farther  out  of  place;  so  that  it  is  of  great 
importance  that  a  dislocation  be  reduced  as  soon  as  possible. 
In  most  cases  this  can  only  be  attempted  with  safety  by  one 


Fig.  44.— A,  arm  in  supination;  B, 
arm  in  pronation.  H,  humerus;  R, 
radius;  U,  ulna. 


STRUCTURE  AND  COMPOSITION  OF  BONE.    JOINTS.      97 

who  knows  the  form  of  the  bones,  and  possesses  sufficient  ana- 
tomical knowledge  to  recognize  the  direction  of  the  displace- 
ment. No  injury  to  a  joint  should  be  neglected.  Inflamma- 
tion once  started  there  is  often  difficult  to  check  and  runs  on, 
in  a  chronic  way,  until  the  synovial  surfaces  are  destroyed, 
and  the  two  bones  perhaps  grow  together,  rendering  the  joint 
permanently  stiff.  A  sprained  joint  should  get  immediate 
and  complete  rest,  for  weeks  if  necessary,  and  if  there  be 
much  swelling,  or  continued  pain,  medical  advice  should  be 
obtained.  An  improperly  cared-for  sprain  is  the  cause  of  many 
a  useless  ankle  or  knee. 


CHAPTER    VIII. 

CARTILAGE   AND   CONNECTIVE  TISSUE. 

Temporary  and  Permanent  Cartilages.  In  earl}-  life  a 
great  many  parts  of  the  supporting  framework  of  the  Body, 
which  afterwards  become  bone,  consist  of  cartilage.     Such  for 

example  is  the  case  with  all  the  vertebrae,  and  with  the  hones 
of  the  limbs.  In  these  cartilages  subsequently  the  process 
known  as  ossification  takes  place,  by  which  a  great  portion  of 

the  original  cartilaginous  model  is  removed  and  replaced  by 
true  osseous  tissue.  Often,  however,  some  of  the  primitive 
cartilage  is  left  throughout  the  whole  of  life  at  the  ends  of 
the  bones  in  joints  where  it  forms  the  articular  cartilages; 
and  in  various  other  places  still  larger  masses  remain,  such  as 
the  costal  cartilages,  those  in  the  external  ears  forming  their 
framework,  others  finishing  the  skeleton  of  the  nose  which  is 
only  incompletely  bony,  and  many  in  internal  parts  of  the 
Body,  as  the  cartilage  of  "  Adam's  apple,"  which  can  be  felt 
in  the  front  of  the  neck,  and  a  number  of  rings  around  the 
windpipe  serving  to  keep  it  open.  These  persistent  masses 
are  known  as  the  permanent,  the  others  as  the  temporary 
cartilages.  In  old  age  many  so-called  permanent  cartilages 
become  calcified — that  is.  hardened  and  made  unyielding  by 
deposits  of  lime-salts  in  them — without  assuming  the  histo- 
logical character  of  bone,  and  this  calcification  of  the  perma- 
nent cartilages  is  one  chief  cause  of  the  want  of  pliability  and 
suppleness  of  the  frame  in  advanced  life. 

Hyaline  Cartilage.  In  its  purest  form  cartilage  is  flexi- 
ble and  elastic,  of  a  pale  bluish-white  color  when  alive  and 
seen  in  large  masses,  and  cuts  readily  with  a  knife.  In  thin 
pieces  it  is  quite  transparent.  Everywhere  except  in  the 
joints  it  is  invested  by  a  tough  adherent  membrane,  the  peri- 
chondrium, which  resembles  in  structure  and  function  the 
periosteum  of  the  bones.  When  boiled  for  a  long  time  in 
water,  such  cartilages  yield  a  solution  of  chondrin,  which 
differs  from  gelatin  in  minor  points,  but  agrees  with  it  in  the 
fact  that  its  solution  in  hot  water  "  sets  "  or  gelatinizes  on  cool 

98 


CARTILAGE  ASD    CONNECTIVE  TISSUE. 


99 


ing.  When  a  thin  slice  of  hyaline  cartilage  is  examined  with 
a  microscope  it  is  found  (Fig.  -15)  to  consist  of  granular  nucle- 
ated cells,  often  collected  into  groups  of  two,  four,  or  more, 
scattered  through  a  homogeneous  or  faintly  granular  ground- 
substance  or  matrix.  Essentially,  cartilage  resembles  bone. 
being  made  up  of  protoplasmic  cells  and  a  proportionately 
large  amount  of  non-protoplasmic  intercellular  substance,  the 


Fig.  45.— A  thin  slice  of  cartilage,  magnified,  to  show  the  cells  imbedded  in  the 
homogeneous  matrix,  n.  a  cell  in  which  the  nucleus  has  divided:  b,  a  cell  in  which 
division  is  just  complete:  c.  e,  a  group  of  four  cells  resulting  from  further  division 
of  a  pair  like  i>;  th>-  new  cells  have  formed  some  matrix  between  them,  separating 
them  from  another:  d.  <l.  cavities  in  the  matrix  from  which  cells  have  dropped  out 
during  the  preparation  of  the  specimen. 

cells  being  the  more  actively  living  part  and  the  matrix  their 
product.  Examples  of  this  hyaline  variety  (so  called  from 
its  glassy  transparent  appearance)  are  found  in  all  the  tempo- 
rary cartilages,  and  in  the  costal  and  articular  among  the 
permanent. 

Cartilages  rarely  contain  blood-vessels  except  at  points 
where  a  temporary  cartilage  is  being  removed  and  replaced 
by  bone;  then  blood-vessels  run  in  from  the  perichondrium 
and  form  loops  in  the  matrix,  around  which  it  is  absorbed 
and   bony  tissue   deposited.     In    consequence   of  the   usual 

•ice  of  blood-vessels  the  nutritive  processes  and  exchanges 
of  material  must  be  small  and  slow  in  cartilage,  as  might  in- 
deed be  expected  from  the  passive  and  merely  mechanical 
role  which  this  tissue  play-. 

Hyaline  cartilage  is  the  type,  or  most  characteristically 
developed  form,  of  a  tissue  found  with  modifications  else- 
where in  the  Body.  One  of  its  other  modifications  is  the  so- 
called  cellular  cartilage,  which  consists  of  the  cells  with 
hardly  any  matrix,  only  just  enough  to  form  a  thin  capsule 
around   each.     This   form    is    that   with   which   all    the  earn- 


100  THE  HUMAN  BODY 

lages  commence,  the  hyaline  variety  being  built  up  by  the  in- 
crease of  the  cell-capsules  and  their  fusion  to  form  the  ma- 
trix. It  persists  throughout  life  in  the  thin  cartilaginous 
plate  of  a  mouse's  external  car.  Other  varieties  of  cartilage 
are  really  mixtures  of  true  cartilage  and  connective  tissues, 
and  will  be  considered  after  the  latter. 

The  Connective  Tissues.  These  complete  the  skeleton, 
marked  out  in  its  coarser  features  by  the  bones  and  cartilages, 
and  constitute  the  final  group  of  the  supporting  tissues. 
They  occur  in  all  forms,  from  broad  membranes  and  stout 
cords  to  the  finest  threads  forming  networks  around  the  other 
ultimate  histological  elements  of  various  organs.  In  addition 
to  subsidiary  forms,  three  main  varieties  of  this  tissue  are 
readily  distinguishable,  viz.,  areolar,  white  fibrous,  and  yellow 
clastic.  Each  consists  of  fibres  and  cells,  the  fibres  being  of 
two  kinds,  mixed  in  nearly  equal  proportions  in  the  areolar 
variety,  while  one  kind  predominates  in  one  and  another  in 
the  second  of  the  remaining  chief  forms. 

Areolar  Connective  Tissue.  This  exists  abundantly  be- 
neath the  skin,  where  it  forms  a  loose  layer  which  permits 
the  skin  to  be  moved,  more  or  less,  to  and  fro  over  the  sub- 
jacent parts.  Areolar  tissue  consists  of  innumerable  bands 
and  cords  interlacing  in  all  directions,  and  can  be  greatly  dis- 
tended by  blowing  air  in  at  any  point,  from  whence  it  travels 
widely  through  the  intercommunicating  meshes:  if  dried 
while  distended  it  is  somewhat  like  raw  cotton  in  appearance 
but  not  so  white.  In  dropsy  of  the  legs  or  feet  the  cavities 
of  this  tissue  are  distended  with  lymph,  which  in  health  is 
present  only  in  sufficient  quantity  to  moisten  them.  From 
beneath  the  skin  the  areolar  tissue  extends  all  through  the 
Body  between  the  muscles  and  around  the  blood-vessels  and 
nerves;  and  still  finer  layers  of  it  enter  into  these  and  other 
organs  and  unite  their  various  parts  together.  It  constitutes 
in  fact  a  soft  packing  material  which  fills  up  the  holes  and 
corners  of  the  Body,  as  for  instance  around  the  blood-vessels 
and  between  the  muscles  in  Fig.  4. 

Microscopic  Structure  of  Areolar  Tissue.  When  exam- 
ined with  the  microscope  areolar  tissue  is  seen  to  consist  of 
nucleated  cells  imbedded  in  a  ground-substance  which  is  per- 
meated by  fibres.  The  fibres  everywhere  form  the  predomi- 
nant feature  of  the  tissue  (the  homogeneous  matrix  and  the 
cells  being  inconspicuous)  and  are  of  two  very  different  kinds. 


CARTILAGE  AND   CONNECTIVE  TISSUE. 


101 


In  a  strict  sense  indeed  the  areolar  tissue  ought  to  be  consid- 
ered as  a  mixture  of  two  tissues,  one  corresponding  to  each 
variety  of  fibres  in  it.  It  is  characterized  by  its  loose  texture 
and  by  the  fact  that  the  two  forms  of  fibres  are  present  in 
about  equal  quantities.  In  many  places  a  tissue  containing 
the  .same  histological  elements  as  the  areolar  tissue  is  found 
in  the  form  of  dense  membranes,  as  for  example  periosteum 
and  perichondrium. 

White  Fibrous  Tissue.  One  of  the  varieties  of  fibres  per- 
vading the  matrix  of  areolar  tissue  exists  almost  unmixed 
with  the  other  kind  in  the  cords  or  tendons  which  unite  mus- 


Fig.  46.  Fig.  46a. 

Fig.  46.— White  fibrous  connective  tissue,  highly  magnified.  The  nucleated  cor- 
puscles, seen  edgewise  and  appearing  spindle-shaped,  are  seen  here  and  there  on 
the  surface  of  the  bundles  of  fibres. 

Fig.  46a.— Yellow  elastic  tissue,  magnified  after  its  fibres  have  been  torn  apart. 

cles  to  the  bones.  This  form,  known  as  the  white  fibrous  con- 
nective tissue,  is  also  found  fairly  pure  in  the  ligaments  around 
most  joints.  Physically  it  is  very  flexible  but  extremely 
tough  and  inextensible,  so  that  it  will  readily  bend  in  any 
direction  but  is  very  hard  to  break;  when  fresh  it  has  an 
opaque  white  color. 

White  fibrous  tissue  (Fig.  46)  consists  of  a  matrix,  contain- 
ing cavities  in  which  cells  lie,  and  pervaded  by  bundles  of 
extremely  fine  fibres.     These  fibres  run  in  each  bundle  toler- 


1<>2  THE  III  MAN  BODY. 

ably  parallel  to  one  another  in  a  wavy  course  (Fig.  46)  and 
never  branch  or  unite.  Their  diameter  varies  from  0.0005  to 
0.001  millimeter  (-„,' to  .,  -,',,,,,  of  an  inch). 

Chemically  this  tissue  is  characterized  by  the  fact  that  its 
fibres  swell  up  and  become  indistinguishable  when  treated 
with  dilute  acetic  acid,  and  by  the  fact  that  it  yields  gelatin 
when  boiled  in  water.  The  substance  in  it,  called  ossein  in 
bones,  which  is  turned  into  gelatin  by  such  treatment,  is  here 
known  as  collagen.  Glue  is  impure  gelatin  obtained  from 
tendons  and  ligaments,  and  calf's-foot  jelly,  so  often  recom- 
mended to  invalids,  is  a  purer  form  of  the  same  substance 
obtained  by  boiling  the  feet  of  calves,  which  contain  the  ten- 
dons of  many  muscles  passing  from  the  leg  to  the  foot. 

Elastic  Tissue.  This  is  almost  invariably  mixed  in  some 
proportion  in  all  specimens  of  white  fibrous  tissue,  even  the 
purest,  such  as  the  tendons  of  muscles;  but  in  certain  places 
it  exists  almost  alone,  as  for  example  in  the  ligaments  (liffa- 
menta  subfiavd)  between  the  arches  of  the  vertebra?,  and  in 
the  coats  of  the  larger  arteries,  In  quadrupeds  it  forms  the 
great  ligament  already  referred  to  (p.  83),  which  helps  to  sus- 
tain the  head.  This  tissue,  in  mass,  is  of  a  dull  yellow  color 
and  extremely  extensible  and  elastic;  when  purest  nearly  as 
much  so  as  a  piece  of  india-rubber.  Sometimes  it  appears 
under  the  microscope  to  be  made  up  of  delicate  membranes, 
but  more  often  it  is  in  the  form  of  fibres  (Fig.  4G«)  which  are 
coarser  than  those  of  white  fibrous  tissue  and  frequently 
branch  and  unite.  It  is  unaffected  by  acetic  acid  and  does 
not  yield  gelatin  when  boiled  in  water. 

Connective-tissue  Corpuscles.  The  fibres  of  white  fi- 
brous tissue,  wherever  it  is  found,  are  united  into  bundles  by 
a  structureless  ground-material  known  as  the  cement-sub- 
stance, which  also  invests  each  bundle,  or  skein  as  we  may 
call  it,  with  a  delicate  coating.  In  this  ground-substance  are 
numerous  cavities,  branched  and  flattened  in  one  diameter, 
and  often  intercommunicating  by  their  branches.  In  these 
cavities  lie  nucleated  masses  of  protoplasm  (Fig.  47),  fre- 
quently also  branched,  known  as  the  connective-tissue  cor- 
puscles. These  it  is  which  build  up  the  tissue,  each 
cell  in  the  course  of  development  forming  around  it  a 
quantity  of  intercellular  substance,  which  subsequently  be- 
comes fibrillated  in  great  part,  the  remainder  forming  the 
cement.     The  cells  do  not  quite  fill  the  cavities  in  which  they 


CARTILAGE  AND   CONNECTIVE  TISSUE. 


103 


lie,  and  these  opening  into  others  by  their  offsets  there  is 
formed  a  set  of  minute  tubes  ramifying  through,  the  con- 
nective tissues;  and  (since  these  in  turn  permeate  nearly  all 
the  Body)  pervading  all  the  organs.  In  these  cell-cavities 
and  their  branches  the  lymph  flows  before  it  enters  definite 
lymphatic  vessels,  and  they  are  accordingly  known  as  lymph 


Fig.  47. — Connective-tissue  corpuscles  :   a,  from  areolar  tissue  ;   b,  from  tendon  ; 
C,  wandering  cells. 

canaliculi.  In  addition  to  the  fixed  branched  connective- 
tissue  corpuscles  there  are  often  found  other  cells,  when  living 
connective  tissue  is  examined.  These  cells  much  resemble 
white  blood-corpuscles,  and  probably  are  such  which  have 
bored  through  the  walls  of  the  finer  blood-vessels.  They 
creep  about  along  the  canaliculi  by  means  of  their  faculty 
of  amoeboid  movement,  and  are  known  as  the  "wandering 
cells." 

Subsidiary  Varieties  of  Connective  Tissue  — In  various 
parts  of  the  Body  are  connective-tissue  structures  which  have 
not  undergone  the  typical  development,  but  have  departed 
from  it  in  one  way  or  another.  The  cells  having  formed  a 
non-fibrillated  intercellular  substance  around  them,  develop- 
ment may  go  no  farther  and  the  mass  remain  permanently  as 
the  jellylike  connective  tissue  ;  or,  as  in  the  vitreous  humor 
of  the  eye  (Chap.  XXXI),  the  cells  having  formed  the  soft 
matrix,  may  disappear  and  leave  the  latter  only.  In  other 
cases  the  intercellular  substance  disappears  and  the  cells 
branching,  and  joining  by  the  cuds  of  their  branches,  form  a 
network  themselves,  nucleated  or  not  at  the  points  answering 
to  the  centre  of  each  originally  separate  cell.  This  is  known 
as  adenoid  connective  tissue.  In  other  cases  the  cells  almost 
alone  constitute  the  tissue,  becoming  flattened,  closely  fitted 
at  their  edges,  and  united  by  a  very  small  amount  of  cement- 
substance.  Membranes  formed  in  this  way  lie  beneath 
epithelium    in    many    places    and     are    known    as   banemeut' 


KM 


THE  HUMAN  BODY 


membranes:  the  il.it  cells  (Fig.  11,  b)  which  form  the 
epithelium  of  the  serous  cavities  are  themselves  a  layer  of 
modified  connective-tissue  corpuscles. 

In  brain  and  spina]  cord,  protecting  and  supporting  the 
nerve-tissues,  are  Eound  branched  cells  forming  the  neuroglia. 
They  are  qo1  true  connective  tissue,  hut  correspond  to  cells 
of  the  horny  layer  of  the  epidermis,  shut  in  when  the 
medullary  canal  was  closed  in  the  embryo. 

Elastic  Cartilage  and  Fibro-cartilage.  We  may  now 
return  to  cartilages  and  consider  those  forms  which  arc  made 
up  of  more  or  less  true  cartilage  mixed  with  less  or  more  con- 
nective tissue  of  one  kind  or  another.  The  cartilages  of  the 
car  and  nose  and  some  others  have  their  matrix  pervaded  by 
fine  branching  fibres  of  yellow  elastic  tissue,  which  form  net- 
works around  the  groups  of  cartilage-cells.  Such  cartilages 
are  pliable  and  tough  and  possess  also  considerable  extensibil- 
ity and  elasticity.  They  are  known  as  elastic  or,  from  their 
color,  as  yellow  cartilages.  Elsewhere,  especially  in  the  carti- 
lages which  lie  between  the  bones  in  some  joints,  we  find 
forms  which  have  the  matrix  pervaded  by  white  fibrous  tissue 
and  known  as  fibro-cartilages.  For  example  the  articular 
cartilage  on  the  end  of  the  lower  jaw  does  not  come  into 


Fig.  48.— Section  through  the  joint  of  the  lower  jaw  strewing  its  interarticular 
fibro-cartilage,  x,  with  the-  synovial  cavity  on  each  side  of  it. 

direct  contact  with  that  covering  its  socket  on  the  skull,  but 
lying  between  the  two  in  the  joint  (Fig.  4S)  is  an  inlerarlir- 
ular  fibro-cartilage :  similar  cartilages  exist  in  the  knee-joint,- 


CARTILAGE  AND    CONNECTIVE  TISSUE.  105 

and  the  intervertebral  disks  are  also  made  up  of  this  tissue. 
Both  elastic  cartilage  and  fibro-cartilage  often  shade  off 
insensibly  into  pure' elastic  or  pure  white  fibrous  connective 

tissue. 

Homologies  of  the  Supporting  Tissues.     Bone,  cartilage, 
and  connective  tissue  all  agree  in  broad  structural  characters, 
and  in  the  uses  to  which  they  are  applied  in  the  Body.     In 
each  of  them  the  cells  which  have  built  up  the  tissue,  with 
few  exceptions,  form  an  inconspicuous  part  of  it  in  its  fully 
developed  state,  the  chief  mass  of  it  consisting  of  intercellular 
substance.     In  hyaline  cartilages  this  latter  is  not  fibrillated; 
but  these  cartilages  pass  insensibly  in  various  regions  of  the 
Body  into  elastic   or   fibro-cartilages,   and    these    latter    in 
turn  into  elastic  or  fibrous  connective  tissue.     The   lamellae 
of  bone,  too,  when  peeled   off  a  bone  softened  in  acid  and 
examined  with  a  very  high  magnifying  power,  are  seen  to  be 
pervaded    by  fine   fibres.      Structurally,  therefore,  one   can 
draw  no  hard  and  fast  line  between  these  tissues.     The  same 
is  true  of  their  chemical  composition ;  bone  and  white  fibrous 
tissue  contain  a  substance  (collagen)  which  is  converted  into 
gelatin  when  boiled  in  water;  and  in  old  people  many  carti- 
lages become  hardened  by  the  deposit  in  their  matrix  of  the 
same  lime-salts  which  give  its  hardness  to  bone.      Further, 
the  developmental  history  of  all  of  them  is  much  alike.     In 
very  early  life  each  is  represented  by  cells  only :  these  form 
an  intercellular  substance,  and  this  subsequently  may  become 
fibrillated,  or  calcified,  or  both.     Finally  they  all  agree  in 
manifesting  in  health  no  great  physiological  activity,  their 
use  in  the  Body  depending  upon  the  mechanical  properties 
of  their  intercellular  portions. 

The  close  alliance  of  all  three  is  further  shown  by  the 
frequency  with  which  they  replace  one  another.  All  the 
bones  and  cartilages  of  the  adult  are  at  first  represented  only 
by  collections  of  connective  tissue.  Before  or  after  birth  this 
is  in  some  cases  substituted  by  bone  directly  (as  in  the  case  of 
the  collar-bone  and  the  bones  on  the  roof  of  the  skull),  while 
in  other  cases  cartilage  supplants  the  connective  tissue,  to  be 
afterwards  in  many  places  replaced  by  bone,  while  elsewhere 
it  remains  throughout  life. 

Moreover  in  different  adult  animals  we  often  find  the 
game  pari  bony  in  one,  cartilaginous  in  a  second,  and  com- 
posed  of  connective  tissue  in  a  third:  so  that  these  tissues 


106  THE  HUMAN  BODY. 

not  only  represent  one  another  at  different  stages  in  the  life 
of  the  same  animal  but  permanently  throughout  the  whole 
life  of  different  animals.  Low  in  the  animal  scale  we  find 
them  all  represented  merely  by  cells  with  structureless  inter- 
cellular substance:  a  little  higher  in  the  scale  the  latter 
becomes  fibrillated  and  forms  distinct  connective  tissue. 
Jn  the  highesi  Mullusks  (as  the  cuttle-fishes)  this  is 
partly  replaced  by  cartilage,  and  the  same  is  true  of  the  low- 
est fishes;  while  in  some  other  fishes  and  the  remaining 
Vertebrates  we  find  more  or  less  bone  appearing  in  place  of 
the  original  connective  tissue  or  cartilage. 

From  the  similarity  of  their  modes  of  development  and 
fundamental  structure,  the  transitional  forms  which  exist 
between  them,  and  the  frequency  with  which  they  replace 
one  another,  histologists  class  the  three  (bone,  cartilage  and 
connective  tissue)  together  as  homologous  tissues  and  regard 
them  as  differentiations  of  the  same  original  structure. 

Hygienic  Remarks.  Since  in  the  new-born  infant  many 
parts  which  will  ultimately  become  bone  consist  only  of  car- 
tilage, the  young  child  requires  food  which  shall  contain  a 
large  proportion  of  the  lime-salts  which  are  used  in  building 
up  bone.  Nature  provides  this  in  the  milk,  which  is  rich  in 
such  salts  (see  Chap.  XXI),  and  no  other  food  can  thoroughly 
replace  it.  Long  after  infancy  milk  should  form  a  large 
part  of  a  child's  diet.  Many  children  though  given  food 
abundant  in  quantity  are  really  starved,  since  their  food  does 
not  contain  in  sufficient  amount  the  mineral  salts  requisite 
for  their  healthy  development. 

At  birth  even  those  bones  of  a  child  which  are  most  ossi- 
fied are  often  not  continuous  masses  of  osseous  tissue.  In  the 
humerus,  for  example,  the  shaft  of  the  bone  is  well  ossified 
and  so  is  each  end,  but  between  the  shafts  and  each  of  the 
articular  extremities  there  still  remains  a  cartilaginous  layer, 
and  at  those  points  the  bone  increases  in  length,  new  cartilage 
being  formed  and  replaced  by  bone.  The  bone  increases  in 
thickness  hy  new  osseous  tissue  formed  beneath  the  perios- 
teum. The  same  thing  is  true  of  the  bones  of  the  leg.  On 
account  of  the  largely  cartilaginous  and  imperfectly  knit 
state  of  its  bones,  it  is  cruel  to  encourage  a  young  child  to 
walk  beyond  its  strength,  and  may  lead  to  "bow-legs"  or 
other  permanent  distortions.  Nevertheless  here  as  elsewhere 
in  the  animal  body,  moderate  exercise  promotes  the  growth  of 


CARTILAGE  AND    CONNECTIVE  TISSUE.  107 

the  tissues  concerned,  and  it  is  nearly  as  bad  to  wheel  a  child 
about  forever  in  a  baby-carriage  as  to  force  it  to  over  exertion. 

The  best  rule  is  to  let  a  healthy  child  use  its  limbs  when 
it  feels  inclined,  but  not  by  praise  or  blame  to  incite  it  to 
efforts  which  are  beyond  its  age,  and  so  sacrifice  its  healthy 
growth  to  the  vanity  of  parent  or  nurse. 

The  final  knitting  together  of  the  bony  articular  ends 
with  the  shaft  of  many  bones  takes  place  only  comparatively 
late  in  life,  and  the  age  at  which  it  occurs  varies  much  in 
different  bones.  Generally  speaking,  a  layer  of  cartilage  re- 
mains between  the  shaft  and  the  ends  of  the  bone,  until  the 
latter  has  attained  its  full  adult  length.  To  take  a  few 
examples  :  the  lower  articular  extremity  of  the  humerus 
only  becomes  continuous  with  the  shaft  by  bony  tissue  in  the 
sixteenth  or  seventeenth  year  of  life.  The  upper  articular 
extremity  only  joins  the  shaft  by  bony  'continuity  in  the 
twentieth  year.  The  upper  end  of  the  femur  joins  the  shaft 
by  bone  from  the  seventeenth  to  the  nineteenth  year,  and 
the  lower  end  during  the  twentieth.  In  the  tibia  the  upper 
extremity  and  the  shaft  unite  in  the  twenty-first  year,  and 
the  lower  end  and  the  shaft  in  the  eighteenth  or  nineteenth  : 
while  in  the  fibula  the  upper  end  joins  the  shaft  in  the 
twenty-fourth  year,  and  the  lower  end  in  the  twenty-first. 
The  separate  vertebrae  of  the  sacrum  are  only  united  to  form 
one  bone  in  the  twenty-fifth  year  of  life;  and  the  ilium, 
ischium,  and  pubis  unite  to  form  the  os  innominatum  about 
the  same  period.  Up  to  about  twenty-five  then  the  skeleton 
is  not  firmly  "knit,"  and  is  incapable,  without  risk  of  injury, 
<>t'  bearing  strains  which  it  might  afterwards  meet  with  im- 
punity. To  let  lads  of  sixteen  or  seventeen  row  and  take 
other  exercise  in  plenty  is  one  thing,  and  a  good  one;  but  to 
allow  them  to  undergo  the  severe  and  prolonged  strain  of 
training  for  and  rowing  a  long  race  is  quite  another,  and  not 
devoid  of  risk. 

Adipose  Tissue.  Fatty  substances  of  several  kinds  exist 
in  considerable  quantity  in  the  Human  Body  in  health,  some 
a-  minute  droplets  floating  in  the  bodily  liquids  or  imbedded 
in  various  cells,  but  most  in  special  cells,  nearly  filled  with 
fat,  and  collected  into  masses  with  supporting  and  nutritive 
part-  to  form  adipose  tissue.  In  fact  almost  in  every  spot 
where  the  widely  distributed  areolar  tissue  is  found,  there  is 
adipose  tissue  in  greater  or   less  proportion  mixed  with  it. 


108 


THE  HUMAN  BODY, 


Fig.  49. — Fat-cells  imbedded 
in  areolar  tissue,  a,  nucleus; 
b,  protoplasm  :  c,  oil-droplet. 


Considerable  quantities  exisl  for  example  in  the  subcuta- 
neous areolar  tissue,  especially  in  the  female  sex,  giving  the 
figure  of  the  woman  its  general  more  graceful  roundness  of 
contour  when  compared  with  thai  of  the  male.  Large  quanti- 
ties commonly  lie  in  the  abdominal  cavity  around  the  kid- 
neys; in  the  eye-sockets,  forming  a 
pad  for  t  he  eyeballs  ;  in  the  mar- 
row of  hones;  around  the  joints, 
and  so  on. 

Examined  with  the  microscope 
(Fig.  49)  adipose  tissue  is  found  to 
consist  of  small  vesicles  from  0.2 
mm.  to  0.09  mm.  (,/,,,  to  .,',„  inch) 
in  diameter,  clustered  together  into 
little  masses  and  hound  to  one  an- 
other hy  connective  tissue  and  blood- 
vessels which  intertwine  around 
them;  in  this  way  the  little  angular  masses  which  are  seen  in 
beef-suet  are  formed,  each  mass  being  separated  by  a  some- 
what coarser  partition  of  areolar  tissue  from  its  neighbors. 
The  individual  fat-cells  are  spherical  or  ovoid  except  when 
closely  packed  ;  then  they  become  polygonal.  Each  consists 
of  a  delicate  envelope  containing  oily  matter,  which  in  life 
is  liquid  at  the  temperature  of  the  Body.  Besides  the  oily 
matter,  a  nucleus  is  commonly  present  in  each  fat-cell;  and 
a  thin  layer  of  protoplasm,  exaggerated  in  Fig.  49,  forms  a 
lining  to  the  cell-wall.  The  oily  matter  consists  of  a  mixture 
of  palmatin,  olein  and  stearin,  which  are  compounds  of  pal- 
mitic, stearic  and  oleic  acids  with  glycerin,  three  molecules 
of  the  acid  being  combined  with  one  of  glycerin,  with  the 
elimination  of  water;  as  for  example: 

3fC18H350  )  Q\      C3HS  >  Q  _  3(C18H„0)  )  0    ,  3H  o 


Stearic  acid. 


Glyceriu. 


Stearin. 


Water. 


CHAPTER  IX. 

THE  STRUCTURE   OF   THE   MOTOR  ORGANS. 

Motion  in  Animals  and  Plants.  If  one  were  asked  to 
point  out  the  most  distinctive  property  of  living  animals,  the 
answer  would  probably  be,  their  power  of  executing  spontane- 
ous movements.  Animals  as  wc  commonly  know  them  are 
rarely  at  rest,  while  trees  and  stones  move  only  when  acted 
upon  by  external  forces,  which  are  in  most  cases  readily  re- 
cognizable. Even  at  their  quietest  times  some  kind  of  motion 
is  observable  in  the  higher  animals.  In  our  own  Bodies 
during  the  deepest  sleep  the  breathing  movements  and  the 
beat  of  the  heart  continue;  their  cessation  is  to  an  onlooker 
the  most  obvious  sign  of  death.  Here,  however,  as  elsewhere 
in  Biology,  we  find  that  precise  boundaries  do  not  exist;  at 
any  rate  so  far  as  animals  and  plants  are  concerned  we  cannot 
draw  a  hard  and  fast  line  between  them  with  reference  to  the 
presence  or  absence  of  apparently  spontaneous  motility.  Many 
a  flower  closes  in  the  evening  to  expand  again  in  the  morning 
sun;  and  in  many  plants  comparatively  rapid  and  extensive 
movements  can  be  called  forth  by  a  slight  touch,  which  in 
itself  is  quite  insufficient  to  produce  mechanically  that  amount 
of  motion  in  the  mass.  The  Venus's  flytrap  (Dionma  musci- 
pula)  for  example  has  fine  hairs  on  its  leaves,  and  when  these 
are  touched  by  an  insect  the  leaf  closes  up  so  as  to  imprison 
the  animal,  which  is  subsequently  digested  and  absorbed  by 
the  leaf.  The  higher  plants  it  is  true  have  not  the  power  of 
locomotion,  they  cannot  change  their  place  as  the  higher  ani- 
mals can;  but  on  the  other  hand  some  of  the  lower  animals 
are  permanently  fixed  to  one  spot;  and  among  the  lowest 
plants  many  are  known  which  swim  about  actively  through 
the  water  in  which  they  live.  The  lowest  animals  and  plants 
are  in  fact  those  which  have  undergone  least  differentiation 
in  their  development,  and  which  therefore  resemble  each 
other  in  possessing,  in  a  more  or  less  manifest  degree,  all  the 
fundamental  physiological   properties  of  that  simple   mass  of 

10!) 


110  THE  III  WAS   BODY. 

protoplasm  which  formed  the  star  tin  g-poinl  of  each  individ- 
ual. With  the  physiological  division  of  labor  which  takes 
place  in  the  higher  forms  we  find  that,  speaking  broadly, 
plants  especially  develop  nutritive  tissues,  while  animals  are 
characterized  by  the  high  development  of  tissues  with  motor 
and  irritable  properties;  bo  that  the  preponderance  of  these 
latter  is  very  marked  when  a  complex  animal,  like  a  dog  or  a 
man,  is  compared  with  a  complex  plant,  like  a  pine  or  a  hick- 
ory. The  higher  animal  possesses  in  addition  to  greatly  de- 
veloped nutritive  tissues  (which  differ  only  in  detail  from 
those  of  the  plant,  and  constitute  what  are  therefore  often 
called  organs  of  vegetative  life)  well-developed  spontaneous. 
irritable  and  contractile  tissues,  found  mainly  in  the  nervous 
and  muscular  systems,  and  forming  what  have  been  called  the 
organs  of  animal  life.  Since  these  place  the  animal  in  clos< 
relationship  with  the  surrounding  universe,  enabling  slight 
external  forces  to  excite  it,  and  it  in  turn  to  act  upon  external 
objects,  they  are  also  often  spoken  of  as  organs  of  relation. 
In  man  they  have  a  higher  development  on  the  whole  than  in 
any  other  animal,  and  give  him  his  leading  place  in  the  ani- 
mate world,  and  his  power  of  so  largely  controlling  and  direct- 
ing natural  forces  for  his  own  good,  while  the  plant  can  only 
passively  strive  to  endure  and  make  the  best  of  what  happens 
to  it;  it  has  little  or  no  influence  in  controlling  the  happening. 

Amoeboid  Cells.  The  simplest  motor  tissues  in  the  adult 
Human  Body  are  the  amoeboid  cells  (Fig.  15)  already  de- 
scribed, which  may  be  regarded  as  the  slightly  modified 
descendants  of  the  undifferentiated  cells  which  at  one  time 
made  up  the  whole  Body.  In  the  adult  they  are  not  attached 
to  other  parts,  so  that  their  changes  of  form  only  affect  them- 
selves and  produce  no  movements  in  the  rest  of  the  Body. 
Hence  with  regard  to  the  whole  frame  they  can  hardly  be 
called  motor  tissues,  and  are  classed  in  the  group  of  undiffer- 
entiated tissues. 

Ciliated  Cells.  As  the  growing  Body  develops  from  its 
primitive  simplicity  we  find  that  the  cells  lining  some  of  the 
tubes  and  cavities  in  its  interior  undergo  a  very  remark- 
able change,  by  which  each  cell  differentiates  itself  into  a  nu- 
tritive and  a  highly  motile  and  spontaneous  portion.  Such 
cells  are  found  for  example  lining  the  windpipe,  and  are 
represented  in  Fig.  50.  Each  has  a  conical  form,  the  base  of 
the  cone  being  turned  to  the  cavity  of  the  air-tube,  and  con- 


THE  STRUCTURE  OF  THE  MOTOR   ORGANS.        Ill 


Fig.  50.— Ciliated  cells. 


tains  an  oval  nucleus  with  a  nucleolus.  On  the  broader  free 
end  are  a  number  (about  thirty  on  the  average)  of  extremely 
fine  processes  called  cilia.  During  life 
these  are  in  constant  rapid  movement, 
lashing  to  and  fro  in  the  liquid  which 
moistens  the  interior  of  the  passage;  and 
as  the  cells  are  very  closely  packed,  a  bit 
of  the  inner  surface  of  the  windpipe,  ex-. 
amined  with  a  microscope,  looks  like  a 
field  of  wheat  or  barley  when  the  wind 
blows  over  it.  Each  cilium  strikes  with 
more  force  in  one  direction  than  in  the  opposite,  and  as  this 
direction  of  more  powerful  stroke  is  the  same  for  all  the  cilia 
on  any  one  surface,  the  resultant  effect  is  that  the  liquid  in 
which  they  move  is  driven  one  way.  In  the  case  of  the  wind- 
pipe for  example  it  is  driven  up  towards  the  throat,  and  the 
tenacious  liquid  or  mucus  which  is  thus  swept  along  is  finally 
coughed  or  "hawked"  up  and  got  rid  off,  instead  of  accumu- 
lating in  the  deeper  air-passages  away  down  in  the  chest. 

These  cells  afford  an  extremely  interesting  example  of  the 
division  of  jmysiological  employments.  Each  proceeds  from 
a  cell  which  was  primitively  equally  motile,  automatic  and 
nutritive  in  all  its  parts.  But  in  the  fully  developed  state 
the  nutritive  duties  have  been  especially  assumed  by  the 
conical  cell-body,  while  the  automatic  and  contractile  prop- 
erties have  been  condensed,  so  to  speak,  in  that  modified 
portion  of  the  primitive  protoplasmic  mass  which  forms  the 
cilia.  These,  being  supplied  with  elaborated  food  by  the  rest 
of  the  cell,  are  raised  above  the  vulgar  cares  of  life  and  have 
the  opportunity  to  devote  their  whole  attention  to  the  per- 
formance of  automatic  movements;  which  are  accordingly  far 
more  rapid  and  precise  than  those  executed  by  the  whole  cell 
before  any  division  of  labor  had  occurred  in  it. 

That  the  movements  depend  upon  the  structure  and  com- 
position  of  the  cells  themselves,  and  not  upon  influences 
reaching  them  from  the  nervous  or  other  tissues,  is  proved  by 
the  fact  that  they  continue  for  a  long  time  in  isolated  cells, 
removed  and  placed  in  a  liquid,  as  hlood-sermn,  which  does 
not  alter  their  physical  constitution.    In  cold-blooded  animals, 

turtles,  whose  constituent  tissues  frequently  retain  their 
individual  vitality  long  after  that  bond  of  union  lias  been 
destroyed  which  constitutes  the  life  of  the  whole  animal  as 


112  THE  HUMAN  BODY. 

distinct  from  the  lives  of  its  different  tissues,  the  ciliated  cells 
in  the  windpipe  have  been  found  still  at  work  three  weeks 
after  the  general  death  of  the  animal. 

The  Muscles.  These  are  the  main  motor  organs  ;  their 
general  appearance  is  well  known  to  every  one  in  the  lean  of 
butcher's  meat.  While  amoeboid  cells  can  only  move  them- 
selves, and  (at  least  in  the  Human  Body)  ciliated  cells  the 
layer  of  liquid  with  which  they  may  happen  to  be  in  contact, 
the  majority  of  the  muscles,  being  fixed  to  the  skeleton,  can, 
by  alterations  in  their  form,  bring  about  changes  in  the  form 
and  position  of  nearly  all  parts  of  the  Body.  With  the  skele- 
ton and  joints,  they  constitute  pre-eminently  the  organs  of 
motion  and  locomotion,  and  are  governed  by  the  nervous 
system  which  regulates  their  activity.  In  fact  skeleton, 
muscles,  and  nervous  system  are  correlated  parts:  the  degree 
of  usefulness  of  any  one  of  them  largely  depends  upon  the 
more  or  less  complete  development  of  the  others.  Man's 
highly  endowed  senses  and  his  powers  of  reflection  and 
reason  would  be  of  little  use  to  him,  were  his  muscles  less 
fitted  to  carry  out  the  dictates  of  his  will  or  his  joints  less 
numerous  or  mobile.  All  the  muscles  are  under  the  control 
of  the  nervous  system,  but  all  are  not  governed  by  it  with  the 
co-operation  of  will  or  consciousness;  some  move  without  our 
having  any  direct  knowledge  of  the  fact.  This  is  especially  the 
case  with  certain  muscles  which  are  not  fixed  to  the  skeleton 
but  surround  cavities  or  tubes  in  the  Body,  as  the  blood-vessels 
and  the  alimentary  canal,  and  by  their  movements  control 
the  passage  of  substances  through  them.  The  former  group. 
or  skeletal  muscles,  are  also  from  their  microscopic  characters 
known  as  striped  muscles,  while  the  latter,  or  visceral  muscles, 
are  called  unstriped  or  plain  muscles.  The  skeletal  muscles 
being  generally  more  or  less  subject  to  the  control  of  the  will 
(as  for  example  those  moving  the  limbs)  are  frequently  spoken 
of  as  voluntary,  and  the  visceral  muscles,  which  change  their 
form  independently  of  the  will,  as  involuntary.  The  heart- 
muscle  forms  a  sort  of  intermediate  link;  it  is  not  directly 
attached  to  the  skeleton,  but  forms  a  hollow  bag  which  drives 
on  the  blood  contained  in  it  and  that  quite  involuntarily;  but 
in  its  microscopic  structure  it  resembles  somewhat  the  skeletal 
voluntary  muscles.  The  muscles  of  respiration  might  perhaps 
be  cited  as  another  intermediate  group.  They  are  striped 
skeletal  muscles  and,  as  we  all  know,  are  to  a  certain  extent 


THE  STRUCTURE  OF  THE  MOTOR   ORGANS.        113 

subject  to  the  will;  any  one  can  draw  a  deep  breath  when  lie 
chooses.  But  in  ordinary  quiet  breathing  we  are  quite  un- 
conscious of  their  working,  and  even  when  attention  is  turned 
to  them  the  power  of  control  is  limited;  no  one  can  voluntar- 
ily hold  his  breath  long  enough  to  suffocate  himself.  As  we 
shall  see  hereafter,  moreover,  any  one  or  all  of  the  striped 
muscles  of  the  Body  may  be  thrown  into  activity  independ- 
ently of  or  even  against  the  will,  as,  to  cite  no  other  instances, 
is  seen  in  the  "fidgets"  of  nervousness  and  the  irrepressible 
trembling  of  extreme  terror;  so  that  the  names  voluntary  and 
involuntary  are  not  good  ones.  The  functional  differences 
between  the  two  groups  depend  really  more  on  the  nervous 
connections  of  each  than  upon  any  essential  difference  in  the 
properties  of  the  so-called  voluntary  or  involuntary  muscular 
tissues  themselves. 

The  Skeletal  Muscles.  In  its  simplest  form  a  skeletal 
muscle  consists  of  a  red  soft  central  part,  the  belly,  which 
tapers  at  each  end  and  there  passes  into  one  or  more  dense 
white  cords  which  consist  almost  entirely  of  white  fibrous 
connective  tissue.  These  terminal  cords  are  called  the  tendons 
of  the  muscle  and  serve  to  attach  it  to  parts  of  the  bony  or 
cartilaginous  skeleton.  In  Fig.  51  is  shown  the  biceps  muscle 
of  the  arm,  which  lies  in  front  of  the  humerus.  Its  fleshy 
belly  is  seen  to  divide  above  and  end  there  in  two  tendons, 
one  of  which,  Bl,  is  fixed  to  the  scapula,  while  the  other,  Bb, 
joins  the  tendon  of  a  neighboring  muscle  (the  cor aco -brachial, 
Cb),  and  is  also  fixed  above  to  the  shoulder-blade.  Near  the 
elbow-joint  the  muscle  is  continued  into  a  single  tendon, 
B',  which  is  fixed  to  the  radius,  but  gives  an  offshoot^  B" ,  to 
the  connective -tissue  membranes  lying  around  the  elbow- 
joint. 

The  belly  of  every  muscle  possesses  the  power  of  shorten- 
ing forcibly  under  certain  conditions.  In  so  doing  it  pulls 
upon  the  tendons,  which  being  composed  of  inextensible 
white  fibrous  tissue  transmit  the  movement  to  the  hard  parts 
to  which  they  are  attached,  just  as  a  pull  at  one  end  of  a  rope 
may  be  made  to  act  upon  distant  objects  to  which  the  other 
end  is  tied.  The  tendons  are  merely  passive  cords  and  are 
sometimes  rery  long,  as  for  instance  in  tin;  case  of  the  mus- 
cles of  tin-  fingers,  the  bellies  of  many  of  which  lie  away  in 
the  forearm. 

If  the  tendons  at  each  end  of  a  muscle  were  fixed  to  the 


114 


THE  HUMAN  BODY. 


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THE  STRUCTURE  OF  THE  MOTOR   ORGANS.        115 


same  bone  the  muscle  would  clearly  be  able  to  produce  no 
movement,  unless  by  bending  or  breaking  the  bone;  the 
probable  result  in  such  a  case  would  be  the  tearing  of  the 
muscle  by  its  own  efforts.  In  the  Body,  however,  the  two 
ends  of  a  muscle  are  always  attached  to  different  parts, 
usually  two  bones,  between  which  more  or  less  movement  is 
permitted,  and  so  when  the  muscle  pulls  it  alters  the  relative 
positions  of  the  parts  to  which  its  tendons  are  fixed.  In  the 
great  majority  of  cases  a  true  joint  lies  between  the  bones  on 
which  the  muscle  can  pull,  and  when  the  latter  contracts  it 
produces  movement  at  the  joint.  Many  muscles  even  pass 
over  two  joints  and  can  produce  movement  at  either,  as  the 
biceps  of  the  arm  which,  fixed  at  one  end  to  the  scapula  and 
at  the  other  to  the  radius,  can  move  the  bones  at  either  the 
shoulder  or  elbow  joint.  Where  a  muscle  passes  over  an  ar- 
ticulation it  is  nearly  always  reduced  to  a  narrow  tendon; 
otherwise  the  bulky  bellies  lying  around  the  joints  would 
make  them  extremely  clumsy  and  limit  their  mobility. 

Origin    and   Insertion    of  Muscles.     Almost    invariably 
that  part  of  the  skeleton  to  which  one  end  of  a  muscle  is 


Fir;.  52, — Tin'  biceps  muscle  and  the  arm-hones,  to  illustrate  how,  under  ordinary 
circumstances,  the  elbow- joint  is  flexed  when  the  muscle  contracts. 

fixed  is  more  easily  moved  than  the  part  on  which  it  pulls  by 
its  other  tendon.  The  less  movable  attachment  of  a  muscle 
ie  called  its  origin,  the  more  movable  its  insertion.  Taking 
for  example  the  biceps  of  the  arm,  we  find  that  when  the 
belly  of  the  muscle  contracts  and  pulls  on  its  upper  and  Lower 
tendons,  it  commonly  moves  only  the  forearm,  bending  the 
elbow-joint  as  shown  in  Fig.  52.  The  shoulder  is  so  much 
more  firm  that  it   serves  as  a  fixed  point,  and  so  that  end  is 


116 


THE  HUMAN   HODY. 


the  origin  of  the  muscle,  and  the  forearm  attachment,  /',  the 
insertion.  It  is  clear,  however,  thai  this  distinction  in  the 
mobility  of  the  points  of  fixation  of  the  muscle  is  only  rela- 
tive, for,  by  changing  the  conditions,  the  insertion  may  beconu 
the  stationary  and  the  origin  the  moved  point;  as  for  instance 
in  going  up  a  rope  "  hand  overhand/'  In  that  case  the  radial 
•  •ml  of  the  muscle  is  fixed  and  the  shoulder  is  moved  through 
space  by  its  contraction. 

Different  Forms  of  Muscles.  Many  muscles  of  the  Body 
have  the  simple  typical  form  of  a  belly  tapering  to  a  single 
Tendon  at  each  end  as  A,  Fig.  53;  but  others  divide  at  one 
end  and  are  called  two-headed  or  biceps  muscles;  while  some 
are  even  three-headed  or  triceps  muscles.  On  the  other  hand 
some  muscles  have  no  tendon  at  all  at  one  end,  the  belly  run- 
ning quite  up  to  the  point  of  attachment;  and  some  have  no 
tendon  at  either  end.  In  many  muscles  a  tendon  runs  along 
one  side  and  the  fibres  of  the  belly  are  at- 
tached obliquely  to  it:  such  muscles  (/>',  Fig. 
53)  are  called  penniform  or  featherlike; 
or  a  tendon  runs  obliquely  down  the  middle 
of  the  muscle  and  has  the  fibres  of  the  belly 
fixed  obliquely  on  each  side  of  it  (C,  Fig.  53), 
forming  a  bipenniform  muscle:  or  even  two 
tendons  may  run  down  the  belly  and  so  form 
a  tripenniform  muscle.  Iu  a  few  cases  a 
tendon  is  found  in  the  middle  of  the  belly 

Fig.   53.— Diasrams  . 

illustrating     typical    as  well  as  at  each  end  ot  it;  such  muscles 

muscle  with  a  central  n-ij-         .     •  »  i         <•  ,  i   •      p 

beiiy  and  two  termi-   are  called  aigast r ir.      A  muscle  oi  this  form 

niform  muscle* cfa    (Fig.  54)  is  found  in  connection  with  the 
bipeuniform muscle.     jower   ^y       h    an<r,   ])y  &  tendon   attached 

to  the  base  of  the  skull;  from  there  its  first  belly  runs  down- 
wards and  forwards  to  the  neck  by  the  side  of  the  hyoid  bone, 
where  it  ends  in  a  tendon  which  passes  through  a  loop  serving 
as  a  pulley.  This  is  succeeded  by  a  sea  ml 
belly  directed  upwards  towards  the  Cilin, 
where  it  ends  in  a  tendon  inserted  into  the 
lower  jaw.  Running  along  the  front  of  the 
abdomen  from  the  pelvis  to  the  chest  is  a  Ions; 
muscle  on  each  side  of  the  middle  line  called 
the  rectus  abdominis:  it  is  polygastric,  con 
sisting  of  four  bellies  separated  by  short  tendons.  Many 
muscles  moreover  are  not  rounded  but  form  wide  flat  masses, 


Fig.  54.— A  digas- 
i  ric  r  1 1 1 1 — » ■  1*  - . 


THE  STRUCTURE  OF  THE  MOTOR   ORGANS.        117 


us  for  example  the  muscle  Ss  seen  on  the  ventral  side  of  the 
shoulder-blade  in  Fig.  51. 

Gross  Structure  of  a  Muscle.  However  the  form  of  the 
skeletal  muscles  and  the  arrangement  of  their  tendons  may 
vary,  the  essential  structure  of  all  is  the  same.  Each  consists 
of  a  proper  striped  muscular  tissue,  which  is  its  essential 
part,  but  which  is  supported  by  connective  tissue,  nourished 
by  blood-vessels  and  lymphatics,  and  has  its  activity  governed 
by  nerves;  so  that  a  great  variety  of  things  go  to  form  the 
complete  organ. 

A  loose  sheath  of  areolar  connective  tissue,  called  the  peri- 
mysium, envelops  each  muscle,  and  from  this  partitions  run 
in  and  subdivide  the  belly  into  bundles  or  fasciculi  which 
run  from  tendon  to  tendon,  or  for  the  whole  length  of  the 
muscle  when  it  has  no  tendons.  The  coarseness  or  fineness 
of  butcher's  meat  depends  upon  the  size  of  these  primary 
fasciculi,  which  differs  in  different  muscles  of  the  same  ani- 
mal. These  larger  fasci- 
culi are  subdivided  by  finer 
connective  tissue  m  e  m  - 
braues  into  smaller  ones 
(as  shown  in  Fig.  55,  which 
represents  a  few  primary 
fasciculi  of  a  muscle  and 
the  secondary  fasciculi  into 
which  these  are  divided), 
each  of  which  consists  of  a 
certain  number  of  micro- 
scopic 'in  a  sen  Jar  fibres 
bound  together  by  very  fine  connective  tissue  and  enveloped 
in  a  close  network  of  blood-vessels.  Where  a  muscle  tapers 
the  fibres  in  the  fasciculi  become  less  numerous,  and  when  a 
tendon  is  formed  disappear  altogether,  leaving  little  but  the 
connective  tissue. 

Histology  of  Muscle.  For  the  present  we  need  only 
concern  ourselves  with  the  muscular  fibres.  Each  of  these  is 
from  eight  to  thirty-five  millimetres  (£  to  14/  inches)  long,  but 
only  from  0.034  to  0.055  mm.  (T4T)  to  ±\Tl  inch)  in  diameter 
in  its  widest  part  and  tapering  to  a  blunt  point  at  each  end. 
hi  cross-section  the  fibres  are  irregularly  polygonal.  In  long 
muscles  witb  terminal  tendons,  no  fibre  runs  the  whole  length 
of  fl   fasciculus,  which  may  be  a  foot  or  more  long,  but  the 


Fig.  55.— A  small  bit  of  muscle  composed 
of  five  primary  fasciculi.  A,  natural  size; 
B.  the  same  magnified  three  diameters,  to 
show  the  secondary  fasciculi  of  which  the 
primary  are  composed. 


118 


Til E  111  MAS   BODY. 


fasciculus  is  made  up  of  many  successive  fibres,  the  narrow 
end  of  each  fitting  in  between  the  ends  of  those  which  follow 
it.     In  muscles  with  short  fasciculi,  the  fibres 

may  run  the  whole  length  of   each   of   the 
latter. 

The  tissue  of  the  skeletal  muscles  is  very 
easily  recognized  under  the  microscope:  even 
when  magnified  only  two  or  three  hundred 
diameters  each  fibre  is  seen  to  be  crossed  for 
its  whole  width  by  regularly  alternating  dim- 
mer and  brighter  bands  (Fig.  50)  or  stripes. 
In  a  relaxed  fibre  each  band  is  about  ^\-g 
mm.  (TT-croir  inch)  in  width,  but  the  brighter 
partofamiucie-fibre. bands  are  a  little  broader  than  the    darker. 

magnified;      showing  T       ,,  l     t   n\  i      ti      1   •      i  e    i         j 

itecross-stYi    in,,  and  In  the  contracted  fibre  both  kinds  of  bands 

a  coup  e  o  iiue  i.     kecome  narrower,  especially  the  brighter,  and 

these  latter  at  the  same  time  undergo  an  optical  change  and 

divert  the  light  so  that  but  little  of  it  reaches  the 

eye  when  the  fibre  is  in  focus;  in  consequence  they 

then  look  darker  than  the  original  dimmer  bands 

lying   between  them   and    now  appearing  as  the 

brighter  of  the  two.     A  fresh  muscle-fibre  shows 

on  close  examination  a  faint  longitudinal  striation. 

This  is  much  more  marked  in  specimens  which 

have  been  preserved  in  alcohol,  and  these  may  be 

teased  out  into  very  fine  threads  which  have  been 

named  fibrillm. 

On  careful  examination  each  fibre  can  be  made 
out  to  possess  an  external  envelope,  the  sarco- 
lemma, enveloping  a  softer  material  which  makes 
up  the  main  mass  of  the  fibre;  but  there  are  in 
addition  a  number  of  oval  nuclei  which  lie  im-  Fig.  57.— a 
mediately  under  the  sarcolemma  and  are  placed  muscular  fl- 
lengthwise  in  the  fibre.  On  account  of  its  extreme  aEre  basteen 
thinness  and  transparency  the  sarcolemma  cannot  twisted  som 
be  recognized  when  lying  in  its  natural  position,  ten^winethe 
closely  applied  to  the  striped  contents,  but  being  f°UKU?r8ar^ 
tougher  than  these  it  sometimes  remains  unbroken    wheresociose- 

0  ly  applied    to 

when  thev  are  crushed  and  then  (Fie\  57)  comes   t"e  re»\  as  to 

.  .  J  H  i.  J.  1  be        ilivisil,le. 

into  view  as    an    apparently  structureless    mem-   remains    un- 

!  .      .  ,     .  ,,  ..,,  .  im  11   and  con- 

brane  bridging  over  the  gap.       I  he   sarcolemma  spicuous. 
is  imperforate  except  at  one    point  where    the   central    por- 


THE  STRUCTURE  OF  THE  MOTOR   ORGANS. 


119 


fcion  {or axis  cylinder,  see  Chap.  XII)  of  a  nerve-fibre  pene- 
trates it. 

The  striped  contents  which  occupy  most  of  the  cavity  en- 
closed by  the  sarcolemma  are  the  essential  contractile  portion 
of  the  fibre  and  during  life  are  soft  or  semi-fluid:  soon  after 
death  they  solidify  or  clot  and  thus  death-stiffening  {rigor 
mortis)  is  produced.  At  intervals,  corresponding  to  the 
middle  zone  of  each  bright  band  of  the  relaxed  fibre,  an 
extremely  delicate  membrane  {membrane  of  Krause,  K,  Fig. 
58)  crosses  the  fibre,  thus  dividing  the  rest  of  the  contents 
into  a  series  of  disks,  each  consisting  of  a  dim  centre  answering 


*'  "'»' Vy  -a 


FlG.  58.— Diagrams  to  illustrate  the  structure  of  a  small  piece  of  a  striped 
muscle-fibre.  A,  in  the  relaxed,  B,  in  the  contracted  condition;  A',  A",  membranes 
of  Krause;  //.  //,  bands  of  Hensen:  aa,  bi>,  parts  of  sarcostyles,  showing  their  con- 
strictions near  Krause's  membranes,  and  the  tubulated  sarcosomes  in  each;  c,d,e,f, 
a  surcous  element;  o,  hyaloplasm;  y,  sarcoplasm 

to  the  whole  of  a  dark  band,  and  two  brighter  ends,  each  cor- 
responding to  half  of  a  bright  band.  Each  disk  is  a  sarcomere. 
Under  certain  conditions,  in  fact,  a  fibre  may  be   split,   up 

-wise  into  a  Dumber  of  such  disks.  When  afresh  muscle- 
fibre  is  artificially  stretched  and  examined  with  a  very  hio'li 
magnifying  power  there  may  further  be  made  out  in  the 
middle  of  each  dim  band  a  transverse  line  (band  of  Hensen, 
If.  Fig.  58)  slightly  brighter  than  the  rest  of  the  dim  hand. 

The  main  bulk  of  each  fibre  consists  of  polygonal  rods,  the 
muscle-columns  or  sarcostyles  (aa,  bb,  Fig.  58),  which  are  inter- 


120  THE  HUMAN  BODY. 

rupted  in  their  course  byKrause'a  membranes.  That  portion 
of  a  Barcostyle,  cdef,  included  between  two  consecutive  mem- 
branes is  a  sarcous  element.  The  terminal  portions  of  each 
sarcous  element  are  of  softer  consistence  than  most  of  t  he  mid- 
dle part  and  correspond  to  the  hyaloplasm  (Fig.  ?)  of  a 
typical  primitive  cell,  and  the  material  composing  them  may 
be  designated  by  the  same  name.  The  central  portion  of  each 
sarcous  element  is  mainly  made  up  of  a  firmer  material  which 
.-tains  with  hemotoxylin  and  answers  in  general  tot  he  ret  iculum 
of  a  primitive  cell :  it  is  named  the  sarcous  substance  or,  better, 
the  sarcosome.  Each  sarcosome  is  permeated  by  tine  longi- 
tudinal tubules  which  commence  at  its  ends  but  do  not  reach 
to  its  centre  and  are  thus  divided  into  two  sets  by  a  median 
transverse  partition  in  which  the  band  of  llensen  lies.  These 
tubules  are  filled  with  hyaloplasm.  The  sarcous  elements  are 
constricted  where  they  abut  on  Krause's  membrane  and  in 
consequence  each  sarcostyle  is  narrowed  at  regular  intervals 
along  its  course.  The  spaces  between  the  sarcostyles  are 
filled  by  a  very  soft  sarcoplasm,  which  is  of  course  more 
abundant  in  the  regions  of  Krause's  membranes,  where  the 
muscle-columns  are  constricted.  In  mammalian  muscle  the 
sarcoplasm  is  present  in  relatively  much  smaller  amount  than 
indicated  in  Fig.  58.  In  fresh  specimens  it  can,  however, 
be  made  out  in  the  form  of  fine  dark  lines  Avith  swollen  ends, 
lying  between  contiguous  sarcous  elements.  Gold  chloride 
stains  the  sarcoplasm  deeply  but  leaves  the  sarcostyles  1111- 
colored :  hence  in  specimens  so  prepared  the  edges  or  ends  of 
the  sarcoplastic  septa  appear  as  very  conspicuous  lines,  which 
look,  especially  in  cross-sections,  as  if  due  to  a  network  of 
fibres,  as  which  they  have  been  described  by  several  observers, 
and  been  regarded  as  the  essential  contractile  part  of  the 
fibre.  In  a  relaxed  muscle-fibre  (A,  Fig.  58)  the  sarcosomes 
are  comparatively  long  and  narrow;  but  during  contraction 
(Z>)  they  become  shorter  and  thicker  and  bulged  out  in  the 
middle,  and  more  hyaloplasm  passes  into  their  tubules,  which 
become  distended,  especially  near  their  deeper  ends:  the  band 
of  llensen  also  ceases  to  be  visible.  Contraction  of  the  whole 
fibre  is  thus  accompanied  by  or,  rather,  is  due  to  a  transfer- 
ence of  hyaloplasm  from  the  ends  of  each  sarcomere  into  the 
interior  of  the  sarcosomes  of  its  central  portion,  in  conse- 
quence of  which  the  whole  fibre  becomes  shorter  and  thicker. 
The  swelling  of    the   sarcosome   pushes   aside    some  of  the 


TIIE  STRUCTURE  OF  THE  MOTOR   ORGANS.        121 

sarcoplasm  lying  between  tliem  and  tlie  displaced  portion  ac- 
cumulates nearer  the  ends  of  the  sarcous  elements,  in  the 
space  left  by  that  portion  of  the  hyaloplasm  which  has  entered 
the  tubules:  compare  gg  in  A  and  B,  Fig.  58. 

Arguing  from  the  analogy  of  the  amoeboid  cell  in  which, 
as  we  have  seen  (p.  27),  parts  consisting  only  of  hyaloplasm 
can  exhibit  movements,  it  would  seem  probable  that  in  the 
muscle-fibre  the  hyaloplasm  is  to  be  regarded  as  the  active 
contractile  portion  and  the  sarcosomes  as  a  framework 
directing  the  form  which  the  contracted  hyaloplasm  shall 
assume,  and  assuring  that  it  shall  be  a  precise  and  definite 
shortening  in  the  direction  of  the  long  axis  of  the  fibre  with 
a  widening  in  the  transverse  direction,  instead  of  such  irreg- 
ular changes  of  form  as  are  exhibited  by  the  amoeboid  cell 
with  its  irregularly  arranged  or,  sometimes,  entirely  absent 
reticulum.  That  the  hyaloplasm  and  not  the  sarcoplasm 
form  the  contractile  part  of  the  fibre  is  proved  by  the  fact 
that  in  some  insect-muscles  in  which  they  are  unusually  large, 
it  is  possible  to  isolate  them  while  alive  and  observe  them  still 
contracting. 

The  nuclei  of  the  fibres  lie  in  the  sarcoplasm,  Avhich  rep- 
resents a  part  of  the  original  protoplasm  of  the  row  of  cells 
from  which  each  muscle-fibre  develops,  that  has  remained 
but  little  changed  while  the  rest  was  differentiated  into 
sarcous  elements. 

The  blood-vessels  and  nerve-fibres  supplied  to  the  skeletal 
muscles  are  numerous.  The  larger  blood-vessels  run  in  the 
coarser  partitions  of  the  connective  tissue  lying  between  the 
fasciculi  and  give  off  fine  branches  which  form  a  network  be- 
tween the  individual  fibres  but  never  penetrate  the  sarcolemma. 

Connected  with  each  muscle-fibre  is  a  nerve-fibre  of  the 
white  variety  (Chap.  XII).  The  central  core  of  the  nerve-fibre 
ends  in  an  oval  expansion  (end  plate)  which  contains  many 
nuclei  and  lies  close  under  the  sarcolemma,  its  deeper  side 
being  in  immediate  contact  and  possibly  continuous  with 
the  striated  contents.  These  nerve-fibres  are  motor  or  con- 
cerned in  exciting  a  contraction  of  the  muscle-fibre.  Other 
white  nerve-fibres  are  connected  with  very  peculiar  bodies 
found  scattered  throughout  the  muscle,  but  especially  numer- 
ous near  the  tendons.  They  are  usually  of  a  size  just  visible 
to    the    unaided    eye  and    from    their   form    have   been    named 

muscle-spindles.    They  appear  to   be   sensory   in   function. 


122 


THE  HUMAN  BODY. 


Somewhat  similar  bodies  (Golgi's  tendon-organs)  are  found 
in  the  tendons  and  arc  also  richly  supplied  with  nerve-fibres. 
In  histological  structure  the  tendon-organs  and  the  muscle- 
spindles  appear  to  be  allied  to  Pacinian  bodies  (Chap.  X  X  X  V). 
Structure  of  the  Unstriped  Muscles.  Of  these  the 
muscular  coat  of  the  stomach  (Fig.  59)  is  a  good  example. 


Fig.  59.— The  muscular  coat  of  the  stomach. 

They  have  no  definite  tendons,  but  form  expanded  membranes 
surrounding  cavities,  so  that  they  have  no 
definite  origin  or  insertion.  Like  the  skel- 
etal muscles  they  consist  of  proper  contractile 
elements,  with  accessory  connective  tissue, 
blood-vessels  and  nerves.  Their  fibres,  how- 
ever, have  a  very  different  microscopic  struc- 
ture. They  present  a  slightly  marked  longi- 
tudinal but  no  cross  striation  and  are  made 
up  of  elongated  cells  (Fig.  GO),  bound  to- 
gether by  a  small#quantity  of  cement iiiLr 
material.  The  cells  vary  considerably  in 
size,  but  on  the  average  are  about  JT  mm. 
(,,,',„  inch)  in  length.  Each  is  flattened 
in  one  plane,  tapers  off  at  each  end,  and 
possesses  a  very  thin  enveloping  membrane; 
in  its  interior  lies  an  elongated  nucleus  with 
one  or  two  nucleoli.  These  cells  have  the 
power  of  shortening  in  the  direction  of  their 
muscie-ceiis.         long  axes,  and  so  of  diminishing  the  capacity 

of  the  cavities  in  the  walls  of  which  they  lie. 


THE  STRUCTURE  OF  THE  MOTOR   ORGANS.        123 

Cardiac  Muscular  Tissue.  This  consists  of  nucleated 
branched  cells  which  unite  to  form  a  network,  in  the  inter- 
stices of  which  blood-capillaries  and 
nerve-fibres  run.  The  cells  present 
transverse  striations,  but  not  so  distinct 
as  tbose  of  the  skeletal  muscles,  and  are 
said  to  have  no  sarcolemma. 

The  Chemistry  of  Muscular  Tissue. 
The  chemical  structure  of  the  living 
muscular  fibre  is  unknown,  but  some 
idea  as  to  it  may  be  obtained  from  ex- 
amination of  the  substances  it  yields  on 
proximate  analysis.  Muscle  contains  75  FlG.7i. -cardiac  muscu- 
per  cent  of  water;  and,  among  other  % t^J^t%&*%£ 
inorganic  constituents,  phosphates   and  boundaries  and  cell-nuclei 

°.  .  .  are   indicated   only   in    the 

Chlorides       of      potassium,     Sodium,     and   right-hand  portion  of  the 

magnesium.  When  at  rest  a  living 
muscle  is  feebly  alkaline,  but  after  hard  work,  or  when  dying, 
this  reaction  is  reversed  through  the  formation  of  sarcolactic 
acid  (C3H603).  Muscles  contain  small  quantities  of  grape- 
sugar  and  glycogen,  and  of  organic  nitrogenous  crystalline 
compounds, 'especially  kreatin  (C4H9Ns0a).  As  in  the  case  of 
all  other  physiologically  active  tissues,  however,  the  main 
post-mortem  constituents  of  the  muscular  fibres  are  proteid 
substances,  and  it  is  probable  that  like  protoplasm  itself  (p. 
27)  the  essential  contractile  part  of  the  tissue  consists  of  a 
complex  body  containing  proteid,  carbohydrate  and  fatty 
residues;  and  that  during  muscular  work  this  is  broken  up, 
yielding  proteids,  carbon  dioxide,  sarcolactic  acid,  and  prob- 
ably other  things. 

During  life  and  for  a  certain  time  after  general  death  the 
muscles  are  soft,  translucent,  extensible  and  elastic,  and 
neutral  or  feebly  alkaline  in  reaction;  after  a  period  which  in 
warm-blooded  animals  is  brief  (varying  from  a  few  minutes 
to  three  or  four  hours)  they  gradually  become  harder,  more 
opaque,  less  extensible  and  less  elastic,  and  distinctly  acid  in 
reaction.  The  result  of  these  changes  is  the  well-known 
cadaveric  rigidity  or  rigor  vior/is.  The  rigid  condition  lasts 
for  a  day  or  longer  and  t  hen  it  gradually  and  finally  disappears 
and  more  marked  decomposition  changes  commence.  Until 
a  Bhort  time  before  the  commencement  of  rigor  the  muscles 
remain  contractile  and  can  he  thrown  into  activity  by  various 


124  THK  HUMAN  BODY. 

excitants,  as  electric  shocks;  that  is  to  say,  although  the  body 
in  genera]  is  dead  and  i  lie  beat  of  the  heart  and  the  How  of 
blood  have  ceased,  yet  the  muscles  retain  their  vitality  for  a 
while.  This  is  especially  the  case  with  the  muscles  of  cold- 
blooded animals,  as  frogs  and  turtles,  the  muscles  of  which 
may,  especially  if  kept  cool,  retain  their  living  properties  for 
several  hours  after  removal  from  the  body  of  the  animal. 

If  muscles  be  taken  in  an  early  stage  of  rigor,  rapidly  freed 
as  much  as  possible  from  tendons,  fats  and  connective  tissue, 
and  then  finely  minced  and  thoroughly  washed  with  water, 
most  of  the  salts  and  crystallizable  muscle  ingredients  can  be 
dissolved  away,  along  with  a  small  amount  of  albumens;  but  by 
far  the  greater  part  of  the  albumen  is  left  behind  in  the  form 
of  myosin,  a  proteid  which  is  insoluble  in  water.  On  treating 
the  residue  with  a  10  per  cent  solution  of  ammonium  chloride 
the  myosin  dissolves  and  may  be  obtained  as  a  flocculent 
white  precipitate  by  allowing  the  solution  to  fall  drop  by  drop 
into  a  large  quantity  of  water,  or  by  adding  to  it  a  consider- 
able proportion  of  common  salt.  Myosin  is  related  chemically 
to  fibrinogen  and  globulin,  and  its  solutions  in  10  per  cent 
neutral  saline  are  coagulated  by  heat  at  the  same  temperature 
(5G°  C.  or  158°  F.)  as  the  former. 

Although  myosin  is  apparently  the  least  altered  form  in 
which  its  chief  proteid  constituent  can  be  separated  from 
muscle,  it  does  not  appear  to  exist,  or  at  least  exists  in  small 
quantity  if  at  all,  in  living  muscle;  it  is  an  early  product  of 
post-mortem  chemical  changes.  Its  precursor  in  living  muscle 
has  been  named  myosinogen,  and  a  solution  containing  that 
substance  may  be  obtained  as  follows:  Perfectly  fresh  and 
still  contractile  muscles  are  cut  out  from  a  frog  which  has 
just  been  killed  by  destruction  of  its  brain  and  spinal  cord,  a 
proceeding  which  entirely  deprives  the  animal  of  conscious- 
ness and  the  power  of  using  its  muscles,  but  leaves  these  lat- 
ter unaltered  and  alive  for  some  time.  The  excised  muscles 
are  thrown  into  a  vessel  cooled  below  0°  C.  by  a  freezing  mix- 
ture and  are  thus  frozen  hard  before  any  great  chemical 
change  has  had  time  to  occur  in  them.  The  solidified  mus- 
cles are  then' cut  up  into  thin  slices,  the  bits  thrown  on  a 
cooled  filter  and  let  gradually  warm  up  to  the  freezing-point 
of  water,  after  the  addition  of  some  ice-cold  0.5  per  cent  solu- 
tion of  common  salt.  Gradually  a  small  quantity  of  a  tena- 
cious alkaline  and  transparent  liquid  filters  through.     This 


THE  CHEMISTRY  OF  MUSCLE.  125 

liquid,  known  as  the  muscle-plasma,  contains  myosinogen  and 
like  blood-plasma  is  spontaneously  coagulable.  It  quickly 
sets  into  a  transparent  jelly  and  this  soon  separates  into  mus- 
cle-serum and  muscle-clot,  the  latter  consisting  of  myosin. 
Dissolved  in  the  muscle-serum  are  found  small  quantities  of 
several  albumens,  oue  much  resembling  the  serum-albumen  of 
blood.  The  spontaneous  clothing  of  the  plasma,  and  presum- 
ably the  natural  formation  of  myosin  during  rigor  mortis,  are 
due  to  the  action  on  myosinogen  of  an  enzyme,  muscle-fer- 
ment, much  resembling  fibrin-ferment.  The  clotting  is 
accompanied  by  a  change  of  reaction  from  the  alkaline  or 
neutral  of  the  plasma  to  a  markedly  acid  one :  this  appears  to 
be  mainly  due  to  the  formation  of  sarcolactic  acid,  the  quan- 
tity of  which  bears  a  proportion  to  that  of  the  myosin  formed, 
suggesting  that  both  may  be  products  of  the  breaking-down 
of  a  pre-existent  more  complex  substance.  It  has  further  been 
shown  that  when  a  muscle  passes  into  the  state  of  rigor  it 
evolves  a  certain  amount  of  carbon  dioxide,  and  that  the 
quantity  of  this  varies  with  the  quantity  of  myosin  and  of 
sarcolactic  acid  formed.  Hence  it  has  been  suggested  that  in 
the  living  muscle  there  is  a  substance  which  after  death 
breaks  up  yielding  (with  possibly  other  things)  myosinogen, 
sarcolactic  acid  and  carbon  dioxide;  and  further  that  this 
chemical  change  is  associated  with  the  liberation  of  energy 
(Chap.  XX)  which  in  the  dead  muscle  is  set  free  mainly  as 
the  heat  which  is  known  to  be  evolved  by  muscles  passing 
into  rigor. 

The  precipitate  produced  when  myosin  solutions  are 
heated  is  coagulated  proteid  (p.  10)  and  insoluble  in  dilute 
acids  and  alkalies  in  which  myosin  itself  is  very  soluble. 
When  dissolved  in  dilute  acids  myosin  is  converted  into  syn- 
tonin,  which  was  formerly  supposed  to  be  the  chief  form  of 
proteid  present  in  dead  muscles.  Syntonin  is  insoluble  m 
water  and  neutral  saline  solutions,  but  soluble  in  dilute  acids 
and  alkalies,  and  its  solutions  are  not  coagulated  by  boiling. 

Beef  Tea  and  Liebig's  Extract.  From  the  above-stated 
facts  it  is  clear  that  when  a  muscle  is  boiled  in  water  its  myo- 
sin is  coagulated  and  left  behind  in  the  meat  :  even  if  cook- 
ing be  commenced  by  soaking  in  cold  water  the  myosin  still 
remains,  as  it  is  as  insoluble  in  cold  water  as  in  hot.  Beef  tea 
ae  ordinarily  made,  then,  contains  little  but  the  flavoring 
matters  and  salts  of  the  meat,  traces  of  some  albumens  and 


126  THE  111. MAX  BODY. 

.-Mine  gelatin,  the  latter  derived  from  the  connective  tissues 
of  the  muscle.  The  flavoring  matters  and  salts  make  it  decep- 
tivelj  taste  as  if  it  were  a  strong  solution  of  the  whole  meat, 
and  the  gelatiu  causes  it  to  "set  "on  cooling,  so  the  cook 
feels  quite  sure  she  has  gol  out  "  all  t  he  si  rengl  ii  <>(  |  he  meat," 
whereas  the  beef  tea  so  prepared  contains  but  little  of  the 
most,  autritious  proteid  portions,  which  in  an  insipid  shrunken 
form  arc  left  when  the  liquid  is  strained  oil'.  Various  pro- 
posals have  been  made  with  the  object  of  avoiding  this  ami 
getting  a  really  nutritive  beef  tea;  as  for  example  chopping 
the  raw  meat  tine  ami  soaking  it  in  strong  brine  for  some 
hours  to  dissolve  out:  the  myosin:  or  extracting  it  with  dilute 
acids  which  turn  the  myosin  into  syntonin  and  dissolve  it  and 
at  the  same  time  render  it  non-coagulable  by  heat  u  hen  subse- 
quently boiled.  Smdi  methods,  however,  make  unpalatable 
compounds  which  invalids  will  not  take.  Beef  tea  is  a  slight 
stimulant,  and  often  extremely  useful  in  temporarily  main- 
taining the  strength  and  in  preparing  the  stomach  for  other 
food,  but  its  direct  value  as  a  food  is  slight,  and  it  cannot  be 
relied  upon  to  keep  up  a  patient's  strength  for  any  length  of 
time.  There  can  be  no  doubt  that  thousands  of  sick  persons 
have  in  the  past  and  are  being  to-day  starved  to  death  on  it. 
Liebig's  extract  of  meat  is  essentially  a  very  strong  beef  tea; 
containing  much  of  the  flavoring  substances  of  the  meat, 
nearly  all  its  salts  and  the  crystalline  nitrogenous  bodies,  such 
as  kreatin.  which  exist  in  muscle,  but  hardly  any  of  its  really 
nutritive  parts,  as  was  pointed  out  by  Liebig  himself.  From 
its  stimulating  effects  it  is  often  useful  to  persons  in  feeble 
health,  but  other  food  should  be  given  with  it.  It  may  also 
be  used  on  account  of  its  flavor  to  add  to  the  "  stock  "  of  soup 
and  for  similar  purposes  ;  but  the  erroneousness  of  the  com- 
mon belief  that  it  is  a  highly  nutritious  food  cannot  be  too 
strongly  ^insisted  upon.  Under  the  name  of  liquid  extracts 
of  meat  other  substances  have  been  prepared  by  subjecting 
meat  to  chemical  processes  in  which  it  undergoes  changes 
similar  to  those  experienced  in  digestion:  the  myosin  is  thus 
rendered  soluble  in  water  and  uncoagulable  by  heat,  and  such 
extracts  if  properly  prepared  are  nutritious  and  can  often  be 
absorbed  when  meat  in  the  solid  form  cannot  be  digested: 
they  may  thus  help  the  stomach  over  a  crisis,  but  are  not, 
even  the  best  of  them,  to  be  depended  on  as  anything  but 
temporary  substitutes  for  other  food;  or  in  some  cases  as  use- 
ful additious  to  it. 


CHAPTER  X. 

THE   PROPERTIES   OF   MUSCULAR  TISSUE. 

Contractility.  The  characteristic  physiological  property 
of  muscular  tissue,  and  that  for  which  it  is  employed  in  the 
Body,  is  the  faculty  possessed  by  its  fibres  of  shortening 
forcibly  under  certain  circumstances.  The  direction  in  which 
this  shortening  occurs  is  always  that  of  the  long  axis  of  the 
fibre  in  both  plain  and  striped  muscles,  and  it  is  accompanied 
by  an  almost  equivalent  thickening  in  other  diameters,  so  that 
when  a  muscle  contracts  it  does  not  shrivel  up  or  diminish 
its  bulk  in  any  appreciable  way;  it  simply  changes  its  form. 
When  a  muscle  contracts  it  also  becomes  harder  and  more 
rigid,  especially  if  it  has  to  overcome  any  resistance.  This 
and  the  change  of  form  can  be  well  felt  by  placing  the  fingers 
of  one  hand  over  the  biceps  muscle  lying  in  front  of  the  hu- 
merus of  the  other  arm.  When  the  muscle  is  contracted  so 
as  to  bend  the  elbow  it  can  be  felt  to  swell  out  and  harden  as 
it  shortens.  Every  schoolboy  knows  that  when  he  appeals  to 
another  to  "  feel  his  muscle"  he  contracts  the  latter  so  as  to 
make  it  thicker  and  apparently  more  massive  as  well  as 
harder.  In  statues  the  prominences  on  the  surface  indicating 
the  muscles  beneath  the  skin  are  made  very  conspicuous 
when  violent  effort  is  represented,  so  as  to  indicate  that  the 
muscles  are  in  vigorous  action.  In  a  muscular  fibre  we  find 
no  longer  the  slow,  irregular,  and  indefinite  changes  of  form 
seen  in  amoeboid  slightly  differentiated  cells;  they  are  replaced 
by  ;i  precise,  rapid  and  definite  change  of  form.  Muscular 
tissue  represent  a  group  of  cells  in  the  bodily  community 
which  have  taken  up  the  one  special  duty  of  executing 
changes  of  form,  and  in  proportion  as  these  cells  have  fewer 
other  things  to  do,  they  do  that  one  better.  This  contractility 
of  the  muscular  fibres  maybe  briefly  described  as  a  passage 
from  the  stated  rest,  in  which  the  fibres  arc  long  and  narrow, 
into  tin-  state  of  activity,  in  which  they  are  shorter  and  thicker: 

this  change  is  made  with  considerable  force,  and  thus  the  mus- 

127 


128  THE  HUMAN  BODY. 

cles  move  parts  attached  to  their  tendons.  When  the  state  of 
activity  lias,  passed  off  the  fibres  suffer  themselves  to  be  ex- 
tended  again  by  any  force  pulling  upon  them,  and  so  regain 
their  resting  shape ;  and  since  in  the  living  Body  almost  in- 
variably other  parts  are  put  upon  the  stretch  when  any  mus- 
cle contracts,  these  by  their  elasticity  serve  to  pull  the  latter 
back  again  to  its  primitive  shape.  No  muscular  fibre  is 
known  to  have  the  power  of  actively  expanding  after  it  has 
contracted  :  in  the  active  state  it  forcibly  resists  extension,  but 
once  the  contraction  is  completely  over,  it  suffers  itself  readily 
to  be  pulled  back  to  its  resting  form.  The  contracted  state 
lasts  always  longer,  however,  than  the  mere  time  occupied  by 
the  muscle  in  shortening:  as  will  be  seen  later,  the  full  state 
of  contraction  is  gradually  attained  and  gradually  disappears. 

Irritability.  With  that  modification  of  the  primitive 
protoplasm  of  an  amoeboid  embryonic  cell  which  gives  rise  to 
a  muscular  fibre  with  its  great  contractility,  there  goes  a  loss 
of  other  properties.  Nearly  all  spontaneity  disappears;  mus- 
cles are  not  automatic  like  primitive  protoplasm  or  ciliated 
cells;  except  under  certain  very  special  conditions  they  remain 
at  rest  unless  excited  from  without.  The  amount  of  external 
change  required  to  excite  the  living  muscular  fibre  is,  how- 
ever, very  small;  muscle  tissue  is  highly  irritable,  a  very 
little  thing  being  sufficient  to  call  forth  a  powerful  contrac- 
tion. In  the  living  Human  Body  the  exciting  force,  or  stim- 
ulus, acting  upon  a  muscle  is  almost  invariably  a  nervous 
impulse,  a  molecular  movement  transmitted  along  the  nerve- 
fibre  attached  to  it,  and  upsetting  the  molecular  equilibrium 
of  the  muscle.  It  is  through  the  nerves  that  the  will  acts 
upon  the  muscle-fibre,  and  accordingly  injury  to  the  nerves  of 
a  part,  as  the  face  or  a  limb,  causes  paralysis  of  its  muscles. 
They  may  still  be  there,  intact  and  quite  ready  to  work,  but 
there  are  no  means  of  sending  commands  to  them,  and  so 
they  remain  idle. 

Although  a  nervous  impulse  is  the  natural  physiological 
muscular  stimulus  it  is  not  the  only  one  known.  If  a  muscle 
be  exposed  in  a  living  animal  and  a  slight  but  sudden  tap  be 
given  to  it,  or  a  hot  bar  be  suddenly  brought  near  it,  or  an  ek-c- 
tric  shock  be  sent  through  it,  or  a  drop  of  glycerin  or  of  solu- 
tion of  ammonia  be  placed  on  it,  it  will  contract;  so  that  in 
addition  to  the  natural  nervous  stimulus,  muscles  are  irritable 
under  the  influence  of  mechanical,  thermal,  electrical,  and 


THE  PROPERTIES  OF  MUSCULAR   TISSUE.  129 

chemical  stimuli.  One  condition  of  the  efficacy  of  each  of 
them  is  that  it  shall  act  with  some  suddenness;  a  very  slowly 
increased  pressure,  even  if  ultimately  very  great,  or  a  very 
slowly  raised  temperature,  or  a  slowly  increased  electrical  cur- 
rent passed  through  it,  will  not  excite  the  muscle;  although 
far  less  pressure,  warmth,  or  electricity  more  rapidly  applied 
would  stimulate  it  powerfully.  Once  an  electric  current  has 
been  set  up  through  a  muscle,  its  steady  passage  does  not  act 
as  a  stimulus;  but  a  sudden  diminution  or  increase  of  it  does 
It  may  perhaps  still  be  objected  that  it  is  not  proved  that  any 
of  these  stimuli  excite  the  muscular  fibres,  and  that  in  all 
these  cases  it  is  possible  that  the  muscle  is  only  excited 
through  its  nerves.  For  the  various  stimuli  named  above 
also  excite  nerves  (see  Chap.  XIII),  and  when  we  apply  them 
to  the  muscle  we  may  really  be  acting  first  upon  the  fine 
nerve-endings  there,  and  only  indirectly  and  through  the 
mediation  of  these  upon  the  muscular  fibres.  That  the  mus- 
cular fibres  have  a  proper  irritability  of  their  owu,  independ- 
ently of  their  nerves,  is,  however,  shown  by  the  action  of  cer- 
tain drugs — for  example  curare,  a  South  American  Indian 
arrow  poison.  When  this  substance  is  introduced  into  a 
wound  all  the  striped  muscles  are  apparently  poisoned,  and 
the  animal  dies  of  suffocation  because  of  the  cessation  of  the 
breathing  movements.  But  the  poison  does  not  really  act  on 
the  muscles  themselves:  it  kills  the  muscle-nerves,  but  leaves 
the  muscle  intact;  and  it  has  been  proved  to  kill  the  very 
endings  of  the  muscle-nerves  right  down  in  the  muscle-fibres 
themselves.  Yet  after  its  administration  we  still  find  that 
the  various  non-physiological  stimuli  referred  to  alove  make 
the  muscles  contract  just  as  powerfully  as  before  the  poison- 
ing, so  we  must  conclude  that  the  muscles  themselves  are 
irritable  in  the  absence  of  all  nerve  stimuli  —or,  what  amounts 
to  the  same  thing,  when  all  their  nerve-fibres  have  been  poi- 
soned. The  experiment  also  shows  that  the  contractility  of  a 
muscle  is  a  property  belonging  to  itself,  and  that  its  contract- 
ing force  is  not  something  derived  from  the  nerves  attached 
to  it.  The  nerve  stimulus  simply  acts  like  the  electric  shock 
or  sudden  blow  and  arouses  the  muscle  to  manifest  a  property 
which  it  already  possesses.  The  older  physiologists  observing 
that  muscular  paralysis  followed  when  the  nervous  connection 
between  a  muscle  and  the  brain  was  interrupted,  concluded 
that   the  nerves  gave  the  muscles  the  power  of  contracting. 


130  THE  HUMAN  BODY. 

They  believed  that  in  the  brain  there  was  a  great  store  of  a 
mysterious  thing  called  vital  spirits,  and  that  some  of  this 
was  ejected  from  the  brain  along  the  nerve  to  the  muscle, 

when  the  latter  was  to  be  set  at  work,  and  gave  it  its  working 
power.  Wy  now  know  that  such  is  not  the  case,  but  that  a 
muscle-fibre  is  a  collection  of  highly  irritable  material  which 
can  have  its  equilibrium  upset  in  a  definite  way,  causing  it  to 
change  its  shape,  under  the  influence  of  certain  slight  disturb- 
ing forces,  one  of  which  is  a  nervous  impulse;  and  since  in 
the  Bodv  no  other  kind  of  stimulus  usually  reaches  the  mus- 
cles, they  remain  at  rest  when  their  nervous  connections  are 
severed.  But  the  muscles  paralyzed  in  this  way  can  still,  in 
the  living  Body,  be  made  to  contract  by  sending  electrical 
shocks  through  them.  Physiologically,  then,  muscle  is  a  con- 
tractile and  irritable,  out  not  an  automatic,  tissue. 

A  Simple  Muscular  Contraction.  Most  of  the  details  con- 
cerning the  physiological  properties  of  muscles  have  been 
studied  on  those  of  cold-blooded  animals.  A  frog's  muscle 
will  retain  all  its  living  properties  for  some  time  after  re- 
moval from  the  body  of  the  animal,  and  so  can  be  experi- 
mented on  with  ease,  while  the  muscles  of  a  rabbit  or  cat 
soon  die  under  those  circumstances.  Enough  has,  however, 
been  observed  on  the  muscles  of  the  higher  animals  to  show 
that  in  all  essentials  they  agree  with  those  of  the  frog  or  ter- 
rapin. 

When  a  single  electric  shock  is  sent  through  a  muscle,  the 
nerves  of  which  have  been  thrown  out  of  action  by  curare,  it 
rapidly  shortens  and  then,  if  a  weight  be  hanging  on  it,  rap- 
idly lengthens  again.  The  whole  series  of  phenomena  from 
the  moment  of  stimulation  until  the  muscle  regains  its  rest- 
ing form  is  known  as  a  simple  muscular  contraction  or  a 
"twitch":  it  occupies  in  frog's  muscle  about  one  tenth  of  a 
second.  So  brief  a  movement  as  this  cannot  be  followed  in 
its  details  by  direct  observation,  but  it  is  possible  to  record  it 
and  study  its  phases  at  leisure.  This  may  be  done  by  firmly 
fixing  the  upper  tendon  of  an  isolated  muscle,  M,  Fig.  62, 
and  attaching  the  other  end  at  d  to  a  lever,  I,  which  can  move 
about  the  fulcrum  /-.  the  end  cf  the  long  arm  of  the  lever 
bears  a  point,/*,  which  scratches  on  a  smooth  smoked  surface, 
8.  Suppose  the  surface  to  be  placed  so  that  the  writing  point 
of  the  lever  is  at  <>:  if  the  muscle  now  contracts  it  will  raise 
the  point  of  the  lever,  and  a  line  ac  will   be   drawn  on  the 


THE  PROPERTIES  OF  MUSCULAR  TISSUE.         131 


132  THE  HUMAN  BODY. 

smoked  surface,  its  vertical  height,  cm,  being  dependent,  first, 

on  the  extent  of  the  shortening  of  the  muscle, and  second,  on 
the  proportion  lid  ween  the  long  and  short  arms  of  the  lever:. 
the  longer//?  is  as  compared  with  />/,  the  more  will  the  actual 
shortening  of  the  muscle  be  magnified.  With  the  lever  shown 
in  the  figure  this  magnification  would  be  about  ten  times,  so 
that  one  tenth  of  cm  would  he  the  extent  of  the  shortening 
of  the  muscle.  Suppose,  next,  the  smoked  surface  to  he  moved 
to  such  position  that  the  writing  point  of  the  lever  touches  it 
at  i,  and,  the  muscle  being  left  at  rest,  the  surface  to  he 
moved  evenly  from  left  to  right;  the  horizontal  line  io  would 
then  be  traced,  its  length  depending  on  the  distance  through 
which  S  moved  during  the  time  the  lever  was  marking  on  it: 
and  it  is  clear  that  if  S  move  uniformly,  and  we  know  its  rate 
of  movement,  we  can  very  readily  calculate  from  the  length  of 
io  how  long  S  was  moving  while  that  line  was  being  traced: 
for  example,  if  we  know  the  rate  of  movement  to  be  ten 
inches  per  second,  and  on  measurement  find  io  to  be  an  inch 
long,  the  time  during  which  the  surface  was  moving  must 
have  been  ^  of  a  second;  and  each  tenth  of  io  correspond 
to  -j-J-y  of  a  second. 

If  we  set  the  recording  surface  in  motion  and  while  the 
lever  point  is  tracing  a  horizontal  line  cause  the  muscle  to 
contract,  the  point  will  be  raised  as  long  as  the  muscle  is 
contracted,  and  the  line  drawn  by  it  will  be  due  to  a 
combination  of  two  simultaneous  movements — a  horizontal, 
due  to  the  motion  of  S,  a  nearly  vertical,  due  to  the  shorten- 
ing of  the  muscle;  the  resulting  line  is  a  curve  known 
as  the  curve  of  a  simple  muscular  Contraction.  Let  the 
surface  S  be  placed  so  that  the  writing  point  is  at  q  and 
then  he  set  in  uniform  motion  from  left  to  right  at  the  same 
rate  as  before  (ten  inches  per  second).  "When  the  point  is 
opposite  t,  stimulate  the  muscle  by  an  electric  shock;  the 
result,  until  the  muscle  has  fully  lengthened  again,  will  be  the 
curve  tuvwzy, from  which  many  things  may  be  learned.  In  the 
first  place  we  see  that  the  muscle  does  not  commence  to  con- 
tract at  the  very  instant  of  stimulation,  but  at  an  appreciably 
later  time,  and  during  the  interval  the  lever  draws  the  hori- 
zontal line  tv;  this  period,  occupied  by  preparatory  changes 
within  the  muscle,  is  known  as  the  period  of  latent  excitation. 
Then  the  muscle  begins  to  shorten  and  the  lever  to  rise,  at  first 
slowly  from  u  to  v,  then  more  rapidly,  and  again  more  slowly 


THE  PROPERTIES  OF  MUSCULAR   TISSUE.  133 

until  the  summit  of  the  contraction  is  reached  at  w.  The 
muscle  does  not  now  instantly  relax,  but  only  gradually  passes 
back  to  the  resting  state:  beginning  at  w,  we  see  the  descent 
of  the  curve  is  for  a  time  slow,  then  more  rapid,  and  finally 
slow  again  from  x  to  y,  when  the  contraction  is  completed 
and  the  lever  once  more  traces  only  the  horizontal  line  yp,  due 
to  the  continued  movement  of  the  recording  surface.  The 
curve  then  shows  three  distinct  phases  in  the  contraction:  the 
period  of  latent  excitation;  the  period  of  shortening;  the 
period  of  elongating.  Kuowing  the  rate  of  horizontal  move- 
ment, we  can  measure  off  the  time  occupied  by  each  phase. 
The  horizontal  distance  from  t  to  u  represents  the  time  taken 
by  the  latent  excitation;  from  u  to  z,  the  time  occupied  in 
shortening;  from  z  to  y,  the  time  taken  in  elongation:  in  a 
fresh  frog's  muscle  these  times  are  respectively  T^,  T^,  T^¥ 
of  a  second.  In  the  muscles  of  warm-blooded  animals  they 
are  all  shorter,  but  the  difficulties  in  the  way  of  accurate  ex- 
periment are  very  great.  If  we  know  the  relative  lengths  of 
the  arms  of  the  lever  we  can  of  course  readily  calculate  from 
the  height,  wz,  of  the  curve  the  extent  of  shortening  of 
the  muscle.  "With  a  single  electrical  stimulation  this  is  never 
more  than  one  fourth  the  total  length  of  the  muscle. 

In  Fig.  62  the  accessory  apparatus  used  in  practice  t<>  in- 
dicate on  the  moving  surface  the  exact  instant  of  stimulation 
and  to  measure  the  rate  at  which  8  moves  have  been  omitted. 

Physiological  Tetanus.  It  is  obvious  that  the  ordinary 
movements  of  the  Body  are  not  brought  about  by  such  tran- 
sient muscular  contractions  as  those  just  described.  Even  a 
wink  lasts  longer  than  one  tenth  of  a  second.  Our  movements 
are,  in  fact,  due  to  more  prolonged  contractions  which  may  be 
described  as  consisting  of  several  simple  contractions  fused 
together,  and  known  as  "tetanic  contractions";  it  might  be 
better  to  call  them  "compound  contractions/'  since  the  word 
tetanus  has  long  been  used  by  pathologists  to  signify  a  dis- 
eased state,  such  as  occurs  in  strychnine  poisoning  and  hydro- 
phobia, in  which  most  of  the  muscles  of  the  Bcdy  are  thrown 
into  prolonged  and  powerful  involuntary  contractions. 

If,  while  a  frog's  muscle  is  still  shortening  under  the  in- 
fluence of  one  electric  shock,  another  stimulus  be  given  it,  it 
will  contract  again  and  the  new  contraction  will  be  added  ob 
co  that  already  existing,  without  any  period  of  elongation 
occurring  between  them,     While  the  muscle  is  still  contract- 


134  THE  HUMAN  BODY. 

ing  under  the  influence  of  the  second  stimulus  a  third  electric 
shock  will  make  it  contract  more,  and  so  on,  until  the  muscle 
is  shortened  as  much  as  is  possible  to  it  for  that  strength  of 
stimulus.  If  now  the  stimuli  be  repeated  at  the  proper  in- 
tervals, each  new  one  will  not  produce  any  further  shortening, 
but,  each  acting  on  the  muscle  before  the  effect  of  the  last 
has  begun  to  pass  off,  the  muscle  will  be  kept  in  a  state  of 
permanent  or  "  tetanic"  contraction;  and  this  can  be  main- 
tained, by  continuing  the  application  of  the  stimuli,  until  the 
organ  begins  to  get  exhausted  or  "  fatigued  ";  elongation  then 
commences  in  spite  of  the  stimulation.  When  our  muscles 
are  stimulated  in  the  Body,  from  the  nerve-centres  through 
the  nerves,  they  receive  from  the  latter  a  sufficient  number 
of  stimuli  in  a  second  (the  exact  number  is  still  doubtful)  to 
throw  them  into  tetanic  contractions.  In  other  words,  not 
even  in  the  most  rapid  movements  of  the  Body  is  a  muscle 
made  to  execute  a  simple  muscular  contraction;  it  is  always 
a  longer  or  a  shorter  tetanus.  When  very  quick  movements 
are  executed,  as  in  performing  rapid  passages  on  the  piano, 
the  result  is  obtained  hy  contracting  two  opposing  muscles 
and  alternately  strengthening  and  weakening  a  little  the 
tetanus  of  each. 

Causes  affecting  the  Degree  of  Muscular  Contraction. 
The  extent  of  shortening  which  can  be  called  forth  in  a  mus- 
cle varies  with  the  stimulus.  In  the  first  place,  a  single  stim- 
ulus can  never  cause  a  muscle  to  contract  as  much  as  rapidly 
repeated  stimuli  of  the  same  strength — since  in  the  latter 
case  we  get,  as  already  explained,  several  simple  contractions 
such  as  a  single  stimulus  would  call  forth,  piled  on  the  top 
of  one  another.  With  powerful  repeated  electrical  stimuli 
a  muscle  can  be  made  to  shorten  to  one  third  of  its  resting 
length,  but  in  the  Body  the  strongest  effort  of  the  Will  never 
produces  a  contraction  of  that  extent.  Apart  from  the  rate 
of  stimulation,  the  strength  of  the  stimulus  has  some  influ- 
ence, a  greater  stimulus  causing  a  greater  contraction;  but 
very  soon  a  point  is  reached  beyond  which  increase  of  stimu- 
lus produces  no  increased  contraction;  the  muscle  has  reached 
its  limit.  The  amount  of  load  carried  by  the  muscle  (or  the 
resistance  opposed  to  its  shortening)  has  also  an  influence, 
and  that  in  a  very  remarkable  way.  Suppose  we  have  a  frog's 
calf-muscle,  carrying  no  weight,  and  find  that  with  a  stimulus 
of  a  certain  strength  it  shortens  two  millimeters  (TV  inch). 


THE  PROPERTIES  OF  MUSCULAR   TISSUE.  135 

Then  if  we  hang  one  gram  (15.5  grains)  on  it  and  give  it  the 
same  stimulus,  it  will  be  found  to  contract  more,  say  four  or 
five  millimeters,  and  so  on,  up  to  the  point  when  it  carries 
eight  or  ten  grams.  After  that  an  increased  weight  will, 
with  the  same  stimulus,  cause  a  less  contraction.  So  that  up 
to  a  certain  limit,  resistance  to  the  shortening  of  the  muscle 
makes  it  more  able  to  shorten :  the  mere  greater  extension  of 
the  muscle  due  to  the  greater  resistance  opposed  to  its  short- 
ening, puts  it  into  a  state  in  which  it  is  able  to  contract  more 
jiowerfully.  Fatigue  diminishes  the  working  power  of  a 
muscle  and  rest  restores  it,  especially  if  the  circulation  of  the 
blood  be  going  on  in  it  at  the  same  time.  A  frog's  muscle 
cut  out  of  the  body  will,  however,  be  considerably  restored 
during  a  period  of  rest,  even  although  no  blood  flow  through  it. 

Cold  increases  the  time  occupied  by  a  simple  muscular 
contraction,  and  also  impairs  the  contractile  power,  as  we 
find  in  our  own  limbs  when  "  numbed"  with  cold,  though  in 
that  case  the  hurtful  influence  of  the  cold  on  the  nerves  no 
doubt  also  plays  a  part.  Moderate  warmth  on  the  other  hand, 
up  to  near  the  point  at  which  death  stiffening  (often  in  this 
case  spoken  of  as  heat  rigor)  occurs,  diminishes  the  time 
taken  by  a  contraction,  and  increases  its  height.  Heat  rigor 
is  produced  in  excised  frog's  muscle  by  heating  it  to  about 
40°  C.  (10-4°  F.)  The  required  temperature  is  higher  in  warm 
blooded  animals,  especially  while  the  circulation  through  the 
muscle  is  maintained:  in  fevers  temperatures  considerably 
greater  than  the  above  have  been  observed  without  the  occur- 
rence of  muscular  rigor. 

The  Measure  of  Muscular  Work.  The  work  done  by  a 
muscle  in  a  given  contraction,  when  it  lifts  a  weight  verti- 
cally against  gravity,  is  measured  by  the  weight  moved,  mul- 
tiplied by  the  distance  through  which  it  is  moved.  When  a 
muscle  contracts  carrying  no  load  it  does  very  little  work, 
lifting  only  its  own  weight;  when  loaded  with  one  gram  and 
lifting  it  five  millimeters  it  does  five  gram-millimeters  of 
work,  just  as  an  engineer  would  say  an  engine  had  done  so 
many  kilogrammeters  or  foot-pounds.  If  loaded  with  ten 
grams  and  lifting  it  six  millimeters  it  would  do  sixty  gram- 
millimeters  of  work.  Even  after  the  weight  becomes  so  great 
that  it  is  lifted  through  a  less  distance,  the  work  done  by  the 
muscle  goes  on  increasing,  for  the  heavier  weight  lifted  more 
than   compensates  for  the  less  distance  through  which  it  is 


136  THE  HUMAN  BODY. 

raised.     For  example,  if  the  above  muscle  were  loaded  with 

fifty  grams  it  would  maybe  lift  that  weight  only  1.5  millime- 

uiit  it  would  then  do  seventy-five  gram-millimeters  of 

work,  which  is  more  than  when  it  lifted  ten  grams  six  milli- 
meters. A  load  is,  however,  at  last  reached  with  which  the 
muscle  does  less  work,  the  lift  becoming  very  little  indi 
until  at  last  the  weight  becomes  so  great  that  the  muscle  can- 
not lift  it  at  all  and  so  does  no  work  when  stimulated.  Starting 
then  from  the  time  when  the  muscle  carried  no  load  ami  did 
no  work,  we  pass  with  increasing  weights,  through  phases  in 
which  it  does  more  and  mure  work,  until  with  one  particular 
load  it  does  the  greatest  amount  possible  to  it  with  that  stim- 
ulus: after  that,  with  increasing  loads  less  work  is  done,  until 
finally  a  load  is  reached  with  which  the  muscle  again  does  no 
work.  What  is  true  of  one  muscle  is  of  course  true  of  all, 
and  what  is  true  of  work  done  against  gravity  is  true  of  all 
muscular  work,  so  that  there  is  one  precise  load  with  which 
a  beast  of  burden  or  a  man  can  do  the  greatest  possible 
amount  of  work  in  a  day.  With  a  lighter  or  heavier  load  the 
distance  through  which  it  can  be  moved  will  be  more  or  less, 
but  the  actual  work  done  always  less.  In  the  living  Body, 
however,  the  working  of  the  muscles  depends  so  much  on 
other  things,  as  the  due  action  of  the  circulatory  and  respira- 
tory systems  and  the  nervous  energy  or  "grit"  (upon  which 
the  stimulation  of  the  muscles  depends)  of  the  individual 
man  or  beast,  that  the  greatest  amount  of  work  obtainable  is 
not  a  simple  mechanical  problem  as  it  is  with  the  excised 
muscle. 

From  what  precedes  it  is  clear  that  the  molecular  changes 
which  take  place  in  a  contracting  muscle  fibre  are  eminently 
eptible  of  modification  by  slight  changes  in  its  environ- 
ment. The  evidence  indicates  that  the  contractility  of  a 
muscle  depends,  not  upon  a  vital  force  entirely  distinct  from 
ordinary  inanimate  forces,  but  upon  an  arrangement  of  its 
material  elements  which  is  only  maintained  under  certain 
conditions  and  is  eminently  modifiable  by  changes  in  the 
surroundinir-. 

Influence  of  the  Form  of  the  Muscle  on  its  Working 
Power.  The  amount  of  work  that  any  muscle  can  do  de- 
pends of  course  largely  upon  its  physiological  state;  a  healthy 
well -nourished  muscle  can  do  more  than  a  diseased  or  starved 
one;  but  allowing  for  such  variations  the  work  which  can  be 


THE  PROPERTIED   OF  MUSCULAR   TISSUE.  137 

done  by  a  muscle  varies  with  its  form.  The  thicker  the  mus- 
cle, that  is  the  greater  the  number  of  fibres  present  in  a  sec- 
tion made  across  the  long  axes  of  the  fasciculi,  the  greater 
the  load  that  can  be  lifted  or  the  other  resistance  that  can  be 
overcome.  On  the  other  hand,  the  extent  through  which  a 
muscle  can  move  a  weight  increases  with  the  length  of  its 
fasciculi.  A  muscle  a  foot  in  length  can  contract  more  than 
a  muscle  six  inches  Ions',  and  so  would  move  a  bone  through 
a  greater  distance,  provided  the  resistance  were  not  too  great 
for  its  strength.  But  if  the  shorter  muscle  had  double  the 
thickness,  then  it  could  lift  twice  the  weight  that  the  longer 
muscle  could.  We  find  in  the  Body  muscles  constructed  on 
both  plans :  some  to  have  a  great  range  of  movement,  others 
to  overcome  great  resistance,  besides  numerous  intermediate 
forms  which  cannot  be  called  either  long  and  slender  or  short 
and  thick:  many  short  muscles  for  example  are  not  speciallv 
thick,  but  are  short  merely  because  the  parts  on  which  they 
act  lie  near  together.  It  must  be  borne  in  mind,  too,  that 
many  apparently  long  muscles  are  really  short  stout  ones — 
those  namely  in  which  a  tendon  runs  down  the  side  or  middle 
of  the  muscle,  and  has  the  fibres  inserted  obliquelv  into  it. 
The  muscle  {gastrocnemius)  in  the  calf  of  the  leg  for  instance 
(Fig.  53,  B)  is  really  a  short  stout  muscle,  for  its  working 
length  depends  on  the  length  of  its  fasciculi  and  these  are 
short  and  oblique,  while  its  true  cross-section  is  that  at  right 
angles  to  the  fasciculi  and  is  considerable.  The  force  with 
which  a  muscle  can  shorten  is  very  great  A  frog's  muscle  of 
1  square  centimeter  (0.39  inch)  hi  section  can  just  lift  2S00 
grams  (98.5  ounces),  and  a  human  muscle  of  the  same  area 
more  than  twice  as  much. 

Muscular  Elasticity.  A  clear  distinction  must  be  made 
between  elasticity  and  contractility.  Elasticity  is  a  physical 
property  of  matter  in  virtue  of  which  various  bodies  tend  to 
■ne  or  retain  a  certain  shape,  and  when  removed  from  it, 
forcibly  to  return  to  it.  When  a  spiral  steel  spring  is  stretched 
it  will,  if  let  go,  '"'contract  n  in  a  certain  sense,  by  virtue  of  its 
elasticity,  but  such  a  contraction  is  clearly  quite  different 
from  a  muscular  contraction.  The  spring  will  only  contract 
afl  a  result  of  previous  distortion;  it  cannot  originate  a  change 
of  form,  while  the  muscle  can  actively  contract  or  change  its 
shape  when  a  stimulus  acts  upon  it.  and  that  without  being 
previously  stretched.     Ir  does  not  merely  tend  to  return 


138  THE  HUMAN  BODY. 

natural  shape  from  which  it  has  been  removed,  but  it  assumes 
a  quite  new  natural  shape,  so  that  physiological  contractility 
is  a  different  thing  from  mere  physical  elasticity;  the  essen- 
tial difference  being  that  the  coiled  spring  or  a  stretched  band 
only  gives  back  mechanical  work  which  has  already  been  spent 
on  it,  while  the  muscle  originates  work  independently  of  any 
previous  mechanical  stretching.  In  addition  to  their  contrac- 
tus v.  however,  muscles  are  highly  elastic.  If  a  fresh  muscle 
be  hung  up  and  its  length  measured,  and  then  a  weight  be 
hung  upon  it,  it  will  stretch  just  like  a  piece  of  india-rubber, 
and  when  the  weight  is  removed,  provided  it  has  not  been  so 
great  as  to  injure  the  muscle,  the  latter  will  return  passively, 
without  any  stimulus  or  active  contraction,  to  its  original 
form.  In  the  Body  all  the  muscles  are  so  attached  that  they 
are  usually  a  little  stretched  beyond  their  natural  resting 
length;  so  that  when  a  limb  is  amputated  the  muscles  divided 
in  the  stump  shrink  away  considerably.  By  this  stretched 
state  of  the  resting  elastic  muscles  two  things  are  gained.  In 
the  first  place  when  the  muscle  contracts  it  is  already  taut, 
there  is  no  "slack"  to  be  hauled  in  before  it  pulls  on  the 
parts  attached  to  its  tendons:  and,  secondly,  as  we  have 
already  seen  the  working  power  of  a  muscle  is  increased  by 
the  presence  of  some  resistance  to  its  contraction,  and  this  is 
always  provided  for  from  the  first,  by  having  the  origin  and 
insertion  of  the  muscles  so  far  apart  as  to  be  pulling  on  it 
when  it  begins  to  shorten. 

The  Electrical  Currents  of  Muscle.  When  a  muscle  is 
exposed  in  the  body  or  carefully  removed  from  it  and  suitable 
electrodes  connected  with  a  sensitive  galvanometer  are  applied 
to  different  parts  of  its  surface,  there  is  nearly  always  to  be 
found  evidence  of  a  difference  of  electric  potential  between 
different  parts  of  the  muscle.  These  differences  give  rise  to 
currents  which  are  shown  by  the  galvanometer  to  travel 
through  the  wires  of  the  circuit  from  any  central  portion  of 
the  muscle  to  any  part  nearer  one  end,  or  from  any  part  of  the 
belly  to  a  tendon.  The  less  injured  the'  muscle  the  more 
feeble  are  these  currents,  and  in  very  fresh  and  very  carefully 
exposed  muscles  they  may  be  absent  altogether.  They  are 
probably  altogether  absent  from  perfectly  uninjured  resting 
muscles,  and  when  present  in  a  resting  muscle  are  due  to  the 
fact  that  any  more  living  part  of  a  muscle  is  electrically  posi- 
tive to  a  more  injured  or  dead.     When  a  muscle  is  exposed 


THE  PROPERTIES  OF  MUSCULAR  TISSUE.         139 

its  thinner  ends  die  more  quickly  than  its  central  parts,  or 
the  ends  are  directly  injured  when  the  muscle  is  cut  across  to 
remove  it  from  the  animal;  and  in  that  way  the  currents  so 
usually  observable  arise.  When  all  of  a  muscle  is  dead,  its 
surface  is  isoelectric;  no  currents  can  be  led  off  from  it. 

Even  a  quite  uninjured  muscle  is  however,  capable,  of  giv- 
ing rise  to  currents  when  it  contracts,  and  these  currents 
pass  in  such  direction  as  to  show  that  a  portion  of  muscle 
in  contraction  is  electronegative  to  a  portion  at  rest.  If  a 
curarized  muscle  be  stimulated  at  one  point,  its  contraction 
commences  at  that  point  and  travels  from  it  over  the  remainder 
of  the  muscle;  so  that  by  the  time  a  distant  portion  is  in  con- 
traction the  part  which  just  contracted  has  come  to  rest.  By 
electrodes  suitably  applied  it  can  be  observed  that  immedi- 
ately after  the  stimulation  the  region  of  muscle  close  to  the 
point  of  stimulation  is  electro-negative  to  a  more  distant  part; 
but  that  afterwards,  when  a  distant  portion  is  in  contraction 
and  the  stimulated  region  has  returned  to  rest,  the  reverse  is 
the  case.  Electrically,  therefore,  any  contracting  part  of  a 
muscle  has  to  any  resting  part  a  relation  similar  to  that  of  a 
dying  or  injured  part  of  a  muscle  to  an  uninjured.  The  cur- 
rents which  arise  in  consequence  of  the  changes  going  on  in 
contracting  muscle  are  known  as  the  action  currents  to  dis- 
tinguish them  from  the  resting  currents  due  to  unequal  rates 
of  death  usually  found  between  different  parts  of  an  exposed 
muscle  in  rest. 

When  a  muscle  is  stimulated  through  its  nerve  the  action 
current  is  less  easy  to  demonstrate,  because  the  nerve  fibres 
branch  all  through  the  muscle  and  stimulate  all  parts  of  it  at 
once,  and  throw  all  simultaneously  into  contraction.  The  cur- 
rent may,  however,  be  shown  indirectly.  A  muscle  is  removed 
with  its  nerve  attached  and  electrodes  put  on  it — one,  for  ex- 
ample, on  the  middle  of  the  belly  and  the  other  on  the  tendon, 
so  as  to  show  on  the  galvanometer  a  resting  current.  If  the 
muscle  be  now  made  to  contract  by  stimulating  its  nerve  the 
current  is  diminished,  or,  as  is  said,  shows  a  negative  varia- 
tion. The  cause  of  this  is  as  follows:  The  amount  of  resting 
current  depends  on  the  difference  between  the  less  injured 
belly  of  the  muscle  and  the  injured  end;  anything  which 
makes  these  two  less  different  electrically  must  diminish  this 
current;  and  as  contracted  muscle  is  electrically  like  dying 
muscle,  when  we  throw  the  whole  into  activity  the  previously 


140  TIIE  HUMAN  BODY. 

existing  difference  is  less   than  it  was,  and   this  the  galva- 
nometer  shews  as  the  negative  variation. 

Secondary  Contraction.  It  is  possible  to  use  the  action 
current  of  one  muscle  to  stimulate  the  nerve  of  a  second  and 
produce  a  contraction.  For  this  purpose  two  frogs'  muscles, 
J  and  />',  are  carefully  dissected  out  with  their  nerves  at- 
tached. The  nerve  of  B  is  laid  over  A  so  that  one  part  of  it 
lies  on  t lie  belly  and  another  on  the  tendon.  If  the  nerve 
of  .1  be  stimulated  by  a  single  induction  shock,  for  each 
contraction  of  A  we  get  a  contraction  of  B,  the  negative 
variation  of  the  muscle  current  of  A  being  the  stimulus  for 
the  nerve  of  B. 

Secondary  Tetanus.  If  the  nerve  of  A  be  given  rapidly 
repeated  stimuli  so  as  to  throw  that  muscle  into  tetanic  con- 
traction, B  is  also  tetanized.  This  is  of  importance,  as  tend- 
ing to  show  that  the  tetanus  of  A  is  really  a  compound  con- 
traction, although  to  the  eye  or  as  recorded  by  a  lever  it  is  one 
unbroken  shortening.  If  the  electrical  condition  of  A 
remained  uniform  during  contraction,  there  should  be  no 
tetanus  of  B,  but  merely  a  simple  contraction  due  to  the  set- 
ting up  of  the  action  current  or  negative  variation  when  A 
commenced  to  contract,  and  a  second  due  to  the  cessation  of 
this  current  when  A  came  to  rest  again.  The  tetanus  of  B 
must  be  due  to  rapidly  repeated  electrical  variations  in  A,  and 
these  probably  correspond  to  the  potentially  separate  con- 
tractile changes  goiug  on  in  A,  and  fused  into  its  apparently 
uniform  tetanic  contraction. 

The  Source  of  Muscular  Energy  will  be  more  fully  dis- 
cussed in  the  chapter  on  nutrition,  but  a  few  of  the  main 
points  may  be  mentioned  here.  A  muscle  where  it  contracts 
is  able  to  do  work  by  using  energy  set  free  by  chemical 
changes  occurring  within  it,  as  a  steam-engine  does  work  by 
using  the  energy  set  free  by  the  chemical  changes  occurring 
in  the  combustion  of  its  fuel;  and  as  in  the  steam-engine, 
so  here,  the  fundamental  change  is  an  oxidative  one,  though  in 
the  muscle  a  very  indirect  oxidation.  A  fresh  frog's  muscle 
deprived  of  blood  contains  no  uncombined  ox}Tgen;  hung  up 
in  an  atmosphere  of  pure  nitrogen  it  can  be  made  to  contract 
and  do  a  great  deal  of  work  before  it  dies  and  passes  into 
rigor  mortis.  While  doing  this  work  it  gives  off  carbon-diox- 
ide gas  and  becomes  acid  from  the  formation  (probably)  of 
sarcolactic  acid,  but  there  does  not  appear  to  occur  any  ap- 


THE  PROPERTIES  OF  MUSCULAR   TISSUE.  141 

preciable  increase  of  oxygen-containing  nitrogen  compounds 
in  it.  As,  under  the  conditions  of  the  experiment,  no  free 
oxygen  is  available,  the  carbon  dioxide  must  be  derived  from 
the  breaking  down  of  something  present  in  the  muscle;  and 
as  the  formation  of  sarcolactic  acid  varies  in  amount  with 
that  of  carbon  dioxide,  and  both  increase  with  the  work  done 
by  the  muscle,  it  would  seem  as  if  the  energy  set  free  were 
obtained  by  the  breaking  down  of  some  highly  unstable 
non-nitrogenous  energy-yielding  matter  stored  in  the  muscle. 
And  such  a  view  gains  support  from  the  fact  that  a  man 
doing  hard  muscular  work  gives  off  per  hour  a  great  deal 
more  carbon  dioxide  through  his  lungs  than  a  man  at  rest, 
and  does  not  give  off  any  or  very  little  more  nitrogenous 
waste  matter. 

But  a  muscle  placed  as  above  described  and  made  to  work 
passes  into  rigor  sooner  than  a  muscle  similarly  situated  and 
left  at  rest:  and  this  shows  that  work  tends  to  favor  the  pro- 
duction of  myosin,  or  rather  of  its  immediate  precursor  myo- 
sinogen,  in  the  muscle:  so  here  we  get  some  evidence  that  the 
nitrogenous  muscle  constituents  are  influenced  and  altered 
though  not  oxidized  during  work.  Further,  when  a  muscle 
passes  into  rigor  it  gives  off  carbon-dioxide  gas,  whether  it 
has  been  worked  previously  or  not;  if  so  situated  as  to  be 
deprived  of  all  exterior  sources  of  supply,  it  gives  off  less 
when  becoming  rigid  after  work  than  when  becoming  rigid 
without  having  been  worked;  but  the  difference  is  almost 
accurately  accounted  for  by  the  greater  quantity  of  carbon 
dioxide  the  working  muscle  had  previously  given  out.  This 
suggests  that  the  chemical  phenomena  of  rigor  and  of  work 
are  essentially  alike,  being  merely  carried  to  an  extreme  in 
the  former. 

Most  of  the  facts  can  be  accounted  for  by  the  supposition 
that  there  is  in  living  muscle  a  store  of  an  unstable  substance 
containing  nitrogen,  hydrogen,  carbon,  and  oxygen.  For  this 
hypothetic  substance  the  name  inogen  has  been  proposed. 
Dnring  work  inogen  is  used  np  and  broken  into  a  highly 
oxidized  part,  carbon  dioxide;  an  oxidized  body  containing 
carbon  and  hydrogen,  as  sarcolactic  acid  (C3II603);  and  a 
third  body  allied  to  myosinogen  and  containing  all  the  nitro- 
gen  and  some  of  the  oxygen,  carbon,  and  hydrogen  of  the 
original  inogen.  Tn  the  products  of  this  alteration  stronger 
chemical  affinities  are  satisfied  than  in  the  original  compound, 


142  THE  HUMAN  BODY. 

and  thus  energy  is  liberated  and  used  by  the  muscle.  In  the 
ordinary  course  of  events  the  carbon  dioxide  is  carried  off  by 
blood  and  lymph  and  eliminated  from  the  Body;  the  sarco- 
lactic  or  other  similar  substance  or  substances  are  also  carried 
off  and  oxidized  elsewhere  to  form  carbon  dioxide  and  water 
and  be  then  eliminated;  but  the  nitrogen-containing  product 
remains  behind,  and  with  the  help  of  fresh  oxygen  and  of 
other  food  material  brought  by  the  blood  is  reconstructed  into 
the  original  inogen.  In  the  excised  muscle  there  is  but 
scant  store  of  material  for  repair;  carbon  dioxide  is  given 
off  when  the  muscle  contracts,  and  the  sarcolactic  acid  and 
nitrogen-containing  product  accumulate:  the  latter  then 
undergoes  further  changes,  and  ultimately  becomes  myosin. 
If  the  excised  muscle  be  thrown  into  rigor  quickly  (as  by 
heat),  then  the  inogen  is  at  once  broken  up,  forming  myosin 
and  carbon  dioxide  and  sarcolactic  acid:  if  it  be  worked 
for  a  time  before  being  thrown  into  rigor,  then  some  of  its 
inogen  will  have  been  already  broken  up,  so  there  will  be  less 
to  give  rise  to  carbon  dioxide  at  the  moment  of  rigor,  but  the 
missing  amount  is  found  in  that  given  off  during  work.  If 
some  such  view  as  this,  which  may  be  called  the  "  inogen 
theory,"  be  the  correct  one,  then  the  energy  liberated  by  a 
resting  muscle  passing  into  rigor  must  take  some  other  form 
than  muscular  work.  As  a  matter  of  fact  a  good  deal  of  heat 
is  liberated  during  death  stiffening,  but  whether  sufficient  to 
account  for  all  the  missing  energy  is  by  no  means  clear.  The 
whole  subject  of  the  immediate  source  of  muscular  work  is 
still  in  much  need  of  elucidation. 

Physiology  of  Plain  Muscular  Tissue.  What  has  hither- 
to been  said  applies  especially  to  the  skeletal  muscles;  but 
in  the  main  it  is  true  of  the  unstriped  muscles.  These  also 
are  irritable  and  contractile,  but  their  changes  of  form  are 
much  more  slow  than  those  of  the  striated  fibres.  Upon 
stimulation,  a  longer  period  of  latent  excitement  elapses 
before  the  contraction  commences  and  when,  finally,  this 
takes  place  it  is  comparatively  very  slow,  gradually  attaining 
a  maximum  and  gradually  passing  away. 

Unstriped  muscular  tissue  has  a  remarkable  power  of 
remaining  in  the  contracted  state  for  long  periods:  the  mus- 
cular coats  of  many  small  arteries,  for  example,  are  rarely 
relaxed ;  sometimes  they  may  be  more  contracted,  sometimes 
less,  but  in  health  seldom  if  ever  completely  at  rest.     There 


THE  PROPERTIES  OF  MUSCULAR   TISSUE.  143 

seems  to  be  some  connection  between  that  arrangement  of 
the  contractile  substance  which  shows  itself  under  the  micro- 
scope as  striation  and  the  power  of  rapid  contraction,  since 
we  find  that  the  heart,  which  is  not  a  skeletal  or  voluntary 
muscle  but  yet  one  that  contracts  rapidly,  agrees  with  these 
in  having  its  fibres  striated.  This  connection  is  further  illus- 
trated by  facts  of  comparative  anatomy:  insects  are,  as  a 
rule,  rapidly  moving  animals,  and  they  are  characterized  by 
very  marked  striation  of  nearly  all  their  muscular  tissue; 
while  in  the  slow-moving  molluscs  nearly  all  the  muscular 
tissue  is  unstriped  except  in  a  few,  as  Pecten,  which  make 
rapid  movements,  and  in  that  genus  the  muscles  concerned 
in  producing  these  movements  are  striated. 


CHAPTER  XL 

MOTION   AND   LOCOMOTION. 

The  Special  Physiology  of  the  Muscles.  Having  now 
considered  separately  the  structure  and  properties  in  general 
of  the  skeleton,  the  joints,  and  the  muscles,  we  may  go  on  to 
consider  how  they  all  work  together  in  the  Body.  Although 
the  properties  of  muscular  tissue  are  everywhere  the  same, 
the  uses  of  different  muscles  are  very  varied,  by  reason  of  the 
different  parts  with  which  they  are  connected.  Some  are 
muscles  of  respiration,  others  of  deglutition;  many  are  known 
as  flexors  because  they  bend  joints,  others  as  extensors  because 
they  straighten  them.  The  exact  use  of  any  particular  mus- 
cle, acting  alone  or  in  concert  with  others,  is  known  as  its 
special  physiology,  as  distinguished  from  its  general  physiol- 
rx/ //,  or  properties  as  a  muscle  without  reference  to  its  use  as 
a  muscle  in  a  particular  place.  The  functions  of  those  mus- 
cles forming  parts  of  the  physiological  mechanisms  concerned 
in  breathing  and  swallowing  will  be  studied  hereafter;  for 
the  present  we  may  consider  the  muscles  which  co-operate  in 
maintaining  postures  of  the  Body;  in  producing  movements 
of  its  larger  parts  with  reference  to  one  another;  and  in  pro- 
ducing locomotion  or  movement  of  the  whole  Body  in  space. 

In  nearly  all  cases  the  striped  muscles  carry  out  their  func- 
tions with  the  co-operation  of  the  skeleton,  since  nearly  all 
are  fixed  to  bones  at  each  end,  and  when  they  contract  pri- 
marily move  these,  and  only  secondarily  the  soft  parts  attached 
to  them.  To  this  general  rule  there  are,  however,  exceptions. 
The  muscle  for  example  which  lifts  the  upper  eyelid  and 
opens  the  eye  arises  from  bone  at  the  back  of  the  orbit,  but 
is  inserted,  not  into  bone,  but  into  the  eyelid  directly;  and 
similarly  other  muscles  arising  at  the  back  of  the  orbit  are 
directly  fixed  to  the  eyeball  in  front  and  serve  to  rotate  it 
on  the  pad  of  fat  on  which  it  lies.  Many  facial  muscles  again 
have  no  direct  attachment  whatever  to  bones,  as  for  example 

144 


MOTION  AND  LOCOMOTION.  145 

the  muscle  (orbicularis  oris)  which  surrounds  the  mouth- 
opening,  and  by  its  contraction  narrows  it  and  purses  out  the 
lips;  or  the  orbicularis  palpebrarum  which  similarly  sur- 
rounds the  eyes  and  when  it  contracts  closes  them. 

Levers  in  the  Body.  When  the  muscles  serve  to  move 
bones  the  latter  are  in  nearly  all  cases  to  be  regarded  as  levers 
whose  fulcra  lie  at  the  joint  where  the  movement  takes  place. 
Examples  of  all  the  three  forms  of  levers  recognized  in  me- 
chanics are  found  in  the  Human  Body. 

Levers  of  the  First  Order.  In  this  form  (Fig.  63)  the 
fulcrum  or  fixed  point  of  support  lies  between  the  "  weight " 


F 


P  jJ^  W 

Fig.   63.— A  lever  of  the  first  order.    F,  fulcrum  ;  P,  power  ;  W,  resistance  or 

weight. 

or  resistance  to  be  overcome  and  the  "  power  "  or  moving 
force,  as  shown  in  the  diagram.  The  distance  PF,  from  the 
power  to  the  fulcrum,  is  called  the  "power-arm;"  the  dis- 
tance FW  is  the  "  weight-arm."  When  power-arm  and 
weight-arm  are  equal,  as  is  the  case  in  the  beam  of  an  ordi- 
nary pair  of  scales,  no  mechanical  advantage  is  gained,  nor  is 
there  any  loss  or  gain  in  the  distance  through  which  the  weight 
is  moved.  For  every  inch  through  which  P  is  depressed,  W 
will  be  raised  an  equal  distance.  When  the  power-arm  is 
longer  than  the  other,  then  a  smaller  force  at  P  will  raise  a 
larger  weight  at  W,  the  gain  being  proportionate  to  the  dif- 
ference in  the  lengths  of  the  arms.  For  example  if  PF  is 
twice  as  long  as  FW,  then  half  a  kilogram  applied  at  P  will 
balance  a  whole  kilogram  at  W,  and  just  more  than  half  a 
kilogram  would  lift  it ;  but  for  every  centimeter  through 
which  P  descended,  If  would  only  be  lifted  half  a  centimeter. 
On  the  other  hand  when  the  weight-arm  in  a  lever  is  longer 
than  the  power-arm,  there  is  loss  in  force  but  a  gain  in  the 
distance  through  which  the  weight  is  moved. 

Examples  of  the  first  form  of  lever  are  not  numerous  in 
the  Human  Body.  One  is  afforded  in  the  nodding  move- 
ments of  the  head,  the  fulcrum  being  the  articulations  be- 
tween the  skull  and  the  atlas.  When  the  chin  is  elevated 
the  power  in  applied  to  tin;  .skull,  behind  the    fulcrum,  by 


146  THE  HUMAN  BODY. 

small  muscles  passing  from  the  vertebral  column  to  the  occi- 
put; the  resistance  is  the  excess  in  the  weight  of  the  part  of 
the  head  in  front  of  the  fulcrum  over  that  behind  it,  and  is 
not  great.  To  depress  the  chin  as  in  nodding  does  not  neces- 
sarily call  for  any  muscular  effort,  as  the  head  will  fall  for- 
ward of  itself  if  the  muscles  keeping  it  erect  cease  to  work, 
as  those  of  us  who  have  fallen  asleep  during  a  dull  discour.se 
on  a  hot  day  have  learnt.  If  the  chin  however  be  depressed 
forcibly,  as  in  the  athletic  feat  of  suspending  one's  self  by 
the  chin,  the  muscles  passing  from  the  chest  to  the  skull  in 
front  of  the  atlanto-occipital  articulation  are  called  into  play. 
Another  example  of  the  employment  of  the  first  form  of  lever 
in  the  Body  is  afforded  by  the  curtsey  with  which  a  lady 
salutes  another.  In  curtseying  the  trunk  is  bent  forward  at 
the  hip-joints,  which  form  the  fulcrum;  the  weight  is  that  of 
the  trunk  acting  as  if  all  concentrated  at  its  centre  of  gravity, 
which  lies  a  little  above  the  sacrum  and  behind  the  hip-joints; 
and  the  power  is  afforded  by  muscles  passing  from  the  thighs 
to  the  front  of  the  pelvis. 

Levers  of  the  Second  Order.  In  this  form  the  weight  or 
resistance  is  between  the  power  and  the  fulcrum.  The 
power-arm  PF  is  always  longer  than  the  weight-arm  WF, 
and  so  a  comparatively  weak  force  can  overcome  a  consider- 
able resistance.  But  it  is  disadvantageous  so  far  as  regards 
rapidity  and  extent  of  movement,  for  it  is  obvious  that  when 
P  is  raised  a  certain  distance  W  will  be  moved  a  less  distance 
in  the  same  time.  As  an  example  of  the  employment  of  such 
levers  (Fig.  64)  in  the  Body,  we  may  take  the  act  of  standing 
on  the  toes.  Here  the  foot  represents  the  lever,  the  fulcrum 
is  at  the  contact  of  its  fore  part  with  the  ground  ;  the  weight 


F 


P 

*  W 


Fig.  64.— A  lever  of  the  second  order.    F,  fulcrum  ;  P,  power  :  W,  weight.    The 
arrows  indicate  the  direction  in  which  the  forces  act. 

is  that  of  the  Body  acting  down  through  the  ankle-joints  at 
Ta,  Fig.  65 ;  and  the  power  is  the  great  muscle  of  the  calf 
acting  by  its  tendon  inserted  into  the  heel-bone  (Ca,  Fig.  65). 
Another  example  is  afforded  by  holding  up  the  thigh  when 
one  foot  is  kept  raised  from  the  ground,  as  in  hopping  on  the 


MOTION  AND  LOCOMOTION. 


147 


other.     Here  the  fulcrum  is  at  the  hip-joint,  the  power  is  ap- 
plied at  the  knee-cap  by  a  great  muscle  {rectus  femor is)  which 


Fig  65. — The  skeleton  of  the  foot  from  the  outer  side.  T<t.  surface  with  which 
the  leg-bones  articulate  ;  C'a,  the  ealcatieum  into  which  the  tendon  {tendo  Achillis} 
of  the  calf  muscle  is  inserted  ;  M5,  the  metatarsal  bone  of  the  fifth  digit  ;  N,  the 
scaphoid  bone  ;  CI,  CII,  CHI,  first,  second,  and  third  cuneiform  bones  ;  Cb,  the 
cuboid  bone. 

is  inserted  there  and  arises  front  the  pelvis;  and  the  weight 
is  that  of  the  whole  lower  limb  acting  at  its  centre  of  gravity,. 
which  lies  somewhere  in  the  thigh  between  the  hip  and 
knee-joints,  that  is  between  the  fulcrum  and  the  point  of  ap- 
plication of  the  power. 

Levers  of  the  Third  Order.  In  these  (Fig.  6C)  the  power- 
is  between  the  fulcrum  and  the  weight.  In  such  levers  the 
weight-arm  is  always  longer  than  the  power-arm,  so  the  power 
works  at  a  mechanical  disadvantage,  but  swiftness  and  range- 
of  movement  are  gained.  It  is  the  lever  most  commonly  used 
in  the  Human  Body.  For  example,  when  the  forearm  is- 
bent  up  towards  the  arm,  the  fulcrum  is  the  elbow-joint,  the- 
power  is  applied  at  the  insertion  of  the  biceps  muscle  (Fig. 
52)  into  the  radius  and  of  another  muscle  (not  represented 
in  the  figure,  the  braehialis  anticus,  into  the  ulna),  and  the 


W 


F 


FlO.  66.     A  lever  of  the  third  order.     V,  fulcrum  ;  P.  power  ;   W,  weight. 

weight  is  thai  of  the  forearm  and  hand,  with  whatever  may 
be  contained  in  the  latter,  acting  at  the  centre  of  gravity  of 
the  whole  somewhere  on  the  distal  side  of  the  point  of  appli- 


148  '////<;  HUMAN  BODY. 

cation  of  the  power.  In  the  Body  the  power-arm  is  usually 
very  short  so  iis  to  gain  speed  and  range  of  movement,  the 
muscles  being  powerful  enough  to  -till  do  their  work  in  spite 
of  the  mechanical  disadvantage  at  which  they  are  then  placed. 

The  limbs  are  thus  made  much  more  shapely  than  would  be 
the  case  were  the  power  applied  near  or  beyond  the  weight. 

It  is  of  course  only  rarely  that  simple  movements  as  those 
described  above  take  place.  In  the  greal  majority  of  those 
executed  several  or  many  muscles  co-operate. 

The  Loss  to  the  Muscles  from  the  Direction  of  their  Pull. 
It  is  worthy  of  note  that,  owing  to  the  oblique  direction  in 
which  the  muscles  are  commonly  inserted  into  the  bones, 
much  of  their  force  is  lost  so  far  as  producing  movement  is 
concerned.  Suppose  the  log  of  wood  in  the  diagram  (Fig. 
67)  to  be  raised  by  pulling  on  the  rope  in  the  direction  a;  it 
is  clear  at  first  that  the  rope  will  act  at  a  great  disadvantage; 
most  of  the  pull  transmitted  by  it  will  be  exerted  against  the 
pivot  on  which  the  log  hinges,  and  only  a  small  fraction  be 
available  for  elevating  the  latter.  But  the  more  the  log  is 
lifted,  as  for  example  into  the  position  indicated  by  the  dotted 
lines,  the  more  useful  will  be  the  direction  of  the  pull,  and  the 
more  of  it  will  be  spent  on  the  log  and  the  less  lost  unavail- 
ing}7 in  merely  increasing  the  pressure  at  the  hinge.  If  we 
now  consider  the  action  of  the  biceps  (Fig.  52)  in  flexing  the 
elbow-joint,  we  see  similarly  that  the  straighter  the  joint  is, 
the  more  of  the  pull  of  the  muscle  is  wasted.     Beginning 


Fig.  67.— Diagram  illustrating  the  disadvantage  of  an  oblique  pull. 

with  the  arm  straight,  it  works  at  a  great  disadvantage,  but 
as  the  forearm  is  raised  the  conditions  become  more  and  more 


MOTION  AND  LOCOMOTION.  149 

favorable  to  the  muscle.  Those  who  have  practised  the  gym- 
nastic feat  of  raising  one's  self  by  bending  the  elbows  when 
hanging  by  the  hands  from  a  horizontal  bar  know  practically 
that  if  the  elbow-joints  are  quite  straight  it  is  very  hard  to 
start;  and  that,  on  the  other  hand,  if  they  are  kept  a  little 
flexed  at  the  beginning  the  effort  needed  is  much  less;  the 
reason  being  of  course  the  more  advantageous  direction  of 
traction  by  the  biceps  in  the  latter  case. 

Experiment  proves  that  the  power  with  which  a  muscle 
can  contract  is  greatest  at  the  commencement  of  its  short- 
ening, the  very  time  at  which,  we  have  just  seen,  it  works 
at  most  mechanical  disadvantage;  in  proportion  as  its  force 
becomes  less  the  conditions  become  more  favorable  to  it. 
There  is,  however,  it  is  clear,  nearly  always  a  considerable 
loss  of  power  in  the  working  of  the  skeletal  muscles,  strength 
being  sacrificed  for  variety,  ease,  rapidity,  extent,  and  ele- 
gance of  movement. 

Postures.  The  term  posture  is  applied  to  those  positions 
of  equilibrium  of  the  Body  which  can  be  maintained  for  some 
time,  such  as  standing,  sitting,  or  lying,  compared  with  leap- 
ing, running,  or  falling.  In  all  postures  the  condition  of 
stability  is  that  the  vertical  line  drawn  through  the  centre  of 
gravity  of  the  body  shall  fall  within  the  basis  of  support 
afforded  by  objects  with  which  it  is  in  contact  ;  and  the 
security  of  the  posture  is  proportionate  to  the  extent  of  this 
base,  for  the  wider  it  is  the  less  is  the  risk  of  the  perpendicu- 
lar through  the  centre  of  gravity  falling  outside  of  it  on  slight 
displacement. 

The  Erect  Posture.  This  is  pre-eminently  characteristic 
of  man,  his  whole  skeleton  being  modified  with  reference  to 
it.  Nevertheless  the  power  of  maintaining  it  is  only  slowly 
learnt  in  the  first  years  after  birth,  and  for  a  long  while  it  is 
unsafe.  And  though  finally  we  learn  to  stand  erect  without 
conscious  attention,  the  maintenance  of  that  posture  always 
requires  the  co-operation  of  many  muscles,  co-ordinated  by 
the  nervous  system.  The  influence  of  the  latter  is  shown  by 
the  fall  which  follows  a  severe  blow  on  the  head,  which  may 
nevertheless  have  fractured  no  bone  nor  injured  any  muscle: 
the  concussion  of  the  brain,  as  we  say,  "stuns"  the  man, 
and  until  its  effects  have  passed  oil'  he  cannot  stand  upright. 
In  standing  with  the  arms  straight  by  the  sides  ami  the  feet 
i  her  the  centre  of  gravity  of  the  whole  adult  Body  lies 


i;.<» 


THE  HUMAN  BODY. 


in  the  articulation  between  the  sacrum  and  the  last  lumbar 
vertebra,  ami  the  perpendicular  drawn  from  it  will  reach  the 
ground  between  the  two  feet,  within  the  basis  of  support  af- 
forded by  them.  With  the  feet  rinse  together,  however,  the 
posture  is  not  very  stable,  and  in  standing  we  commonly 
make  it  more  so  by  .-lightly  separating  them  so  as  to  increase 
the  base.  The  more  one  foot  is  in  front 
of  the  other  the  more  swaying  back  and 
forward  will  be  compatible  with  safety:  and 
the  greater  the  lateral  distance  separating 
them  the  greater  will  be  the  lateral  sway 
which  is  possible  without  falling.  Conse- 
quently we  see  that  ;i  man  about  to  make 
great  movements  with  the  upper  part  of 
his  Body,  as  in  fencing  or  boxing,  or  a  sol- 
dier preparing  for  the  bayonet  exercise, 
always  commences  by  thrusting  one  foot 
forwards  obliquely,  so  as  to  increase  his 
basis  of  support  in  both  directions. 

The  ta.-e  with  which  we  can  stand  is 
largely  dependent  upon  the  way  in  which 
the  head  is  almost  balanced  on  the  top  of 
the  vertebra]  column,  so  that  but  little 
muscular  effort  is  needed  to  keep  it  up- 
right. In  the  same  way  the  trunk  is  almost 
balanced  on  the  hip  joints,  but  not  quite, 
its  centre  of  gravity  falling  rather  behind 
them;  so  that  just  as  some  muscular  effort 
is  needed  to  keep  the  head  from  falling 
forwards, some  is  needed  to  keep  the  trunk 
from  toppling  backwards  at  the  hips.  In 
a  similar  manner  other  muscles  are  called 
into  play  at  other  joints:  as  between  the 
lines)  which  pass  before  vertebral   column   and  the  pelvis,  and  at 

and    behind    the    joints  x 

and  by  their  balanced  the  knees  and  ankles;  and  thus  a  certain 

activity    keep    the  joints        ..-.._  t  a- 

rigid  and  the  body  erect,  rigidity,  due  to  muscular  effort,  extends  all 
along  the  erect  Body:  which,  on  account  of  the  flexibility  of  its 
joints,  could  not  otherwise  be  balanced  on  its  feet,  as  a 
statue  can.  Beginning  (Fig.  68)  at  the  ankle-joint,  we  find 
it  kept  stiff  in  standing  by  the  combined  and  balanced  con- 
traction of  the  muscles  passing  from  the  heel  to  the  thigh, 
and  from  the  dorsum  of  the  foot  to  the  shin-bone  (tibia). 


II 


Fig.  68.—  Dia<rram  il 
lustratinjj  the  muscles 
(drawn    in    thick    black 


MOTION  AND  LOCOMOTION  151 

Others  passing  before  and  behind  the  knee-joint  keep  it  from 
yielding;  and  so  at  the  hip-joints:  and  others  again,  lying  in 
the  walls  of  the  abdomen  and  along  the  vertebral  column, 
keep  the  latter  rigid  and  erect  on  the  pelvis;  and  finally  the 
skull  is  kept  in  position  by  muscles  passing  from  the  sternum 
and  vertebral  column  to  it,  in  front  of  and  behind  the  occipi- 
tal condyles. 

Locomotion  includes  all  the  motions  of  the  whole  Body 
in  space,  dependent  on  its  own  muscular  efforts:  such  as 
walking,  running,  leaping,  and  swimming. 

"Walking.  In  walking  the  Body  never  entirely  quits  the 
ground,  the  heel  of  the  advanced  foot  touching  the  ground  in 
each  step  before  the  toe  of  the  rear  foot  leaves  it.  The  ad- 
vanced limb  supports  the  Body,  and  the  foot  in  the  rear  at 
the  commencement  of  each  step  propels  it. 

Suppose  a  man  standing  with  his  heels  together  to  com- 
mence to  walk,  stepping  out  with  the  left  foot;  the  whole 
Body  is  at  first  inclined  forwards,  the  movement  taking  place 
mainly  at  the  ankle-joints.  By  this  means  the  centre  of 
gravity  would  be  thrown  in  front  of  the  base  formed  by  the 
feet  and  a  fall  on  the  face  result,  were  not  simultaneously  the 
left  foot  slightly  raised  by  bending  the  knee  and  then  swung 
forwards,  the  toes  just  clear  of  the  ground  and,  in  good 
walking,  the  sole  nearly  parallel  to  it.  "When  the  step  is 
completed  the  left  knee  is  straightened  and  the  sole  jilaced 
on  the  ground,  the  heel  touching  it  first,  and  the  base  of  sup- 
port being  thus  widened  from  before  back,  a  fall  is  prevented. 
Meanwhile  the  right  leg  is  kept  straight,  but  inclines  for- 
wards above  with  the  trunk  when  the  latter  advances,  and  as 
this  occurs  the  sole  gradually  leaves  the  ground,  commencing 
with  the  heel.  When  the  step  of  the  left  leg  is  completed  the 
great  toe  of  the  right  alone  is  in  contact  with  the  support. 
With  this  a  push  is  given  which  sends  the  trunk  on  over  the 
left  leg,  which  is  now  kept  rigid,  except  at  the  ankle-joint; 
and  the  right  knea  being  bent  that  limb  swings  forwards, 
its  for,!  just  clearing  the  ground  as  the  left  did  before.  The 
Pxidy  is  meanwhile  supported  on  the  left  foot  alone,  hut  when 
the  right  completes  its  step  the  knee  of  that  leg  is  straight- 
ened and  the  foot  thus  placed,  heel  first,  on  the  ground. 
Meanwhile  the  left  fool  haa  been  gradually  leaving  the 
ground,  and  its  toes  only  are  al  thai  momenl  upon  it:  from 
these  a  push  w  given,  as  before,  with  the  right  foot,  and  the 


152  THE  HUMAN  BODY. 

knee  being  bent  so  as  to  raise  the  foot,  the  left  leg  swings  for 
wards  at  the  hip-joint  to  make  a  fresh  step. 

During  each  step  the  whole  Body  sways  up  and  down 
and  also  from  side  to  side.  It  is  highest  at  the  mo- 
ment when  the  advancing  trunk  is  vertically  over  the 
foot  supporting  it,  and  then  sinks  until  the  moment 
when  the  advancing  foot  touches  the  ground,  when  it  is 
lowest.  From  this  moment  it  rises  as  it  swings  forward 
on  this  foot,  until  it  is  vertically  over  it,  and  then  sinks 
again  until  the  other  touches  the  ground;  and  so  on.  At 
the  same  time,  as  its  weight  is  alternately  transferred  from 
the  right  to  the  left  foot  and  vice  versa,  there  is  a  slight 
lateral  sway,  commonly  more  marked  in  women  than  in  men, 
and  which  when  excessive  produces  an  ugly  "waddling" 
gait. 

The  length  of  each  step  is  primarily  dependent  on  the 
length  of  the  legs;  but  can  be  controlled  within  wide  limit- 
by  special  muscular  effort.  In  easy  walking  little  muscular 
work  is  employed  to  carry  the  rear  leg  forwards  after  it  has 
given  its  push.  When  its  foot  is  raised  from  the  ground  it 
swings  on,  like  a  pendulum;  but  in  fast  walking  the  muscles, 
passing  in  front  of  the  hip-joint,  from  the  pelvis  to  the  limb. 
by  their  contraction  forcibly  carry  the  leg  forwards.  The 
easiest  step,  that  in  which  there  is  most  economy  of  labor,  is 
that  in  which  the  limb  is  let  swing  freely,  and  since  a  short 
pendulum  swings  faster  than  a  longer,  the  natural  step  of 
short-legged  people  is  quicker  than  that  of  long-legged  ones. 

hi  fast  walking  the  advanced  or  supporting  leg  also  aids  in 
propulsion;  the  muscles  passing  in  front  of  the  ankle-joint 
contracting  so  as  to  pull  the  Body  forwards  over  that  foot 
and  aid  the  push  from  the  rear  foot.  Hence  the  fatigue  and 
pain  in  front  of  the  shin  which  is  felt  in  prolonged,  very  fast 
walking.  From  the  fact  that  each  foot  reaches  the  ground 
heel  first,  but  leaves  it  toe  last,  the  length  of  each  stride  is 
increased  by  the  length  of  the  foot. 

Running.  In  this  mode  of  progression  there  is  a  moment 
in  each  step  when  both  feet  are  off  the  ground,  the  Body 
being  unsupported  in  the  air.  The  toes  alone  come  in  con- 
tact with  the  ground  at  each  step,  and  the  knee-joint  is  not 
straight  when  the  foot  reaches  the  ground.  When  the  rear 
foot  is  to  leave  the  support,  the  knee  is  suddenly  straight- 
ened, and  at  the  same  time  the  ankle-johit  is  extended  so  as 


MOTION  AND  LOCOMOTION.  153 

to  push  the  toes  forcibly  on  the  ground  ami  give  the  whole 
Body  a  powerful  push  forwards  and  upwards.  Immediately 
after  this  the  knee  is  greatly  flexed  and  the  foot  raised  from 
the  ground,  and  this  occurs  before  the  toes  of  the  forward 
foot  reach  the  latter.  The  swinging  leg  in  each  step  is  vio- 
lently pulled  forwards  and  not  suffered  to  swing  naturally,  as 
in  walking.  By  this  the  rapidity  of  the  succession  of  steps 
is  increased,  and  at  the  same  time  the  stride  is  made  greater 
by  the  sort  of  one-legged  leap  that  occurs  through  the  jerk 
given  by  the  straightening  of  the  knee  of  the  rear  leg  just 
before  it  leaves  the  ground. 

Leaping.  In  this  mode  of  progression  the  Body  is  raised 
completely  from  the  ground  for  a  considerable  period.  In  a 
powerful  leap  the  ankles,  knees,  and  hip-joints  are  all  flexed 
as  a  preparatory  measure,  so  that  the  Body  assumes  a  crouch- 
ing attitude.  The  heels,  next,  are  raised  from  the  ground  and 
the  Body  balanced  on  the  toes.  The  centre  of  gravity  of  the 
Body  is  then  thrown  forwards,  and  simultaneously  the  flexed 
joints  are  straightened,  and  by  the  resistance  of  the  ground, 
the  Body  receives  a  propulsion  forwards;  much  in  the  same 
way  as  a  ball  rebounds  from  a  wall.  The  arms  are  at  the 
same  time  thrown  forwards.  In  leaping  backwards,  the  Body 
and  arms  are  inclined  in  that  direction;  and  in  jumping  ver- 
tically there  is  no  leaning  either  way  and  the  arms  are  kept 
by  the  sides. 

Hygiene  of  the  Muscles.  The  healthy  working  of  the 
muscles  needs  of  course  a  healthy  state  of  the  Body  gener- 
ally, so  that  they  shall  be  supplied  with  proper  materials  for 
growth  and  repair,  and  have  their  wastes  rapidly  and  effi- 
ciently removed.  In  other  words,  good  food  and  pure  air  are 
necessary  for  a  vigorous  muscular  system,  a  fact  which  train- 
ers recognize  in  insisting  upon  a  strict  dietary,  and  in  super- 
vising generally  the  mode  of  life  of  those  who  are  to  engage 
in  athletic  contests.  The  muscles  should  also  not  be  exposed 
to  any  considerable  continued  pressure,  since  this  interferes 
with  the  flow  of  blood  and  lymph  through  them. 

Afi  far  as  the  muscles  themselves  are  directly  concerned, 
exercise  is  the  necessary  condition  of  their  best  development. 
A  muscle  which  is  permanently  unused  degenerates  and  is 
absorbed,  little  finally  being  left  but  the  connective  tissue  of 
the  organ  and  a  few  muscle  fibres  filled  with  oil-drops.  This 
is  well  seen  in  cases  of  paralysis  dependent  on  injury  to  the 


154  TIIE  HUMAN  BODY. 

nerves.  In  such  cases  the  muscles  may  themselves  be  per- 
fectly healthy  at  first,  but  lying  unused  for  weeks  they  become 
altered,  and  finally,  when  the  nervous  injury  lias  been  healed, 
the  muscles  may  be  found  incapable  of  functional  activity. 
The  physician  therefore  is  often  careful  to  avoid  this  by  exer- 
cising the  paralyzed  muscles  daily  by  means  of  electrical 
shocks  sent  through  the  part,  while  at  the  same  time  he  tries 
to  restore  the  nerves;  passive  exercise,  as  by  proper  massage, 
is  frequently  of  great  use  in  such  cases.  The  same  fact  is 
illustrated  by  the  feeble  and  wasted  condition  of  the  muscles 
of  a  limb  which  has  been  kept  for  some  time  in  splints.  After 
the  latter  have  been  removed  it  is  only  slowly,  by  judicious 
and  persistent  exercise,  that  the  long-idle  muscles  regain 
their  former  size  and  power.  The  great  muscles  of  the 
"brawny"  arm  of  the  blacksmith  or  wrestler  illustrate  the 
reverse  fact,  the  growth  of  the  muscles  by  exercise.  Exer- 
cise, however,  must  be  judicious;  repeated  frequently  to  the 
point  of  exhaustion  it  does  harm;  the  period  of  repair  is  not 
sufficient  to  allow  replacement  of  the  parts  used  in  work,  and 
the  muscles  thus  waste  under  too  violent  exercise  as  with  too 
little.  Rest  should  alternate  with  work,  and  that  regularly, 
if  benefit  is  to  be  obtained.  Moreover,  violent  exercise  should 
never  be  suddenly  undertaken  by  one  unused  to  it,  not 
only  lest  the  muscles  suffer,  but  because  muscular  effort 
greatly  increases  the  work  of  the  heart,  not  merely  because 
more  blood  has  to  be  sent  to  the  muscles  themselves,  but  they 
produce  great  quantities  of  carbon  dioxide,  which  must  be 
carried  off  in  the  blood  to  the  lungs  for  removal  from  the 
Body,  and  the  heart  must  work  harder  to  send  the  blood  faster 
through  the  lungs,  and  at  the  same  time  the  breathing  be 
hastened  so  as  to  renew  the  air  in  those  organs  faster.  The 
least  evil  result  of  throwing  too  violent  work  on  the  heart 
and  lungs  in  this  way  is  represented  by  being  "  out  of 
breath,"  which  is  advantageous  insomuch  as  it  may  lead  to  a 
cessation  of  the  exertion.  But  much  more  serious,  and 
sometimes  permanent,  injuries  of  either  the  circulatory  or 
respiratory  organs  may  be  caused  by  violent  and  prolonged 
efforts  without  due  previous  training.  No  general  rule  can 
be  laid  down  as  to  the  amount  of  exercise  to  be  taken;  for  a 
healthy  man  in  business  the  minimum  would  perhaps  be  rep- 
resented by  a  daily  walk  of  five  miles. 

Varieties  of  Exercise.      In    walking   and    running:   the 


MOTION  AND  LOCOMOTION.  155 

muscles  chiefly  employed  are  those  of  the  lower  limbs  and 
trunk.  This  is  in  part  true  of  rowing,  which  when  good  is 
performed  much  more  by  the  legs  than  the  arms:  especially 
since  the  introduction  of  sliding  seats.  Hence  any  of  these 
exercises  alone  is  apt  to  leave  the  muscles  of  the  chest  and 
arms  imperfectly  exercised.  Indeed,  no  one  exercise  employs 
equally  or  proportionately  all  the  muscles:  therefore  gym- 
nasia in  which  various  feats  of  agility  are  practised,  so  as  to 
call  different  parts  into  play,  have  very  great  utility.  It 
should  be  borne  in  mind,  however,  that  the  legs  especially 
need  strength;  while  the  upper  limbs, in  which  delicacy  of 
movement,  as  a  rule,  is  more  desirable  than  power,  do  not  re- 
quire so  much  exercise;  and  the  fact  that  gymnastic  exercises 
are  commonly  carried  on  indoors  is  a  great  drawback  to  their 
value.  When  the  weather  permits,  out-of-door  exercise  is  far 
better  than  that  carried  on  in  even  the  best  ventilated  and 
lighted  gymnasium.  For  those  who  are  so  fortunate  as  to 
possess  a  garden  there  is  no  better  exercise,  at  suitable  sea- 
sons, than  an  hour's  daily  digging  in  it;  since  this  calls  into 
play  nearly  all  the  muscles  of  the  Body;  while  of  games,  the 
modern  one  of  lawn  tennis  is  perhaps  the  best  from  a  hygienic 
view  that  has  ever  been  invented,  since  it  not  only  demands 
great  muscular  agility  in  every  part  of  tne  Body,  but  trains 
the  hand  to  work  with  the  eye  in  a  way  that  walking,  run- 
ning, rowing,  and  similar  pursuits  do  not.  For  the  same 
reasons  baseball,  cricket,  and  boxing  are  excellent. 

Exercise  in  Infancy  and  Childhood.  Young  children 
have  not  only  to  strengthen  their  muscles  by  exercise,  but 
also  to  learn  to  use  them.  Watch  an  infant  trying  to  con- 
vey something  to  its  mouth,  and  you  will  see  how  little 
control  it  has  over  its  muscles.  On  the  other  hand,  the 
healthy  infant  is  never  at  rest  when  awake;  it  constantly 
throws  its  limbs  around,  grasps  at  all  objects  within  its 
reach,  coils  itself  about,  and  so  gradually  learns  to  exercise  its 
powers.  It  is  a  good  plan  to  leave  every  healthy  child  more 
than  a  few  months  old  several  times  daily  on  a  large  bed,  or 
even  on  a  rug  or  carpeted  floor,  with  as  little  covering  as  is 
safe,  and  that  as  loose  as  possible,  and  let  it  wriggle  about  as  it 
pleases.  In  this  way  it  will  not  only  enjoy  itself  thoroughly, 
but  gain  strength  and  a  knowledge  of  how  to  use  its  limbs. 
To  keep  ::  healthy  child  swathed  all  day  in  tight  and  heavy 
clothes  i-  cruelty. 


l.r)6  THE  HUMAN  BODY. 

When  ;i  little  later  the  infant  commences  to  crawl  it  is  safe 
to  permil  it  to  as  much  as  it  wishes,  but  unwise  to  tempi  it  to 

do  so  when  disinclined:  the  bones  and  muscles  are  still  feeble 
and  may  be  injured  by  too  much  work.  The  same  is  true  of 
learning  to  walk. 

From  four  or  five  to  twelve  years  of  age  almost  any  form 
of  exercise  should  be  permitted,  or  even  encouraged.  During 
this  time,  however,  the  epiphyses  of  many  hones  are  not  firmly 
united  to  their  shafts,  and  so  anything  tending  to  throw  too 
great  a  strain  on  the  joints  should  be  avoided.  After  that  up 
to  commencing  manhood  or  maidenhood  any  kind  of  out- 
door exercise  for  healthy  persons  is  good,  and  girls  are  all  the 
better  for  being  allowed  to  join  in  their  brothers'  sports. 
Half  of  the  debility  and  general  ill-health  of  so  many  of  our 
women  is  the  consequence  of  deficient  exercise  during  early 
life;  and  the  day,  which  fortunately  seems  approaching, 
which  will  see  dolls  as  unknown  to  or  as  despised  by  healthy 
girls  as  by  healthy  boys  will  see  the  beginning  of  a  great  im- 
provement in  the  stamina  of  the  female  portion  of  our  popu- 
lation. 

Exercise  in  Youth  should  be  regulated  largely  by  sex;  not 
that  women  are  to  be  shut  up  and  made  pale,  delicate,  and 
unfit  to  share  the  duties  or  participate  fully  in  the  pleasures 
of  life;  but  the  other  calls  on  the  strength  of  the  young  woman 
render  vigorous  muscular  work  often  unadvisable,  especially 
under  conditions  where  it  is  apt  to  be  followed  by  a  chill. 

A  healthy  boy  or  young  man  may  do  nearly  anything;  but 
until  twenty-two  or  twenty-three  very  prolonged  effort  is  un- 
advisable. The  frame  is  still  not  firmly  knit  or  as  capable  of 
endurance  as  it  will  subsecpiently  become. 

Girls  should  be  allowed  to  ride  or  play  out-door  games  in 
moderation,  and  in  any  case  should  not  be  cribbed  in  tight 
stays  or  tight  boots.  A  flannel  dress  and  proper  lawn  tennis 
shoes  are  as  necessary  for  the  healthy  and  safe  enjoyment  of  an 
afternoon  at  that  game  by  a  girl  as  they  are  for  her  brother  in  the 
baseball  field.  Rowing  is  excellent  for  girls  if  there  be  any 
one  to  teach  them  to  do  it  properly  with  the  legs  and  back, 
and  not  with  the  arms  only,  as  women  are  so  apt  to  row. 
Properly  practised  it  strengthens  the  back  and  improves  the 
carriage. 

Exercise  in  Adult  Life.  Up  to  forty  a  man  may  carry  on 
safely  the  exercises  of  youth,  but  after  that  sudden  efforts 


MOTION  AND  LOCOMOTION  157 

should  be  avoided.  A  lad  of  twenty-one  or  so  may,  if  trained, 
safely  run  a  quarter-mile  race,  but  to  a  man  of  forty-five  it 
would  be  dangerous,  for  witli  the  rigidity  of  the  cartilages 
and  blood-vessels  which  begins  to  show  itself  about  that  time 
comes  a  diminished  power  of  meeting  a  sudden  violent  de- 
mand. On  the  other  hand,  the  man  of  thirty  would  more 
safely  than  the  lad  of  nineteen  or  twenty  undertake  one  of 
the  long-distance  walking  matches  which  have  lately  been  in 
vogue;  the  prolonged  effort  would  be  less  dangerous  to  him, 
though  a  six-days'  match,  with  its  attendant  loss  of  sleep, 
cannot  fail  to  be  more  or  less  dangerous  to  any  one.  Prob- 
ably for  one  engaged  in  active  business  a  walk  of  two  or 
three  miles  to  it  in  the  morning  and  back  again  in  the  after- 
noon is  the  best  and  most  available  exercise.  The  habit 
which  Americans  have  everywhere  acquired,  of  never  walking 
when  they  can  take  a  street  car,  is  certainly  detrimental  to 
the  general  health;  though  the  extremes  of  heat  and  cold  to 
which  we  are  subject  often  render  it  unavoidable. 

For  women  during  middle  life  the  same  rules  ajiply:  there 
should  be  some  regular  but  not  violent  daily  exercise. 

In  Old  Age  the  needful  amount  of  exercise  is  less,  and  it 
is  still  more  important  to  avoid  sudden  or  violent  effort. 

Exercise  for  Invalids.  This  should  be  regulated  under 
medical  advice.  For  feeble  persons  gymnastic  exercises  are 
especially  valuable,  since  from  their  variety  they  permit  of 
selection  according  to  the  condition  of  the  individual;  and 
their  amount  can  be  conveniently  controlled. 

Training.  If  any  person  attempt  seme  unusual  exercise 
he  soon  finds  that  he  loses  breath,  gets  perhaps  a  "  stitch 
in  the  side/'  and  feels  his  heart  beating  with  unwonted 
violence.  If  he  persevere  he  will  probably  faint — or  vomit, 
as  is  frequently  seen  in  the  case  of  imperfectly  trained  men  at 
the  end  of  a  hard  boat-race.  These  phenomena  are  avoided 
by  careful  gradual  preparation  known  as  "  training."  The 
immediate  cause  of  them  lies  in  disturbances  of  the  circula- 
tory and  respiratory  organs,  on  which  excessive  work  is 
thrown. 


CHAPTEE  XII. 
ANATOMY  OF  THE  NERVOUS  SYSTEM. 

Nerve-Trunks.  In  dissecting  the  Human  Body  numerous 
white  cords  are  found  which  at  first  sight  might  be  taken  for 
tendons.  That  they  are  something  else  however  soon  becomes 
clear,  since  a  great  many  of  them  have  no  connection  with 
muscles  at  all,  and  those  which  have  usually  enter  somewhere 
into  the  belly  of  the  muscle,  instead  of  being  fixed  to  its  ends 
as  most  tendons  are.  These  cords  arc  nerve-trunks  :  followed 
in  one  direction  each  (Fig.  69)  will  be  found  to  break  up  into 
finer  and  finer  branches,  until  the  subdivisions  become  too 
small  to  be  followed  without  the  aid  of  a  microscope.  Traced 
the  other  way  the  trunk  will  in  most  cases  be  found  to  in- 
crease by  the  union  of  others  with  it,  and  ultimately  to  join 
a  much  larger  mass  of  different  structure,  from  which  other 
trunks  also  spring.  This  mass  is  a  nerve-centre.  That  end 
of  a  nerve  attached  to  the  centre  is  naturally  its  central, 
and  the  other  its  distal  or  peripheral  end.  Nerve-centres, 
then,  give  origin  to  nerve-trunks;  these  latter  spread  all  over 
the  Body,  usually  branching  and  becoming  smaller  and  smaller 
as  they  proceed  from  the  centre;  they  finally  become  very 
small,  and  how  they  ultimately  end  is  not  in  all  cases  certain, 
but  it  is  known  that  some  have  sense-organs  at  their  termina- 
tions and  others  muscular  fibres.  The  general  arrangement 
of  the  larger  nerve-trunks  of  the  Body  is  shown  in  Fig.  69. 
Physically  a  nerve  is  not  so  tough  or  strong  as  a  tendon  of 
the  same  size;  it  may  readily  be  split  up  into  longitudinal 
strands,  each  of  which  consists  of  a  number  of  microscopic 
threads,  the  nerve-fibres,  bound  together  by  connective  tissue. 

Plexuses.  Very  frequently  several  neighboring  nerve- 
trunks  send  off  communicating  branches  to  one  another,  each 
branch  carrying  fibres  from  one  trunk  to  the  other.  Such 
networks  are  called  plexuses  (Fig.  72),  and  through  the  inter- 
changes taking  place  in  them  it  often  happens  that  the  distal 

158 


ANATOMY  OF  THE  NERVOUS  SYSTEM.  159 

branches  of  a  nerve-trunk  contain  fibres  which  it  does  not 
possess  as  it  leaves  the  centre  to  which  it  is  connected. 


IV..  69.— Diagram  illustrating  the  general  arrangement  of  the  nervous  system. 

Nerve-Centres.  The  great  majority  of  the  nerves  take 
their  origin  from  the  brain  and  spinal  curd,  which  together 
form  the  great  cerebrospinal  centre.    Some,  however,  coin- 


160  THE  HUMAN  BODY. 

mence  in  rounded  or  oval  masses  which  vary  in  size  from  that 
of  the  kernel  of  an  almond  down  to  microscopic;  dimensions, 
and  which  are  widely  distributed  in  the  liody.  Each  of  these 
smaller  scattered  centres  is  called  a  ganglion,  and  the  whole 
of  them  are  arranged  in  three  sets.  A  considerable  num- 
ber of  the  largest  are  united  directly  to  one  another  by 
nerve-trunks,  and  also  give  off  nerves  to  various  organs,  espe- 
cially to  the  blood-vessels  and  the  viscera  in  the  thoracic  and 
abdominal  cavities.  These  ganglia  and  their  branches  form 
the  sympathetic  nervous  system,  as  distinguished  from  the 
cerebro-spinal  nervous  system  consisting  of  the  brain  and 
spinal  cord  and  the  nerves  springing  from  them.  Of  the  re- 
maining ganglia  some  are  connected  with  various  cerebro- 
spinal trunks  near  their  origin,  while  the  rest,  for  the  most 
part  very  small  and  connected  with  the  peripheral  branches 
of  sympathetic  or  other  nerves,  are  known  as  the  sporadic 
ganglia. 

The  Cerebro-Spinal  Centre  and  its  Membranes.  Lying 
inside  the  skull  is  the  brain  and  in  the  neural  canal  of  the 
vertebral  column  the  spinal  cord  or  spinal  marrow,  the  two 
being  continuous  through  the  foramen  magnum  of  the  oc- 
cipital oone  and  forming  the  great  cerebro-spinal  nerve-centre. 
This  centre  is  bilaterally  symmetrical  throughout  except  for 
slight  differences  on  the  surfaces  of  parts  of  the  brain,  which 
are  often  found  in  the  higher  races  of  mankind.  Both  brain 
and  spinal  cord  are  very  soft  and  easily  crushed,  the  con- 
nective tissue  and  a  peculiar  supporting  tissue  (neuroglia ) 
which  pervade  them  being  delicate;  accordingly  both  organs 
are  placed  in  nearly  completely  closed  bony  cavities  and  are 
also  enveloped  by  membranes  which  give  them  support.  These 
membranes  are  three  in  number.  Externally  is  the  dura 
mater,  very  tough  and  strong  and  composed  of  white  fibrous 
and  elastic  connective  tissues.  In  the  cranium  the  dura 
mater  adheres  by  its  outer  surface  to  the  inside  of  the  skull 
chamber,  serving  as  the  periosteum  of  its  bones;  this  is 
not  the  case  in  the  vertebral  column,  where  the  dura  mater 
forms  a  loose  sheath  around  the  spinal  cord  and  is  only  at- 
tached here  and  there  to  the  surrounding  bones,  which  have 
a  separate  periosteum  of  their  own.  The  innermost  membrane 
of  the  cerebro-spinal  centre,  lying  in  immediate  contact  with 
the  proper  nervous  parts,  is  the  pia  mater,  also  made  up  of 
white  fibrous  tissue  interwoven    with   elastic  fibres,  but  less 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


161 


closely  than  in  the  dura  mater,  so  as  to  form  a  less  dense  and 
tough   membrane.     The    pia   mater  r> 

contains  many  blood-vessels  which 
break  up  in  it  into  small  branches 
before  entering  the  nervous  mass 
beneath.  Covering  the  outside  of 
the  pia  mater  is  a  layer  of  flat  closely 
fitting  cells;  a  similar  layer  lines  the 
inside  of  the  dura  mater,  and  these 
two  layers  are  described  as  the  third 
membrane  of  the  cerebro-spinal  cen- 
tre, called  the  arachnoid.  In  the 
space  between  the  two  layers  of  the 
arachnoid  is  contained  a  small  quan- 
tity of  watery  cerebrospinal  liquid. 
The  surface  of  the  brain  is  folded 
and  the  pia  mater  follows  closely  these 
folds;  the  arachnoid  often  stretches 
across  them :  in  the  spaces  thus  left 
between  it  and  the  pia  mater  is  con- 
tained some  of  the  cerebro-spinal 
liquid. 

(Fig.    70)    is 

form,   being 

from  side  to 


10 


-8    F 


Tf 


The    Spinal  Cord 

nearly  cylindrical  in 
however  a  little  wider 
side  than  dorsoventrally,  and  taper- 
ing off  at  its  posterior  end.  Its 
average  diameter  is  about  19  milli- 
meters (|  inch)  and  its  length  0.43 
meter  (17  inches).  It  weighs  42.5 
grams  (H  ounces).  There  is  no 
marked  limit  between  the  spinal  cord 
and  the  brain,  the  one  passing  grad- 
ually into  the  other  (Fig.  77),  but 
the  cord  is  arbitrarily  said  to  com- 
mence opposite  the  outer  margin  of 
the  foramen  magnum  of  the  occipital 
bone:  from  there  it  extends  to  the 
articulation   between   the   first  and      ?"• ,  rn,TThe  l"vina}  cford  &.\Hl 

medulla  oblongata.    A.  from  the 
Second     lumbal-    vertebrae,    where    it  ventral,  and  B,  from  th >rsal 

aspect  :  C  to  //  cross-sections  at 

narrows  <>fi  to  a  slender  filament,  the  different  i.-veis. 

fil a  in  terminals  (cut  ofE  and  represented  separately  at  B  in  Fig. 


II 


162 


THE  HUMAN  BODY. 


70),  which  runs  bacls  to  the  end  of  the  neural  canal  ln-hind 
tli-  sacrum.  In  its  course  the  cord  presents  two  expansions, 
an  upper,  10,  the  cervical  enlargement,  reaching  from  the  third 
cervical  to  the  firs!  dorsal  vertebrae,  and  a  lower  or  lumbar 
enlargement,  9,  opposite  the  last  dorsal  vertebra. 

Running  along  the  middle  line  on  both  the  ventral  and  the 
dorsal  aspects  of  the  cord  is  a  groove,  and  a  cross-seci  inn  -hows 
that  these  grooves  are  the  surface  indications  of  fissures  which 
extend  deeply  into  the  cord  (('.  Fig.  U)  and  nearly  divide  it 
into  right  and  left  halves. 

The  anterior  fissure  (1,  Fig.  71)  is  wider  and  shallower 
than  the  posterior,  2,  which  indeed  is  hardly  a  true  fissure, 
being  completely  filled  up  by  an  ingrowth  of  pia  mater.  The 
transverse  section,   C,  shows  also  that  the  substance  of  the 


Fig.  71. — The  spinal  cord  and  nerve-roots.  A,  a  small  portion  of  the  cord  seen 
from  the  ventral  sHe;  B.  tin-  same  seen  laterally:  C.  a  cross-section  of  ill*'  cord; 
]>,  the  two  roots  of  a  spinal  nerve;  1.  anterior  (ventral)  fissure;  £,  posterior  (dorsal) 
fissure;  :',.  surface  groove  along  the  line  of  attachment  of  the  anterior  nerve-roots: 
4.  line  of  origin  of  the  posterior  roots;  5.  anterior  root  filaments  of  spinal  nerve; 
6.  posterior  root  filaments,'  C,  ganglion  of  the  posterior  root;  7,  7',  the  first  two 
divisions  of  the  nerve-trunk  after  its  formation  by  the  union  of  the  two  roots.  The 
grooves  are  much  exaggerated 

cord  is  not  alike  throughout,  but  that  its  white  superficial 
layers  envelop  a  central  gray  substance  arranged  somewhat  in 
the  form  of  a  capital  H.  Each  half  of  the  gray  matter  is 
crescent-shaped,  and  the  crescents  are  turned  back  to  back  and 
united  across  the  middle  line  by  the  gray  commissure.     The 


ANATOMY  OF  THE  NERVOUS  SYSTEM.  163 

tips  of  each  crescent  are  called  its  horns  or  cornua,  and  the 
ventral,  or  anterior  cornu,  on  each  side  is  thicker  and  larger 
than  the  posterior.  In  the  cervical  and  lumbar  enlargements 
the  proportion  of  white  to  gray  matter  is  greater  than  else- 
where; and  as  the  cord  approaches  the  medulla  oblongata  its 
central  gray  mass  becomes  irregular  in  form  and  begins  to 
break  up  into  smaller  portions.  If  lines  be  drawn  on  the 
transverse  section  of  the  cord  from  the  tip  of  each  horn  of  the 
gray  matter  to  the  nearest  point  of  the  surface,  the  white  sub- 
stance in  each  halt  will  be  divided  into  three  portions:  one 
between  the  anterior  fissure  and  the  anterior  cornu,  and 
called  the  anterior  white  column;  one  between  the  posterior 
fissure  and  the  posterior  cornu,  and  called  the  posterior  white 
column  ;  while  the  remaining  one  lying  in  the  hollow  of  the 
crescent  and  between  the  two  horns  is  the  lateral  column  :  the 
anterior  and  lateral  columns  of  the  same  side  are  frequently 
named  the  antero -lateral  column.  A  certain  amount  of  white 
substance  crosses  the  middle  line  at  the  bottom  of  the  ante- 
rior fissure;  this  forms  the  anterior  white  commissure.  There 
is  no  posterior  white  commissure,  the  bottom  of  the  posterior 
fissure  being  the  only  portion  of  the  cord  where  the  gray  sub- 
stance is  uncovered  by  white.  Running  along  the  middle 
of  the  gray  commissure,  for  the  whole  length  of  the  cord,  is 
a  tiny  channel,  just  visible  to  the  unaided  eye;  it  is  known  as 
the  central  canal  {canalis  centralis). 

The  Spinal  Nerves.  Thirty-one  pairs  of  spinal  nerve- 
trunks  enter  the  neural  canal  of  the  vertebral  column  through 
the  intervertebral  foramina  (p.  71).  Each  divides  in  the  fora- 
men into  a  dorsal  and  ventral  portion  known  respectively 
as  the  posterior  and  anterior  roots  of  the  nerve  (6  and  5,  Fig. 
71),  and  these  again  subdivide  into  finer  branches  which  are 
attached  to  the  sides  of  the  cord,  the  posterior  root  at  the 
point  where  the  posterior  and  lateral  white  columns  meet, 
and  the  anterior  root  at  the  junction  of  the  lateral  and  anterior 
columns.  At  the  lines  on  which  the  roots  are  attached  there 
are  superficial  furrows  on  the  surface  of  the  cord.  On  each 
posterior  roof  is  a  spinal  ganglion  (6',  Fig.  71),  placed  just  be- 
fore it  joins  the  anterior  root  to  make  up  the  common  nerve- 
trunk.  Immediately  after  its  formation  by  the  mixture  of 
fibres  from  both  roots,  the  trunk  divides  {D,  Fig.  71),  into 
a  posterior  primary,  an  anterior  primary,  and  a  communi- 
cating branch.    The  branches  of  the  firs!  sel  go  for  the  most 


1C4  THE  HUMAN  BODY. 

part  to  the  skin  and  muscles  on  the  back,  the  second  form 
a  series  of  plexuses  from  which  the  nerves  for  the  sides  and 

ventral  region  of  the  neck  and  trunk  and  for  the  limbs  arise; 
the  communicating  branches  go  to  neighboring  sympathetic 
ganglia. 

The  various  spina]  nerves  are  named  from  the  portions  of 
the  vertebral  column  through  the  intervertebral  foramina  of 
vvhiidi  they  pass  out  ;  and  as  a  general  rule  each  nerve  is  named 
from  the  vertebra  in  front  of  it.  For  example  the  nerve  pass- 
ing out  between  the  fifth  and  sixth  thoracic  vertebrae  is  the 
"  fifth  thoracic"  nerve,  and  that  between  the  last  thoracic  and 
first  lumbar  vertebrae,  the  "twelfth  thoracic."  In  the  cervi- 
cal region,  however,  this  rule  is  not  adhered  to.  The  nerve 
passing  out  between  the  occipital  bone  and  the  atlas  is  called 
the  "first  cervical"  nerve,  that  between  the  atlas  and  axis  the 
second,  and  so  on;  that  between  seventh  cervical  and  first 
thoracic  vertebrae  being  the  "eighth  cervical**  nerve.  The 
thirty-one  pairs  of  spinal  nerves  are  then  thus  distributed:  S 
cervical,  12  thoracic,  5  lumbar,  5  sacral,  and  1  coccygeal;  the 
latter  passing  out  between  the  sacrum  and  coccyx.  Since  the 
spinal  cord  ends  opposite  the  upper  lumbar  vertebrae  while 
the  sacral  and  coccygeal  nerves  pass  out  from  the  neural  canal 
much  farther  back,  it  is  clear  that  the  roots  of  those  nerves, 
on  their  way  to  unite  in  the  foramina  of  exit  and  form  nerve- 
trunks,  must  run  obliquely  backwards  in  the  spinal  canal  for 
a  considerable  distance.  One  finds  in  fact  the  neural  canal 
in  the  lumbar  and  sacral  regions,  behind  the  point  where  the 
spinal  cord  has  tapered  off  to  form  the  filum  terminate,  oc- 
cupied chiefly  by  a  great  bunch  of  nerve-roots  forming  the 
so-called  "horse's  tail"  or  cauda  equina. 

Distribution  of  the  Spinal  Nerves.  It  would  be  out  of 
place  here  to  go  into  detail  as  to  the  exact  portions  of  the 
Body  supplied  by  each  spinal  nerve,  but  the  following  general 
statements  may  be  made.  The  anterior  primary  branches  of 
the  first  four  cervical  nerves  form  on  each  side  the  cervical 
plexus  (Fig.  72)  from  which  branches  are  supplied  to  the 
muscles  and  integument  of  the  neck:  also  to  the  outer  ear 
and  the  back  part  of  the  scalp.  The  anterior  primary 
branches  of  the  remaining  cervical  nerves  and  the  first  dorsal 
form  the  brachial  plexus,  from  which  the  upper  limb  is 
supplied.  The  roots  of  the  trunks  which  form  this  plexus 
arise  from  the  cervical  enlargement  of  the  spinal  cord. 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


165 


From  the  fourth  and  fifth  cervical  nerves  on  eacli  side,  small 
branches  arise  and  unite  to  make  the  phrenic  nerve  (4',  Fig. 
72)  which  runs  down  through  the  chest  and  ends  in  the 
diaphragm. 

The  anterior  primary  branches  of  the  dorsal  nerves,  except 
part  of  the  first  which  enters  the  brachial  plexus,  form  no 


Fio.  72— Tho  cervical  and  brachial  plexuses  of  the  left  side  of  the  Body. 


plexus,  but  each  runs  along  the  posterior  border  of  a  rib  and 
supplies  branches  to  the  chest-walls,  and  the  lower  ones  to 
th08e  of  the  abdomen  al80. 

The  anterior  primary  branches  of  the  four  anterior  lumbar 
nerves  are  united  by  branches  to  form  the   lumbar  plexus. 


166 


THE  II UMAX  BODY. 


It  supplies  t lie  lower  part  of  the  trunk,  the  buttocks,  the 
front  of  the  thigh,  and  inner  side  of  the  leg. 

The  sacral  plexus  is  formed  by  the  anterior  primary 
branches  of  the  fifth  lumbar  and  the  first  four  sacral  nerves, 
which  unite  in  one  great  cord  and  so  form  the  sciatic  nerve, 
which  is  the  largest  in  the  Body  and,  running  down  the 
back  of  the  thigh,  ends  in  branches  for  the  lower  limb.  The 
roots  of  the  trunks  which  form  the  sacral  plexus  arise  from 
the  lumber  enlargement  of  the  cord. 

The  Brain  (Fig.  I'd)  is  far  larger  than  the  spinal  cord 
and  more  complex  in  structure.     It  weighs  on  the  average 


Fig  73. — Diagram  illustraiir.gr  the  general  relationships  of  the  parts  of  the  brain. 
A.  fore-brain:  b.  midbrain  ;  B.  cerebellum;  ('.  pons  Varolii  ;  D,  medulla  oblon- 
gata ;  B,  C,  and  D  together  constitute  the  hind-brain. 

about  1415  grams  (50  ounces)  in  the  adult  male,  and  about 
155  grams  (5.5  ounces)  less  in  the  female.  In  its  simpler 
forms  the  vertebrate  brain  consists  of  three  masses,  each  with 
subsidiary  parts,  following  one  another  in  series  from  before 
back,  and  known  as  the  fore-brain,  mid-brain,  and  hind- 
brain  respectively.  In  man  the  fore-brain.  A,  weighing 
about  1245  grams  (44  ounces),  is  much  larger  than  all  the 
rest  put  together  and  laps  over  them  behind.  It  consists 
mainly  of  two  large  convoluted  masses,  separated  from  one 
another  by  a  deep  median  fissure,  and  known  as  the  cerebral 
hemispheres.  The  immense  proportionate  size  of  these  is 
very  characteristic  of  the  human  brain.     Beneath  each  cere- 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


167 


bral  hemisphere  is  an  olfactory  lobe,  inconspicuous  in  man 
but  in  many  animals  larger  than  the  cerebral  hemispheres. 
Buried  in  the  fore-brain  on  each  side'  are  two  large  gray 
masses,  the  corpora  striata  and  optic  thalami.  The  mid- 
brain forms  a  connecting  isthmus  between  the  two  other 
divisions  and  presents  on  its  dorsal  side  four  hemispherical 
eminences,  the  corpora  quadrigemina.  On  its  ventral  side 
it  exhibits  two  semicylindrical  pillars  (seen  under  the  nerve 
IV.  in  Fig.  77),  known  as  the  crura  cerebri.  The  hind- 
brain  consists  of  three  main  parts  :  on  its  dorsal  side  is  the 
cerebellum,  B  (Fig.  73),  consisting  of  a  right,  a  left,  and  a 
median  lobe;  on  the  ventral  side  is  the  pons  Varolii,  C 
(Fig.  73),  and  behind  that  the  medulla  oblongata,  D  (Fig.  73), 
which  is  continuous  with  the  spinal  cord. 

In  nature,  the  main  divisions  of  the  brain  are  not  sepa- 
rated so  much  as  has  been  represented   in   the  diagram  for 


Fig.  74.— The  brain  from  the  left  side.  Cb.  the  cerebral  hemispheres  forming 
tliH  main  bulk  of  the  fore-brain;  Cbl,  the  cerebellum  ;  Mo,  the  medulla  oblon- 
gata; f,  the  pons  Varolii  ;*  the  Assure  of  S.ylvius. 

the  sake  of  clearness,  but  lie  close  together,  as  represented 
in  Fig.  74,  only  some  folds  of  the  membranes  extending  be- 
tween them  ;  and  the  mid-brain  is  entirely  covered  in  on  its 
dorsal  aspect.  Nearly  everywhere  the  surface  of  the  brain 
is  folded,  the  folds,  known  as  gyri  or  convolutions  being 
deeper  and  more  numerous  in  the  brain  of  man  than  in  that 
of  the  animals  nearesl  allied  to  him;  and  in  the  human 
species  more  marked  in  the  higher  than  in  the  lower  races. 

It   should    however    be   added    that    some   species    of    animals 


168 


THE  HUMAN  BODY. 


which  are  not  markedly  intelligent  have  much  convoluted 
cerebral  hemispheres. 

The  brain  like  the  spinal  cord  consists  of  gray  and  white 
nervous  matter,  but  somewhat  differently  arranged,  for  while 
the  brain,  like  the  cord,  contains  gray  matter  in  its  interior, 
a  great  part  of  its  surface  is  also  covered  with  it.  By  the 
external  convolutions  of  the  cerebellum  and  the  cerebral 
hemispheres  the  surface  over  which  this  gray  substance  is 
spread  is  very  much  increased  (see  Fig.  74). 
^  The  Ventricles  of  the  Brain.  The  minute  central  canal 
of  the  spinal  cord  is  continued   into  the  brain  and  expands 


Fig.  75. — Diagram  of  the  right  half  of  a  vertical  median  section  of  the  brain. 
H.  H,  convoluted  inner  surface  of  right  cerebral  hemisphere;  5.  the  fifth  ventricle; 
the  figure  is  placed  on  the  thin  inner  wall  of  the  right  lateral  ventricle;  Cc,  cor- 
pus callosum;  '■},  the  third  veutriele  ;  the  partition  separating  it  from  the  fifth  ven- 
tricle is  the  fornix,  and  just  behind  the  anterior  thickened  end  of  the  fornix  is 
shown  part  of  the  right  foramen  of  Monro  m.  leading  to  the  right  lateral  ven- 
tricle ;  t,  the  soft  commissure  cut  across;  in  the  fore  part,  of  the  fornix  is  the 
anterior  commissure ;  the  anterior  portion  of  the  floor  of  the  third  ventricle 
shows  two  downward  prolongations,  one  directed  to  the  optic  commissure,  z,  the 
other  (infundibnlurri)  to  the  pituitary  body.pt,'  ".  the  pineal  body ;  the  thickening 
immediately  beneath  its  root  is  the  posterior  commissure:  the  mass  forming  the 
exposed  wall  of  the  ventricle  and  on  which  the  figure  3  is  placed  is  the  inner  side 
of  the  right  optic  thalamus ;  o,  <l,  the  anterior  and  posterior  corpora  quadri- 
gemina  of  the  right  side:  4.  the  fourth  ventricle  lying  near  the  dorsal  side  of  the 
medulla  oblongata,  Mo,  and  connected  by  the  iter  with  the  third  ventricle:  pos- 
teriorly it  is  continued  to  join  the  central  canal  of  the  spinal  cord:  Or,  right  crus 
cerebri  ;  F,  pons  Varolii;  Cl>,  cerebellum;  where  it  is  divided  in  the  middle  line 
the  radial  arrangement  of  its  central  white  matter  forming  the  so-called  arbor 
viice  is  seen :  op.  light  optic  nerve  proceeding  from  the  optic  commissure  ;  oc,  the 
third  cranial  nerve  arising  from  the  crus   cerebri;   1,  callosal  convolution. 

there  at  several  points  into  chambers  known  as  the  ventri- 
cles. Entering  the  medulla  oblongata  it  approaches  its 
upper  surface  and  dilates  into  the  fourth  ventricle,  -i,  Fig.  75, 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


169 


which  has  a  very  thin  roof,  lapped  over  by  the  cerebellum. 
From  the  front  of  the  fourth  ventricle  rims  a  narrow  pas- 
sage (aqueduct  of  sylvius  or  iter)  which  enters  another  dila- 
tation, 3,  Fig.  75,  lying  in  the  middle  line  near  the  under 
side  of  the  fore-brain  and  known  as  the  third  ventricle. 
From  the  third  ventricle  two  apertures  (the  foramens  of 
Monro),  one  of  which  is  partly  seen  at  m  in  the  diagram, 
lead  into  the  first  and  second,  or  lateral  ventricles,  one  of 
which  lies  in  each  of  the  cerebral  hemispheres.  The  front 
ends  of  these  two  ventricles  are  seen  in  the  vertical  trans- 
verse section  of  the  brain  represented  in  Fig.  76. 


Fig.  76. — A  vertical  section  across  the  cerebral  hemispheres  taken  in  front  of  the 
fifth  ventricle.  C'c/Q,  anterior  part  of  coifrus  callosum  ;  VI,  the  anterior  end  of  the 
right  lateral  ventricle:  the  gray  mass  on  its  exterior  is  the  front  end  of  the  corpus 
striatum.  On  the  leftside  the  superficial  gray  matter  covering  the  convolutions 
is  shaded. 

The  ventricles  contain  a  small  amount  of  cerebrospinal 
liquid,  and  are  lined  by  epithelium  which  is  ciliated  in  early 
life.  Part  of  the  posterior  wall  of  the  third  ventricle  is  ex- 
fcremely  thin,  consisting  of  little  but  this  epithelium  sup- 
ported by  a  thin  layer  of  pia  mater:  this  part  is  pushed  in  or 
doubled  into  the  cavity  of  the  ventricle  in  the  form  of  a 
triangular  membrane,  the  velum  inter positum,  which  lies 
beneath  the  fornix  and  scuds  offshoots  into  the  lateral  ven- 
tricles. Between  the  upper  and  lower  layers  of  the  indupli- 
cated  velum  interpositnm  arteries  enter  and  there  breakup 
into  plexuses-    the  choroid  plexuses — covered  everywhere  by 


170  TIIN  HUMAN  BODY. 

the  pushed-in  epithelium.  These  plexuses  occupy  a  consid- 
erable part  of  the  third  and  lateral  ventricles:  and  a  pair  of 
similar  vascular  tufts  drive  in  before  them  part  of  the  thin 

roof  of  the  fourth  ventricle  and  encroach  on  its  cavity. 

Note.  A  frequent  cause  of  apoplexy  is  a  hemorrhage 
into  one  of  the  lateral  ventricles;  the  outpoured  blood  accu- 
mulating and  pressing  upon  the  cerebral  hemispheres,  their 

functions  are  suppressed  and  unconsciousness  produced. 
When  a  person  is  found  in  an  apoplectic  tit  therefore  the 
best  thing  to  do  is  to  leave  him  perfectly  quiet  until  medical 
aid  is  obtained:  for  any  movement  may  start  afresh  a,  bleed- 
ing into  the  ventricle  which  had  been  stopped  by  clots 
formed  in  the  mouths  of  the  torn  blood-vessels. 

Sections  of  the  Brain.  Having  got  a  general  idea  of  the 
parts  composing  the  brain,  the  best  way  to  continue  the  study 
of  its  anatomy  is  to  examine  sections  taken  in  various  direc- 
tions. Two  such  are  given  in  Figs.  75  and  76.  Fig.  75  rep- 
resents the  right  half  of  a  vertical  section  of  the  brain,  taken 
from  before  back  in  the  middle  line  and  viewed  from  the 
inner  side.  Above,  the  knife  has  passed  between  the  two 
cerebral  hemispheres,  in  the  longitudinal  fissure,  without  cut- 
ting either,  and  the  convoluted  inner  surface  of  the  right  one 
is  seen.  The  sickle-shaped  mass  lower  down,  Gc  to  Cc,  rep- 
resents the  cut  surface  of  a  connecting  band  of  white  nervous 
tissue  called  the  corpus  callosum,  which  runs  across  the  mid- 
dle line  from  one  cerebral  hemisphere  to  the  other  and  puts 
them  in  communication.  Beneath  the  corpus  callosum 
the  knife  has  opened  a  cavity,  the  fifth  ventricle,  5, 
bounded  on  each  side  by  a  very  thin  wall,  which  forms  part 
of  the  inner  wall  of  the  corresponding  lateral  ventricle;  the. 
median  partition  formed  by  these  two  walls  and  containing 
the  slit-like  fifth  ventricle  is  the  septum  lucidum.  The  fifth 
is  quite  different  in  origin  from  the  remaining  cerebral  ven- 
tricles, not  being  a  continuation  of  the  canalis  centralis  of 
the  spinal  cord. 

Forming  the  floor  of  the  fifth  ventricle  and  separating  it 
from  the  third  ventricle,  :'>,  is  the  fornix,  mainly  made  lip  of 
fibres  running  from  before  back.  The  anterior  downward- 
curved  end  of  the  fornix  is  thickened,  and  contains  the  an- 
terior commissure,  a  small  cord  of  transverse  nerve-fibres. 
The  cavity  of  the  third  ventricle  is  narrow  from  side  to  side, 
and  is  bounded  laterally  by  the  optic  thnhtr.ii.  of  which  the 


ANATOMY  OF  THE  NERVOUS  SYSTEM.  171 

right,  having  the  figure  3  placed  on  it,  has  its  median  side 
exposed  in  the  section.  The  third  ventricle  is  crossed  about 
its  middle  by  the  middle  commissi! re,  t,  and  from  its  anterior 
end  the  foramina  of  Monro,  of  which  the  right,  m,  is  partly 
exposed  in  the  section,  lead  to  the  lateral  ventricles.  From 
the  fore  part  of  the  third  ventricle  two  conical  extensions  pass 
downward, one  directed  to  z,  the  optic  commissure,  from  which 
the  optic  nerves  pass,  and  the  other,  named  the  infundibulum, 
to  the  pituitary  body,  pt.  The  latter  consists  of  an  anterior 
and  posterior  lobe,  and  in  the  human  brain  contains  no  ner- 
vous elements.  The  anterior  lobe,  indeed,  is  an  outgrowth 
from  the  pharynx  of  the  embryo,  and  only  secondarily  be- 
comes attached  to  the  brain.  It  is  not  known  to  have  any 
function  in  existing  vertebrates.  From  the  posterior  part  of 
the  floor  of  the  third  ventricle  the  iter  leads  as  a  narrow  pas- 
sage dorsal  to  the  crura  cerebri,  (Jr,  and  ventral  to  the  corpora 
quadrifjemina,  o,  d,  to  the  fourth  ventricle,  4.  Projecting 
from  the  posterior  wall  of  the  third  ventricle  is  a  small  coni- 
cal non-nervous  mass,  the  pineal  body,  which,  though  of  no 
functional  importance,  is  of  interest,  in  the  first  place  be- 
cause the  philosopher  Descartes  considered  it  the  special  seat 
of  the  soul,  and  in  the  second  because  embryology  and  com- 
parative anatomy  show  that  it  is  the  remnant  of  a  third 
median  eye,  which  primitive  vertebrates  possessed  on  the 
dorsal  side  of  the  head.  In  some  existing  reptiles  its  original 
structure  is  more  complete  than  in  man,  but  in  none  is  it 
functional.  Just  beneath  the  attachment  of  the  pineal  body 
is  a  slight  thickening  of  the  posterior  wall  of  the  third  ven- 
tricle containing  transverse  fibres,  and  named  the  posterior 
commissure.  The  third  ventricle  and  the  parts  immediately 
surrounding  it  constitute  the  inter-brain  or  thalamencephalon, 
which  with  the  two  cerebral  hemispheres  and  the  corpus  cal- 
losum  and  fornix  makes  up  the  fore  brain. 

The  mid-brain,  consisting  mainly  of  the  crura  cerebri,  Cr, 
and  the  corpora quadrigemina,  o,  d,  and  traversed  by  the  nar- 
row iter,  is  continuous  posteriorly  with  the  hind  brain,  con- 
sisting of  pons  Varolii,  I';  cerebellum,  Cb;  and  medulla  oblon- 
gata,  Mo.  The  thin  roofed  cavity  of  the  fourth  ventricle,  4, 
lies  near  its  dorsal  side  Where  cut  in  making  the  section 
the  cerebellum  shows  a  curious  branching  core  of  white  nerve 
matter,  surrounded  by  gray,  named  arbor  vitce  by  the  old 
anatomists.      The  pons  consists  mainly  of  transverse  fibres 


172  THE  HUMAN  BODY. 

uniting  fche  righl  and  left  halves  of  the  cerebellum;  the 
medulla  oblongata  and  crura  contain  mainly  longitudinal 
fibres,  but  there  are  many  transverse. 

Fig.  76  represents  a  vertical  transverse  section  of  the  brain 
taken  through  the  forepart  of  the  corpus  callosum  (CcF)  and 
altogether  in  front  of  the  third  ventricle,  [tshows  the  foldings 
of  the  cerebrum  and  its  superficial  layer  of  gray  substance;  the 
anterior  ends  of  the  lateral  ventricles,  VI,  with  a  gray  mas.-,  the 
corpus  striatum  lying  beneath  and  on  the  outer  side  of  each. 
If  the  section  had  been  taken  a  little  farther  back  the  optic 
tlialami  would  have  been  found  reaching  the  floor  of  each  ven- 
tricle. Like  the  optic  thalamus,  to  the  front  of  and  partly  to 
the  outer  side  of  which  it  lies,  the  corpus  striatum  is  mainly 
composed  of  gray  nerve  matter.  It  i.-.  however,  divided  in 
its  posterior  region  into  an  inner  and  outer  portion  by  a  well 
marked  hand  of  white  suhstance,  consisting  of  nerve  fibres, 
passing  through  on  the  way  to  or  from  the  surface  of  the 
cerebral  hemispheres:  this  band  is  the  internal  capsule. 

The  Base  of  the  Brain  and  the  Cranial  Nerves.  Twelve 
pairs  of  nerves  leave  the  skull  by  apertures  in  its  base,  and 
are  known  as  the  cranial  nerves.  Most  of  them  spring  from 
the  under  side  of  the  brain,  and  so  they  are  best  studied  in 
connection  with  the  hase  of  that  organ,  which  is  represented 
in  Fig.  77.  The  first  pair, or  olfactory  serves,  spring  from 
the  under  sides  of  the  olfactory  lobes,  /.and  pass  out  through 
the  roof  of  the  nose.  They  are  the  nerves  of  smell.  The 
second  pair,  or  optic  nerves,  II,  spring  from  the  optic  thalami 
and  corpora  quadrigemina,  and.  under  the  name  of  optic  tracts, 
run  down  to  the  base  of  the  brain,  where  they  appear  passing 
around  the  crura  cerebri,  as  represented  in  the  figure.  In  the 
middle  line  the  two  optic  tracts  unite  to  form  the  optic  com- 
missure (seen  in  section  at  z,  in  Fig.  75),  from  which  an  optic 
nerve  proceeds  to  each  eyeball.  Behind  the  optic  commis- 
sure is  seen  the  conical  stalk  of  the  pituitary  body  or  hy- 
pophysis cerebri  (pt  in  Fig.  75),  and  still  further  hack  a  pair  of 
hemispherical  masses,  about  the  size  of  split  peas,  known  as 
the  corpora  albicantia. 

All  the  remaining  cranial  nerves  arise  from  the  hind- 
brain.  The  third  pair  {motores  oculi)  arise  from  the  front  of 
the  pons  Varolii,  and  are  distributed  to  most  of  the  muscles 
which  move  the  eyeball  and  also  to  that  which  lifts  the  upper 
eyelid.     The   four-sided  space    bounded   by  the  ojitic  tracts 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


173 


and  commissure  in  front  and  the  third  pair  of  nerves  behind, 
and  having  on  it  the  pituitary  body  and  the  corpora  albi- 
cantia,  lies  beneath  the  third  ventricle,  so  that  a  probe  pushed 
in  there  would  enter  that  cavity  (see  Fig.  75). 


Fig.  77. — The  base  of  the  brain.    The  cerebral  hemispheres  are  seen  overlap- 

Fiing  all  the  rest.  /,  olfactory  lobes;  II.  optic  tract  passing  tothe  optic  commissure 
rom  which  the  optic  nerves  proceed;  ///.  the  third  nerve  or  motor  ocidi  ;  I  V,  the 
fourth  nerve  or patheticus ;  r,  the  fifth  nerve  or  trigeminalis;  \'l.  the  sixth  nerve 
or  abducens;  ill,  the  seventh  or  facial  nerve  or portio  dura;  fill,  the  auditory 
nerve  or portio  mollis:  IX,  the  ninth  or  glosso-pharyngeal;   X,  the  tenth  or  pneu- 

mOR&Stric  or  varjwt;  XI.  the  spinal  accessory  ;  XII,  the  hypoglossal;  nd,  the  first 
cervical  spinal  nerve. 

The  fourth  pair  of  nerves,  IV  (pathetici),  arise  from  the 

front  part  of  the  roof  of  the  fourth  ventricle.  From  there, 
each  curls  around  ;i  cms  cerebri  (the  cylindrical  mass  seen 
beneath  it  in  the  figure,  running  from  the  pons  Varolii  to 
enter  the  under  surface  of  the  cerebral  hemispheres)  and  ap- 
pears on  the  base  of  the  brain.  Each  goes  to  one  muscle  of 
the  eyeball. 

The   fifth   pair   of    nerves    (Iri i/rm i inih's),    \',  resemble   the 


174  THE  HUMAN  BODY. 

spinal  nerves  in  having  two  roots;  one  of  these  is  much 
larger  than  the  other  and  possesses  a  ganglion  (the  Gasserian 
ganglion)  like  the  dorsal  root  of  a  spinal  nerve.  Beyond 
the  ganglion  the  two  roots  form  a  common  trunk  which 
divides  into  three  main  branches.  Of  these,  the  ophthalmic 
is  the  smallest  and  is  mainly  distributed  to  the  muscles  and 
skin  over  the  forehead  and  upper  eyelid;  but  also  gives 
branches  to  the  mucous  membrane  lining  the  nose,  and  to 
the  integument  over  it.  The  second  division  {superior  maxil- 
lary tier  re)  of  the  trigeminal  gives  branches  to  the  skin  over 
the  temple,  to  the  cheek  between  the  eyebrow  and  the  angle 
of  the  mouth,  and  to  the  upper  teeth;  as  well  as  to  the 
mucous  membrane  of  the  nose,  pharynx,  soft  palate  and  roof 
of  the  mouth.  The  third  division  [inferior  maxillary)  is  the 
largest  branch  of  the  trigeminal;  it  receives  some  fibres  from 
the  larger  root  and  all  of  the  smaller.  It  is  distributed  to 
the  side  of  the  head  and  the  external  ear,  the  lower  lip  and 
lower  part  of  the  face,  the  mucous  membrane  of  the  mouth 
and  the  anterior  two  thirds  of  the  tongue,  the  lower  teeth, 
the  salivary  glands,  and  the  muscles  which  move  the  lower 
jaw  in  mastication. 

The  sixth  pair  of  cranial  nerves  (Fig.  77),  VI,  or  abdu- 
centes  arise  from  the  posterior  margin  of  the  pons  Varolii, 
and  each  is  distributed  to  one  muscle  of  the  eyeball. 

The  seventh  pair  {facial  nerves),  VII,  appear  also  at  the 
posterior  margin  of  the  pons.  They  are  distributed  to  most 
of  the  muscles  of  the  face  and  scalp. 

The  eighth  pair  (auditory  nerves)  arise  close  to  the  facial. 
They  are  the  nerves  of  hearing  and  are  distributed  entirely 
to  the  internal  ear. 

The  ninth  pair  (glossopharyngeal,  IX,  arising  close  to 
the  auditories,  are  distributed  to  the  mucous  membrane  of 
the  pharynx,  the  posterior  part  of  the  tongue,  and  the  middle 
ear. 

The  tenth  pair  (pneumo gastric  nerves  or  vagi),  X,  arise 
from  the  sides  of  the  medulla  oblongata.  Each  gives  branches 
to  the  pharynx,  gullet  and  stomach,  the  larynx,  windpipe 
and  lungs,  and  to  the  heart.  The  vagus  runs  farther  through 
the  body  than  any  other  cranial  nerve. 

The  eleventh  pair  (spinal  accessory  nerves),  XI,  do  not 
arise  mainly  from  the  brain  but  by  a  number  of  roots  attached 
to  the  lateral  columns  of  the  cervical  portion  of  the  spinal 


ANATOMY  OF  THE  NERVOUS  SYSTEM.  175 

cord,  between  the  anterior  and  posterior  roots  of  the  proper 
cervical  spinal  nerves.  Each,  however,  runs  into  the  skull 
cavity  alongside  of  the  spinal  cord  and,  getting  a  few  fila- 
ments from  the  medulla  oblongata,  passes  out  along  with  the 
glossopharyngeal  and  pneumogastric  nerves.  Outside  the 
skull  it  divides  into  two  branches,  one  of  which  joins  the 
pneumogastric  trunk,  while  the  other  is  distributed  to  mus- 
cles about  the  shoulder. 

The  twelfth  pair  of  cranial  nerves  (hypoglossi),  XII,  arise 
from  the  sides  of  the  medulla  oblongata;  they  are  distributed 
mainly  to  the  muscles  of  the  tongue  and  hyoid  bone. 

if  Deep  Origins  of  the  Cranial  Nerves.  The  places  referred 
to  above,  at  which  the  various  cranial  nerves  appear  on  the 
surface  of  the  brain,  are  known  as  their  superficial  origins. 
From  them  the  nerves  can  be  traced  for  a  lesser  or  greater  way 
in  the  substance  of  the  brain  until  each  is  followed  to  one  or 
more  masses  of  gray  matter,  which  constitute  its  proper  start- 
ing-point and  are  known  as  its  deep  origin.  The  deep  origins 
of  all  except  the  first  and  second  and  part  of  the  eleventh  lie 
in  the  medulla  oblongata,  midbrain,  and  thalamen  cephalon. 

The  Ganglia  and  Communications  of  the  Cranial  Nerves. 
Besides  the  Gasserian  ganglion  above  referred  to,  many  others 
are  found  in  connection  with  the  cranial  nerves.  Thus  for 
example  there  is  one  on  each  of  the  main  divisions  of  the 
trigeminal,  two  are  found  on  each  pneumogastric  and  two  in 
connection  with  the  glossopharyngeal.  At  these  ganglia  and 
elsewhere,  the  various  nerves  often  receive  branches  from 
neighboring  cranial  or  spinal  nerves,  so  that  very  soon  after 
it  leaves  the  brain  hardly  any,  except  the  olfactory,  optic,  and 
auditory,  remains  free  from  fibres  derived  from  other  trunks. 
This  often  makes  it  difficult  to  say  from  where  the  nervej  of 
a  special  part  have  come;  for  example,  the  nerve-fibres  going 
to  the  submaxillary  salivary  gland  from  the  trigeminal  leave 
the  brain  first  in  the  facial  and  only  afterwards  enter  the 
fifth;  and  many  of  the  fibres  going  apparently  from  the 
pneumogastric  to  the  heart  come  originally  from  the  spinal 
accessory. 

The  Sympathetic  System.  The  ganglia  which  form  the 
main  centres  of  the  sympathetic  nervous  system  lie  in  two 
rows  (*,  Fig.  2,  and  sy,  Fig.  3),  one  on  either  side  of  the 
bodies  of  the  vertebra.  Each  ganglion  is  united  by  a  nerve- 
trunk  with  the  one  in  front  of  it,  and  so  two  great  chains  are 


176  THE  HUMAN  BODY. 

formed  reaching  from  the  base  of  the  skull  to  the  coccyx. 
In  the  trunk  region  these  chains  lie  in  the  ventral  cavity, 
their  relative  position  in  which  is  indicated  by  the  dots  sy  in 
the  diagrammatic  transverse  section  represented  od  p.  6  in 
Fig.  ">.  The  ganglia  on  these  chains  arc  forty-nine  in  num- 
ber, viz.,  twenty-four  pairs,  and  a  single  one  in  front  of  the 
coccyx  in  which  both  chains  terminate.  They  are  named 
from  the  regions  of  the  vertebral  column  near  which  they  lie; 
there  being  three  cervical,  twelve  thoracic,  four  lumbar,  and 
five  sacral  pairs. 

Each  sympathetic  ganglion  is  united  by  communicating 
branches  with  the  neighboring  spinal  nerves,  and  near  the 
skull  with  various  cranial  nerves  also;  while  from  the  gan- 
glia and  their  uniting  cords  arise  numerous  trunLs,  many  of 
which,  in  the  thoracic  and  abdominal  cavities,  form  plexuses, 
from  which  in  turn  nerves  are  given  oil'  to  the  viscera. 
These  plexuses  frequently  possess  numerous  ganglia  of  their 
own;  two  of  the  most  important  are  the  cardiac  plexus 
which  lies  on  the  dorsal  side  of  the  heart,  and  the  solar  plexus 
which  lies  in  the  abdominal  cavity  and  supplies  nerves  to  the 
stomach,  liver,  kidneys,  and  intestines.  .Many  of  the  sympa- 
thetic nerves  finally  end  in  the  walls  of  the  blood-vessels  of 
various  organs.  To  the  naked  eye  they  are  commonly  grayer 
in  color  than  the  eerebro-spinal  nerves. 

The  Sporadic  Ganglia.  These  are  found  scattered  in 
nearly  all  parts  of  the  Body  except  the  limbs.  They  are  for 
the  most  part  small,  even  microscopic  in  size,  though  several 
large  ones  exist  in  the  abdominal  cavity.  They  are  especially 
abundant  in  the  neighborhood  of  secretory  tissues  ami  about 
blood-vessels,  Avhile  a  very  important  set  is  found  in  the 
heart.  Nerves  unite  them  with  the  cerebro-spinal  and  sym- 
pathetic centres,  and  probably  most  of  them  should  be  classi- 
fied as  belonging  to  the  sympathetic  system. 

The  Histology  of  Nerve-Fibres.  The  microscope  shows 
that  in  addition  to  connective  tissue  and  other  accessory 
parts,  such  as  blood-vessels,  the  nervous  organs  contain  tis- 
sues peculiar  to  themselves  and  known  as  nerve-fibres  and 
nerve-cells.  The  cells  are  found  in  the  centres  only;  while 
the  fibres,  of  which  there  are  two  main  varieties  known  as 
the  white  and  the  gray,  are  found  in  both  trunks  and  cen- 
tres: the  white  variety  predominating  in  most  cerebro-spinal 
nerves  and   in  the  white  substance  of  the  centres,  and  the 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


177 


gray  in  the  sympathetic  trunks  and  the  gray  portions  of  the 
central  organs. 

If  an  ordinary  eerebro-spinal  nerve-trunk  be  examined  it 
will  be  found  to  be  enveloped  in  a  loose  sheath  of  areolar 
connective  tissue,  which  forms  a  packing  for  it  and  unites 
it  to  neighboring  parts.  From  this  sheath,  or  perineurium, 
bands  of  connective  tissue  penetrate  the  nerve  and  divide  it 
up  into  a  number  of  smaller  cords  or  funiculi,  much  as  a 
muscle  is  subdivided  into  fasciculi;  each  funiculus  has  a 
sheath  of  its  own  called  the  neurilemma,  composed  of  several 


Fig.  78.  Fig.  79. 

Fig.  78.—  White  nerve-fibres  soon  after  removal  from  the  Body  and  when  they 
have  acquired  their  double  contour. 

Fig.  79.— Diagram  illustrating  the  structure  of  a  white  or  medallated  nerve-fibre. 
I,  1.  primitive  sheath;  •■>,  2,  medullary  sheath;  3,  axis  cylinder. 


concentric  layers  of  a  delicate  membrane,  within  which  the 
true  nerve-fibres  lie.  These,  which  would  be  nearly  all  of 
the  white  kind,  consist  of  extremely  delicate  threads,  on  the 
average,  0.01'i5  mm.  (2(?(t-lt  inch)  in  diameter,  though  often 
considerably  smaller,  and  of  a  length  which  is  in  proportion 
very  great.  The  core  of  each  nerve-fibre  in  fact  is  continuous 
from  ;i  nerve-centre  to  the  organ  in  which  it  ends,  so  that  the 
fibres,  eg.,  which  pass  out  through  the  sacral  plexus  and  then 
run  on  through  the  sciatic  nerve  and  its  branches  to  the  skin 


178 


THE  HUMAN   I'.ODY. 


of  the  toes,  are  three  to  four  feel  long.  Efa  fresh  white  nerve' 
fibre  be  examined  with  the  microscope  it  presents  the  appear- 
ance  of  a  perfectly  homogeneous  glassy  thread ;  but  soon  it 
acquires  a  characteristic  double  contour  (Fig. 
78)  from  the  coagulation  of  a  portion  of  its 
substance.  By  proper  treatment  with  re- 
agents three  layers  may  be  broughl  into  view. 
Outside  is  a  fine  transparent  envelope  (1, 
Fig.  79)  called  the  primitive  sheath  ;  inside 
this  is  a  fatty  substance,  2,  forming  the 
medullary  sheath  (the  coagulation  of  which 
gives  the  fibre  its  double  border),  and  in  the 
centre  is  a  core,  the  axis  cylinder,  3,  which 
is  longitudinally  fibrillated  and  is  clearly  the 
essential  part  of  the  fibre,  since  near  the  end- 
ing the  primitive  and  medullary  sheaths  are 
frequently  absent.  At  intervals  of  about 
one  millimeter  (^  inch)  along  the  fibre  are 
found  nuclei  (c,  Fig.  80),  around  each  of 
which  lies  a  little  protoplasm.  These  are 
indications  of  the  primitive  cells  which  have 
elongated  and  formed  an  envelope  for  the 
axis  cylinder,  which  itself  is  a  branch  given 
off  by  a  nerve-cell  in  some  centre.  The 
medullary  sheath  is  interrupted  half-way 
between  each  pair  of  nuclei  at  a  point  called 
the  node  of  Ranvier  (R,  Fig.  SO),  which  is 
the  boundary  between  two  of  the  enveloping 
cells.  In  the  course  of  a  nerve-trunk  its 
fibres  rarely  divide;  when  a  branch  is  given 
on  wo  whiTe  or rtm?a-  off   some   fibres    merely   separate    from   the 

SSLdtZfti.™  rest>  much  as  a  skein  of  silk  misht  be  sePa- 

four  hundred  diame-  nitc(|  0llt  at   one  enfl  jnt0  smaller  bundles 

ters:  they  nave  linen 

treated  with,  osmic  containing  fewer  threads.     Near  their  ends, 

acid,  which  stains  the  ° 

medullary      sheath  however,  nerve-fibres  frequently  branch,  and 

black  and   brines  in-  .    .    .  ,  .  ..     , 

to  view  the  nuclei,  then  a  division  of  the  axis  cylinder  goes  to 


c.    c,   and    Les    of 

Ranvier.  R.  The  axis 
cylinder  is  seen  to  he 
continuous  through 
the  nodes. 


each  branch. 

Gray  Nerve-Fibres.     Some  of  these  are 

merely  white  fibres  which  near  their  peri- 
pheral ends  have  lost  their  medullary  sheaths;  others  have  no 
medullary  sheath  throughout  their  whole  course,  and  consist 
merely  of  an  axis  cylinder  (often  striated)  and  nuclei,  with 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


179 


or  without  a  primitive  sheath.  Such  fibres  are  especially 
abundant  in  the  sympathetic  trunks;  and  they  alone  form 
the  olfactory  nerve.  In  the  communicating  branches  between 
the  sympathetic  ganglia  and  the  spinal  nerves  both  white 
and  gray  fibres  are  found ;  the  former  are  cerebro-spinal 
fibres  passiug  into  the  sympathetic  system,  while  the  gray 
fibres  originate  in  the  sympathetic  system  and  pass  to  the 
membranes  and  blood-vessels  of  the  spinal  cord  and  spinal 
column.  Another  group  of  gray  nerve-fibres  may  be  called 
nerve-fibrils  r  they  are  extremely  fine,  and  result  from  the 
subdivision  of  axis  cylinders,  close  to  their  final  endings  in 
many  parts  of  the  Body,  after  they  have  already  lost  both 
primitive  and  medullary  sheaths.  Many  fine  gray  fibres  exist 
in  the  nerve-centres. 

The  Histology  of  Nerve-Cells.  The  only  structures 
known  with  certainty  to  be  connected  with  the  central  ends 
of  nerve-fibres  are  nerve-cells,  and  so  many  nerve-fibres  have 


Fig.  81— Nerve-cell  from  anterior  born  of.  grey  matter  of  spinal  cord;  u,  axis- 
cylinder  process.     ~,  Cell  from  posterior  born  of  ppinal  cord. 

been  traced  into  continuity  with  nerve-cells,  that  it  is  fairly 
certain  all  arise  in  this  way.  The  latter  may  therefore  be  re- 
garded as  the  central  organs  of  the  nerve-fibres. 


180  THE  HUMAN  BODY. 

At  1,  Fig.  81,  is  shown  a  typical  nerve-cell  such  as  may 
be  found  in  an  anterior  horn  of  the  gray  matter  of  the  spinal 
cord.  It  consists  of  the  cell  body,  or  cell  protoplasm,  in 
which  is  a  large  nucleus  containing  a  nucleolus.  From  the 
body  of  the  cell  arise  several  branches,  the  great  majority  of 
which  are  granular  and  divide  frequently  in  a  forking  or 
" dichotomous "  manner.  These  are  known  as  the  "proto- 
plasmic '*  branches  of  the  cell,  and  possibly  serve  merely  to 
absorb  nourishment  for  it.  One  branch,  however,  a,  gives  off 
at  right  angles  smaller  filaments,  but  still  maintains  its  in- 
dividuality and  ultimately  becomes  the  axis  cylinder  of  a 
nerve-fibre.  Its  side  branches  probably  put  it  in  anatomical 
continuity  with  other  nerve-fibres  and  other  nerve-cells. 
Nerve  cells  from  the  posterior  horn  of  the  grey  matter  of 
the  spinal  cord  (2,  Fig.  81)  also  possess  numerous  granular 
protoplasmic  processes  and  a  nerve-fibre  process  (b)\  but  the 
latter,  instead  of  continuing  directly  into  an  axis  cylinder, 
breaks  up  into  a  network  of  fine  branches  which  frequently 
unite  with  one  another  and  also,  no  doubt,  with  fibrils  from 
neighboring  cells.  It  is  almost  certain  that  one  or  more  of 
these  fibrils  or  a  bunch  of  them  forms  the  axis  cylinder  of  a 
fibre  in  a  dorsal  root  of  a  spinal  nerve. 

As  we  shall  learn  later,  the  dorsal  roots  are  concerned  in 
carrying  impulses  from  the  skin  and  other  sensitive  parts  to 
the  spinal  cord;  the  anterior  roots  in  conveying  impulses  from 
the  nerve-centres  to  the  organs  (muscles,  glands,  etc.)  of  the 
Body.  Therefore,  in  general  te|ms,  we  may  speak  of  the  type 
of  nerve-cell  1,  Fig.  81,  as  a  motor  nerve-cell;  and  the  type 
of  cell  2,  Fig.  81,  as  a  sensory  nerve-cell.  Both  varieties  of 
cells  are  found  abundantly  in  the  gray  matter  of  the  brain 
(Fig.  83),  along  with  other  forms,  of  which  the  pear-shaped 
cells  of  Purkinje  existing  in  the  cerebellum  may  be  mentioned 
(Fig.  82). 

In  the  sympathetic  and  sporadic  ganglia  somewhat  simjfier 
forms  of  nerve-cells,  having  fewer  branches,  occur.  As  a  rule 
nerve-cells  are  comparatively  large  and  have  conspicuous 
nuclei,  but  in  the  brain  many  small  ones  exist. 

Neuroglia.  In  the  brain  and  spinal  cord  the  true  nervous 
elements  are  intertwined  with  and  supported  by  connective 
tissue  and  minute  blood-vessels,  but  in  addition  there  is  found 
closely  twisted  around  the  cells  and  fibres  a  peculiar  tissue 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


181 


made  of   greatly  branched   cells   (Fig.  S3),  and  named    the 
neuroglia  or  sustentacula?-  tissue. 

Nerve-Centres  consist  of  white  and  gray  nerve-fibres,  of 
nerve-cells,  of  neuroglia,  and 
of  connective  tissue  and 
blood-vessels  arranged  in 
different  ways  in  the  differ- 
ent centres.  They  are  es- 
sentially collections  of  nerve- 


Fig.  82.— A  thin  section  of  the  cere-  Fig.  83.— Cells  from  the  surface  gray  mat- 
bellum  abowing  p^ar-shaped  cells  of  ter  of  a  cerebral  convolution  :  p,  nerve-cells 
Purkinj'-.  and  numerous  other  small  with  axis  cylinder  processes,  o  ;  n,  non-ner- 
Derre-cellS.  vous  neuroglia  cells.     Tli^  method  of  prepa- 

ration (Golgi's)  stains  the  cells  an  uniform 
black. 

cells  and  nerve-fibres,  some  of  the  latter  being  connected  with 
the  cells,  while  others  may  merely  pass  through  on  their  way 
to  or  from  other  centres.  As  an  illustration  of  the  structure 
of  a  more  complex  nerve-centre  we  may  study  the  spinal  cord. 
Histology  of  the  Spinal  Cord.  If  a  thin  transverse  sec- 
tion of  the  spinal  cord  be  examined  with  a  microscope  it  will 
be  found  that  enveloping  the  whole  is  a  delicate  layer  of 
connective   tissue,  the   pin  milter.     Fine   bands  of  it  ramify 


182 


THE  HUMAN  ISO  It  Y. 


through  the  cord,  supporting  the  nervous  elements;  some  of 
the  coarser  of  these  are  represented  ai  6,  7,  and  elsewhere  in 
Fig.  84,  but  from  these  still  finer  processes  arise,  as  represented 
at  d  and  e  in  Fig.  85.  The  ultimate  finest  tissue  directly 
supporting  the  nervous  elements  directly,  is  the  neuroglia. 
In  the  white  columns,  the  cord  (Fig.  85)  will  be  seen  to  be 


.  ig.  84.— A  thin  transverse  section  of  half  of  the  spinal  cord  magnified  about 
10  oiameters.  1,  anterior  fissure  ;  g,  posterior  fissure  :  3.  canalis  centralis  :  8,  pia 
mater  enveloping  the  cord  ;  6,  7,  bands  of  pia  mater  penetrating  the  cord  and  sup- 
porting its  nerve  elements  ;  '.<,  a  posterior  root  :  10,  bundles  of  an  anterior  root  :  a, 
b,  c,  d,  e,  groups  of  nerve-cells  in  the  gray  matter. 

mainly  made  up  of  medullated  nerve-fibres  which  run  longi- 
tudinally and  therefore  appear  in  the  transverse  section  as 
circles,  with  a  dot  in  the  centre,  which  is  the  axis  cylinder. 
At  o  in  Fig.  85  these  fibres  are  represented,  the  intermediate 
connective  tissue  being  omitted,  while  at  e  this  latter  alone  is 
represented  in  order  to  show  more  clearly  its  arrangement. 
At  the  levels  of  the  nerve-roots  horizontal  white  fibres  are 
found  (9  and  10,  Fig.  84,  and  a,  Fig.  S~>),  running  into  the 
gray  matter,  and  others  exist  at  the  bottom  of  the  anterior 
fissure,  running  from  one  side  of  the  cord  to  the  other.     In 


ANATOMY  OF  THE  NERVOUS  SYSTEM. 


183 


the  gray  substance  the  same  supporting  network  of  connec- 
tive tissue  is  found,  but  in  it  the  majority  of  the  nerve-fibres 
are  non-medullated,  and  at  certain  points  nerve-cells,  such  as 
are  totally  absent  in  the  white  substance,  are  found.  One 
collection  of  these  nerve-cells  is  seen  at  c  in  Fig.  84,  and 
others  are  represented  at  a.  e,f,  and  elsewhere.  The  nerve- 
fibres  in  the  gray  matter  are  for  the  most  part  branches  of 
the  axis  cylinder  processes  of  these  cells  (see  Fig.  81),  and  as 
they  unite  with  one  another  freely  they  form  a  structurally 
continuous  network  through  the  whole  gray  substance.  The 
fibres  of  the  anterior  roots  of  the  spinal  nerves  enter  the  gray 
matter  and  there  most  of  them  soon  become  continuous  with 
the  axis  cylinder  process  of  a  nerve-cell;  the  ending  of  the 
posterior  root-fibres  is  not  quite  certain,  but  they  appear  to 
break  up  and  join  the  gray  network,  to  be  by  it  placed  indi- 
rectly in  connection  with  nerve-cells.  In  any  case  the  funda- 
mental fact  remains  that  every  nerve-fibre  joining  the  spinal 
cord  is  directly  or  indirectly  in  continuity  with  the  gray  net- 
work, and  so  with  all  the  other  fibres  of  all  the  spinal  nerves. 


©    ~    Q 
0«0f? 


Fig.  85. — A  small  bit  of  'lie  section  represented  in  Fig,  84  more  magnified,  a,  a 
bundle  of  fibres  from  an  anterior'  root  passing  through  the  white  substance  on  its 
way  to  the  gray.  Towards  the  right  of  the  figure  t  lie-  nerve-fibres  of  the  anterior 
column  have  been  omitted  so  an  to  render  more  conspicuous  the  supporting  con- 
nective tissue, '/  and  •■ .  Elsewhere  the  nerve-fibres  alone  are  represented  ;  c,  envel- 
oping pia  mater.    The  neuroglia  is  nol  indicated. 

From  the  sides  of  the  gray  substance,  fibres  continually  pass 
out  into  the  white  portion  and  become  medullated;  some  of 
these  enter  the  gray  network  again  ;tt  another  level  and  so 
bring  parts  of  the  cord  into  especially  close  union,  while 
other,-:  pass  on  into  the  brain.  At  the  top  of  the  neck,  more- 
over, the  gray  matter  of  the  cord  is  continuous  with  that  of 


184 


THE  HUMAN  BODY. 


the  medulla  oblongata  and  through  it  with  the  rest  of  the 

brain,  so  that  nervous  disturbances  can   na.-s  by  anatomically 

continuous  paths  from  one  to  the  other. 

The  Structure  of  a  Spinal  Ganglion.     When  one  of  these 

ganglia  is  cut  lengthwise,  and  the  section  examined  micro- 
scopically, it  is  seen  thai 
connective  tissue  forms  an 
envelope  for  it,  and  sends 
ramifying  bundles  through 
it.  The  fibres  of  the  poste- 
rior root  become  separated 
into  bundles  when  they 
enter  a  ganglion  and  unite 
into  a  single  bunch  when 
they  leave  it  to  join  the 
mixed  spinal  nerve  trunk. 
Between  the  bundles  of 
nerve-fibres  within  the  gan- 
glion are  groups  of  nerve- 
cells,  and  probably  each  fibre 
on  its  way  through  the  gan- 
glion is  connected  with  a 
cell.      This   connection    oc- 


Fig.  86. — Diagram  of  a  spinal  ganglion 
cell:  ac,  its  fibrillated  process,  which  ac- 
quiring primitive  sheath,  ps,  and  medullary  •  «nmp\vlinr  nppnlinr 

sheath,  ms,  becomes  a  fibre   which  at   th«  curs    ln   a  SOUieU  1U1T    pel  Ulldl 

node  of  Ranvier   nr  joins  a  posterior  roof.  W.1V         TllP      Pp11<4 

fibre,  part  of   its  axis  cylinder,  c,  running  maJ'        x  Ilt'      <-CI1° 

centrally  in  this,  and  part,  d,  distally.  are      pear-shaped, 


(Fig.   86) 
granular, 

contain  a  large  nucleus  and  nucleolus,  and  average  T\T  mm. 
(^"o  i'lch)  in  long  diameter.  Near  its  narrow  end  the  cell 
substance  is  fibrillated,  and  a  bundle  of  these  fine  fibres.  ac, 
passes  from  it,  something  like  the  stalk  from  a  pear.  This 
stalk  is  an  axis  cylinder,  and  has  on  it  small  nuclei.  A  little 
way  from  the  cell  the  axis  cylinder  acquires  a  primitive  slieat  h, 
ps,  and  a  little  farther  on  a  medullary  sheath,  ms,  so  that  it 
becomes  a  fully  formed  white  nerve-fibre.  At  a  node  of  Ran- 
vier (usually  that  one  nearest  the  cell),  nr,  this  divides,  its 
branches  diverging  from  it  at  right  angles:  one  branch  runs 
to  the  posterior  root  and  enters  the  spinal  cord ;  the  other 
continues  through  the  ganglion  as  a  fibre  of  the  mixed 
nerve-trunk.  The  axis  cylinders  of  these  branches,  c  and  //. 
in  some  cases  at  least,  contain  fibrillse  not  derived  from  the 
pear-shaped  cell  in  addition  to  those  which  are.  Each  cell  as 
it  lies  in  the  ganglion  is  encased  in  a  delicate  envelope  of 


ANATOMY  OF  THE  NERVOUS  SYSTEM.  185 

flattened  nucleated  cells  (not  indicated  in  the  figure)., 
probably  belonging  to  the  surrounding  connective  tissue. 
Blood-vessels  are  distributed  in  the  ganglion,  the  capillaries 
being  especially  numerous  about  the  nerve-cells. 

Most  of  the  cells  of  sympathetic  and  other  peripheral  gan- 
glia seem  to  have  several  branches,  no  one  of  which  differs 
essentially  from  the  rest;  probably  each  branch  becomes  part 
of  the  axis  cylinder  of  a  different  fibre,  the  cell  thus  placing 
several  distinct  fibres  in  communication. 


166  THE  HUMAN  BODY. 


CHAPTER   XIII. 

THE   GENERAL   PHYSIOLOGY   OF   THE    NERVOUS   SYSTEM. 

The  Properties  of  the  Nervous  System.  General  Con- 
siderations. If  the  finger  of  any  one  unexpectedly  touches 
a  very  hot  object,  pain  is  felt  and  the  hand  is  suddenly 
snatched  away;  that  is  to  say,  sensation  is  aroused  and  cer- 
tain muscles  are  caused  to  contract.  If,  however,  the  nerves 
passing  from  the  arm  to  the  spinal  cord  have  been  divided,  or 
if  they  have  been  rendered  incapable  of  activity  by  disease,  no 
such  results  follow.  Pain  is  not  then  felt  on  touching  the 
hot  body  nor  does  any  movement  of  the  limb  occur;  even 
more,  under  such  circumstances  the  strongest  effort  of  the 
will  of  the  individual  is  unable  to  bring  about  movement 
of  his  hand.  If,  again,  the  nerves  of  the  limb  have  uninjured 
connection  with  the  spinal  cord,  but  parts  of  the  latter, 
higher  up,  between  the  brain  and  the  point  of  junction  of  the 
nerves  of  the  brachial  plexus  with  the  cord,  are  injured,  then 
a  sudden  contact  with  the  hot  body  will  cause  the  arm  to  be 
snatched  away,  but  no  pain  or  other  sensation  due  to  the 
contact  will  be  felt,  nor  can  the  will  act  upon  the  muscles  of 
the  arm.  Prom  the  comparison  of  what  happens  in  such 
cases  (which  have  been  observed  over  and  over  again  upon 
wounded  or  diseased  persons)  with  what  occurs  in  the  natural 
condition  of  things,  several  important  conclusions  may  be 
arrived  at: 

1.  The  feeling  of  pain  does  not  reside  in  the  burnt  part  it- 
self; although  that  may  be  perfectly  normal,  no  sensation 
will  be  aroused  by  any  external  force  acting  upon  it,  if  the 
nervous  cords  uniting  it  with  the  centres  be  previously 
divided. 

2.  The  hot  body  has  originated  some  change  which,  when 
propagated  along  the  nerve-trunks,  has  excited  a  condition  of 
the  nerve-centres  which  is  accompanied  by  a  sensation,  in  this 
particular  case  a  painful  one.  This  is  clear  from  the  fact 
that  the  loss  of  sensation  immediately  follows  division  of  the 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     187 

nerves  of  the  limb,  but  not  the  injury  of  any  of  its  other 
parts;  unless  of  such  a  character  as  to  cut  oft'  the  supply  of 
blood,  when  of  course  the  nerves  soon  die,  with  the  rest. 
Even,  however,  some  time  after  tying  the  vessels  which  carry 
blood  to  a  limb  one  can  observe  in  experiments  upon  the 
lower  animals  that  sensibility  is  still  retained  if  the  nerves 
be  not  directly  injured. 

3.  When  a  nerve  in  the  skin  is  excited  by  a  burn,  or  other- 
wise,  it  does  not  directly  call  forth  muscular  contractions;  for 
if  so,  touching  the  hot  body  would  cause  the  limb  to  be  moved 
even  when  the  nerve  had  been  divided  high  up  in  the  arm,  and 
as  a  matter  of  observation  and  experiment  we  find  that  no  such 
result  follows  if  the  nerve-fibres  have  been  cut  in  any  part  of 
their  course  from  the  burned  part  to  the  spinal  marrow.  It 
is  therefore  through  the  nerve-centres  that  the  change  trans- 
mitted from  the  excited  part  of  the  skin  is  reflected  or  sent 
back,  to  act  upon  the  muscles. 

4.  The  last  deduction  makes  it  probable  that  nerve-Jib?  es 
must  pass  from  the  centre  to  muscles  as  ivell  as  from  the  skin 
to  the  centre.  This  is  confirmed  by  the  fact  that  if  the  nerves 
of  the  limb  be  divided  the  will  is  unable  to  act  upou  its 
muscles,  showing  that  these  are  excited  to  contract  through 
the  nerves.  That  the  nerve-fibres  concerned  in  arousing 
sensation  and  muscular  contractions  are  different,  is  shown 
also  by  cases  of  disease  in  which  the  sensibility  of  the  limb  is 
lost  while  the  power  of  voluntarily  moving  it  remains,  and  by 
other  cases  in  which  the  reverse  is  seen,  objects  touching  the 
hand  being  felt  while  it  cannot  be  moved  by  the  will.  We  con- 
clude then  that  certain  nerve-fibres  when  stimulated  convey 
something  {a  nervous  impulse)  to  the  centres,  and  that  these 
when  excited  may  radiate  impulses  through  other  nerve-fibres 
to  distant  parts,  the  centre  serving  as  a  connecting  link  be- 
tween the  fibres  which  carry  impulses  from  without  in,  and 
those  which  convey  them  from  within  out. 

5.  Further  we  conclude  that  the  spinal  cord  can  act  as 
an  intermediary  between  the  fibres  carrying  in  nervous  im- 
pulses and  those  carrying  I  hem  out,  but  that  sensations  can- 
not be  aroused  by  impulses  reaching  the  spinal  cord  only, 
nor  has  the  Will  its  seat  there;  volition  and conscious?iess  are 
dependent  n/ion  stales  of  the  brain.  This  follows  from  the 
unconscious  movements  of  the  limbs  which  follow  stimula- 
tion of   its  skin  after  such  injury  to  the  spinal  cord  as  pre- 


188  THE  HUMAN  BODY. 

vents  the  farther  transmission  of  nervous  impulses  (show* 
ingthal  the  cord  is  a  reflex  centre),  and  from  the  absence,  in 

such  cases,  of  sensation  in  the  pari  whose  nerves  have  been 
injured,  and  the  loss  of  the  power  of  voluntarily  causing  its 
muscles  to  contract. 

(J.  Finally  we  conclude  that,  the  spinal  cord  in  addition  to 
being  a  centre  for  rrflex  actions  serves  to  transmit  nervous  im- 
pulses to  and  from  the  brain;  a  fact  which  is  confirmed  by  the 
histological  observation  that  in  addition  to  the  nerve-cells, 
which  are  the  characteristic  constituents  of  nerve-centres,  it 
contains  the  simply  conductive  nerve-fibres,  many  of  which 
pass  on  to  the  brain.  In  other  words,  the  spinal  cord,  besides 
containing  fibres  which  enter  it  from  and  pass  from  it  to  peri- 
pheral parts, contains  many  which  unite  it  to  other  centres; 
and  others  which  connect  the  various  centres  in  it,  as  those 
for  the  arms  and  legs,  together.  This  is  true  not  only  of  the 
spinal  cord,  but  of  the  brain  (which  contains  many  fibres 
uniting  different  centres  in  it),  and  probably  of  all  nerve- 
centres. 

The  Functions  of  Nerve-Centres  and  Nerve-Trunks. 
From  what  has  been  stated  in  the  previous  paragraphs  it  is 
clear  that  we  may  distinctly  separate  the  nerve-trunks  from 
the  nerve-centres.  The  fibres  serve  simply  to  convey  impulses 
either  from  without  to  a  centre  or  in  the  opposite  direction, 
while  the  centres  conduct  and  do  much  more.  Some,  as  the 
spinal  cord,  are  merely  reflex  centres  and  have  nothing  to  do 
with  states  of  consciousness.  A  man  with  iis  spinal  cord 
cut  or  diseased  in  the  thoracic  region  will  kick  violently  if  the 
soles  of  his  feet  be  tickled,  but  will  feel  nothing  of  the  tick- 
ling, and  if  he  did  not  see  his  legs  would  not  know  that  they 
were  moving.  Reflex  centres  moreover  do  not  act,  as  a  rule, 
indifferently  and  casually,  but  rearrange  the  impulses  reach- 
ing them,  so  as  to  produce  a  protective  or  in  some  way  advan- 
tageous result.  In  other  words,  these  centres,  acting  in 
health,  commonly  co-ordinate  the  incoming  impulses  and  give 
rise  to  outward-going  impulses  which  produce  an  apparently 
purposive  result.  The  burnt  hand  or  the  tickled  foot,  in  the 
absence  of  all  consciousness,  is  snatched  away  from  the  irri- 
tant; and  food  chewed  in  the  mouth  excites  nerves  there 
which  act  on  a  centre  which  causes  certain  cells  in  the  salivary 
glands  to  make  and  pour  into  the  mouth  more  saliva.  In 
addition  to  the  reflex  centres  we  have  others,  placed  in  the 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     189 

brain,  the  excitation  of  which  is  accompanied  in  us  by  various 
states  of  consciousness,  as  sensations,  emotions,  and  the  will; 
concerning  these  centres  of  consciousness  our  physiological 
knowledge  is  still  very  incomplete;  what  we  know  about  them 
is  based  rather  on  psychological  than  physiological  observa- 
tion. The  brain  also  contains  a  great  many  important  reflex 
centres,  as  that  for  the  muscles  of  swallowing,  an  act  which 
goes  on  perfectly  without  our  consciousness  at  all.  In  fact 
if  we  pay  attention  to  our  swallowing  we  fail  to  perform  it  as 
well  as  if  we  let  the  nervo-muscular  apparatus  alone,  as  is 
illustrated  by  the  difficulty  many  persons  experience  on  trying 
to  swallow  a  pill.  To  complete  the  statement  of  the  functions 
of  the  nerve-centres  we  must  probably  add  two  other  groups. 
The  first  of  these  is  that  of  the  automatic  centres,  which  are 
centres  excited  not  directly  by  nerve-fibres  conveying  impulses 
to  them,  but  in  other  ways.  For  example  the  breathing 
movements  go  on  independently  of  our  consciousness,  being 
dependent  on  stimulation  of  a  nerve-centre  in  the  brain  by 
the  blood  which  flows  through  it  (see  Chap.  XXVII);  and 
the  beat  of  the  heart  is  also  much  dependent  (Chap.  XVIII) 
upon  nerve-centres,  the  excitant  of  which  is  unknown.  The 
final  group  of  nerve-centres  is  represented  by  certain  sporadic 
sympathetic  and  cerebro-spinal  ganglia  which  are  not  known 
to  be  either  reflex,  automatic,  or  conscious  in  function.  They 
may  be  called  relay  and  junction  centres,  since  in  them  prob- 
ably an  impulse  entering  by  one  nerve-fibre  excites  a  cell, 
which  by  its  communicating  branches  arouses  many  others, 
and  these  then  send  out  impulses  by  the  many  nerve-fibres 
connected  with  them.  By  such  means  a  single  nerve-fibre  can 
act  upon  an  extended  region  of  the  Body.  In  other  cases  it 
seems  likely  that  a  feeble  nervous  impulse  reaching  an  irri- 
table nerve-cell  excites  changes  in  this  comparable  to  those 
produced  in  a  muscle  when  it  is  stimulated;  the  cell  is  in 
fact  a  store  of  readily  decomposable  material  which  breaks 
down  when  stimulated  through  one  branch,  with  the  liberation 
of  energy,  the  discharge  of  which  takes  the  form  of  reinforced 
nerve  impulses  sent  along  other  branches  or  one  of  them. 

That  nerve-cells  are  the  seats  of  considerable  metabolic 
changes  is  indicated  by  the  abundant  supply  of  blood  always 
sent  to  regions  where  they  are  numerous:  and  that  some  of 
their  material  is  used  up,  or  undergoes  katabolism,  during 
their  activity  and  is  replaced   by  anabolic  processes  during 


1 90  I 'HE  HUMAN  BOD) '. 

rest,  can  be  demonstrated  histologically.  If  the  sensory 
nerves  of  one  fore  limb  of  an  animal  be  Left  ai  rest,  and  those 
of  the  other  simultaneously  excited  for  several  hours,  it  will 
be  found,  at  the  cud  of  that  time,  that  the  nuclei  of  manj 

cells  of  the  spinal  ganglia  of  the  brachial  nerves  on  the  stim- 
ulated side  are  shrunken  and  distorted  when  compared  with 
those  of  the  other  side.  But  if  some  hours  be  suffered  to 
elapse  before  the  animal  is  killed  and  the  ganglia  examined, 
the  nuclei  of  the  cells  on  both  sides  will  be  found  equally 
large  and  well  rounded.  In  carrier-pigeons  after  a  long  flighl 
and  in  wild  sparrows  shot  at  the  close  of  day,  the  nuclei  of 
the  nerve-cells  connected  with  the  origin  of  motor  nerve-fibres 
are  found  to  be  shrivelled,  and  the  whole  cell  frequently  dimin- 
ished in  size  when  compared  with  specimens  taken  from  birds 
after  a  period  of  rest.  In  old  age  the  nuclei  of  many  nerve- 
cells  are  small  and  distorted,  even  after  prolonged  rest. 

Nerve-trunks  and  the  white  portions  of  nerve-centres  art; 
sparsely  supplied  with  blood  and  undergo  but  small  and  slow 
nutritive  changes  in  health.  Their  activity  appears  to  consist 
in  the  transmission  of  some  molecular  motion  not  affecting 
the  nutrition  and  chemical  composition  of  the  fibre,  and  not 
using  up  its  material. 

Excitant  and  Inhibitory  Nerves.  The  great  majority 
of  the  nerve-fibres  of  the  Body  when  they  convey  nervous 
impulses  to  a  part  arouse  it  to  activity;  they  are  excitant 
fibres.  There  is,  however,  in  the  Body  another  very  impor- 
tant set  which  arrest  the  activity  of  parts  and  which  are 
known  as  inhibitory  nerve-fibres.  Some  of  these  check  the 
action  of  central  nervous  organs,  and  others  the  work  of 
peripheral  parts.  For  instance,  taking  a  pinch  of  snuff  will 
make  most  persons  sneeze;  it  excites  centrally  acting  fibres 
in  the  nose,  these  excite  a  centre  in  the  brain,  and  this  in 
turn  sends  out  impulses  by  motor  fibres  which  cause  various 
muscles  to  contract.  But  if  the  skin  of  the  upper  lip  be 
pinched  immediately  after  taking  the  snuff,  in  most  cases  the 
reflex  act  of  sneezing,  which  the  "Will  alone  could  not  pre- 
vent, will  not  take  place.  The  afferent  impulses  conveyed 
from  the  skin  of  the  lip  have  "  inhibited  "  what  we  may  call 
the  "  sneezing  centre;"  and  afford  us  therefore  an  example 
of  inhibitory  fibres  checking  a  centre.  On  the  other  hand, 
the  heart  is  a  muscular  organ  which  goes  on  beating  steadily 
throughout  life;  but  if  certain  branches  of  the  pneumogastric 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     101 

nerve  going  to  it  be  excited,  the  beat  of  the  heart  will  be 
stopped;  it  will  cease  to  work  and  lie  in  a  relaxed  resting 
condition:  in  this  we  have  an  instance  of  an  inhibitory  nerve 
checking  the  activity  of  a  peripheral  organ. 

Classification  of  Nerve-Fibres.  Nearly  all  the  nerve- 
fibres  of  the  Body  fall  into  one  of  two  great  groups  corre- 
sponding to  those  which  carry  impulses  to  the  centres  and 
those  which  carry  them  ont  from  the  centres.  The  former 
are  called  afferent  or  centripetal  fibres,  and  the  latter  eff'  rent 
or  centrifugal.  .Since  the  impulses  reaching  the  centres 
through  the  afferent  fibres  frequently  cause  sensations  they  are 
often  called  sensory  fibres  ;  and  as  many  of  those  which  carry 
ont  impulses  from  the  centres  excite  movements,  they  are 
frequently  called  motor  fibres ;  but  these  last  names  are  bad, 
since  even  excluding  inhibitory  nerves,  many  afferent  fibres 
are  not  sensory  and  many  efferent  are  not  motor. 

We  may  distinguish  as  subdivisions  of  afferent  fibres  the 
following  groups.  1.  Sensory  fibres  proper,  the  excitement 
of  which  is  followed  by  a  sensation  when  they  are  connected 
with  their  brain-centre,  which  sensation  may  or  may  not  give 
rise  to  a  voluntary  movement.  2.  Keflex  fibres,  the  excitation 
of  which  may  be  attended  with  consciousness  but  gives  rise 
to  involuntary  efferent  impulses.  Thus  for  example  light 
falling  on  the  eye  causes  not  only  a  sensation,  but  also  a  nar- 
rowing of  the  pupil,  which  is  entirely  independent  of  the 
control  of  the  will.  No  absolute  line  can,  however,  be  drawn 
between  these  fibres  and  those  of  the  last  group:  any  sudden 
excitation,  as  an  unexpected  noise,  will  cause  an  involuntary 
movement,  while  the  same  sound  if  expected  would  cause  a 
movement  or  not  according  as  was  willed.  '.).  Excito  motor 
fibres.  The  excitation  of  these  when  reaching  a  nerve-centre 
'•auses  the  stimulation  of  efferent  fibres,  but  without  the  par- 
ticipation of  consciousness.  During  fasting,  for  instance,  bile 
accumulates  in  the  gall-bladder  and  remains  there  until  some 
semi-digested  food  passes  from  the  stomach  into  the  intestine. 
This  is  acid  and  stimulates  nerves  in  the  mucous  membrane 
lining  the  intestine,  and  these  convey  an  impulse  to  a  centre, 
which  in  consequence  sends  out  impulses  to  the  muscular  coat 
of  the  gall-bladder  causing  it  to  contract  and  expel  its  con- 
tent- into  the  intestine:  but  of  all  this  we  are  entirely  un- 
conscious. 4  Oentro-inhibitory fibres.  Whether  these  exist 
as  a  distinct  class   is  at  present  doubtful,     it  may  be  that 


192  THE  HUMAN  BODY. 

they  are  only  ordinary  sensory  or  reflex  fibres,  and  thai  the 
inhibition  is  due  only  to  the  interference  of  two  impulses 
reaching  a  central  organ  at  the  same  time  and  impeding  or 
hindering  the  full  productioD  of  the  normal  result  of  cither. 

In  efferent  nerve-fibres  physiologists  also  distinguish  sev- 
eral groups.  1.  Motor  fibres,  which  are  distributed  to  the 
muscles  and  govern  their  coin ract ions.  2.  Vaso-moi 'or  fibres. 
These  are  not  logically  separable  from  other  motor  fibres; 
nut  they  are  distributed  to  the  muscles  of  the  blood-vessels, 
and  by  governing  the  Mood-supply  of  various  parts,  indirectly 
produce  such  secondary  results  as  entirely  overshadow  their 
primary  effect  as  merely  producing  muscular  contractions. 
3.  Secretory  fibres.  These  are  distributed  to  the  cells  of  the 
Body  which  form  various  liquids  \\>fd  in  it.  as  the  saliva  and 
the  gastric  juice,  and  arouse  them  to  activity.  The  salivary 
glands,  for  instance,  may  he  made  to  form  saliva  by  stimulat- 
ing nerves  going  to  them,  and  the  same  is  true  of  the  cells 
which  form  the  sweat  poured  out  upon  the  surface  of  the 
Body.  4.  Trophic  nerve-fibres.  Under  this  head  are  included 
nerve-fibres  which  have  been  supposed  to  govern  the  nutri- 
tion of  the  various  tissues,  and  so  to  control  their  healthy 
life.  It  has  been  doubted  if  any  such  nerve-fibres  exist  as  a 
distinct  class,  and  no  doubt  many  of  the  facts  which  have  been 
cited  to  prove  their  existence  are  otherwise  explicable.  For 
instance,  shingles  is  a  disease  characterized  by  an  eruption  on 
the  skin  along  the  line  of  certain  nerves,  oftenest  those  which 
run  between  the  ribs;  but  it  may  be  dependent  upon  disease 
of  the  vaso-motor  nerves  which  control  the  blood-supply  of 
the  part.  In  other  cases  diseases  ascribed  to  injury  of  trophic 
nerves  have  been  shown  to  be  due  to  injury  of  the  sensory 
nerves  of  the  part,  which  having  lost  its  feeling,  is  exposed  to 
injuries  from  which  it  would  otherwise  have  been  protected. 
There  are,  however,  cases  which  seem  to  indicate  a  direct  nu- 
tritive influence  of  the  nervous  system  on  the  tissues;  as  for 
example  the  acute  bedsores  seen  in  some  diseased  states  of  the 
spinal  cord  and  leading  to  extensive  destruction  of  the  skin  in 
a  very  few  hours;  and  there  is  direct  experimental  evidence 
to  show  that  stimulation  of  the  branches  of  the  pneumogastric 
nerve  going  to  the  heart  tends  to  restore  that  organ  when  ex- 
hausted, while  stimulation  of  the  sympathetic  branches  has  a 
precisely  opposite  effect  (see  Chapter  XVIII).  There  is  also 
no  doubt  that  each  nerve  fibre  depends  for  the  maintenance 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     193 

of  its  nutrition  on  a  nerve-cell  since,  if  divided  in  its  course, 
the  part  separated  from  the  cell  rapidly  degenerates.  It 
might  also  he  urged  that  secretory  nerves  are  trophic  nerves 
in  the  true  sense  of  the  word,  since  when  excited  they 
cause  the  secretory  cells  to  live  in  a  special  way,  and  produce 
substances  which  when  unacted  upon  by  their  nerves  they 
do  not  form.  But  if  we  call  secretory  nerves  trophic  we 
must  include  also  under  that  name  all  other  efferent  nerves; 
the  nutritive  processes  going  on  in  a  muscular  fibre  when  at 
work  are  different  from  those  in  the  same  fibre  when  at  rest, 
and  the  same  is  true  of  all  other  cells  the  activity  of  which  is 
governed  by  nerve-fibres.  5.  P  crip  he  rally -acting  inhibitory 
nerves;,  such  as  those  which  slow  or  stop  the  beat  of  the  heart. 

Intercentral  Nerve-Fibres.  These,  which  do  not  convey 
impulses  between  peripheral  parts  and  nerve-centres,  but 
connect  one  centre  with  another,  form  a  final  group  in  addi- 
tion to  efferent  and  afferent  nerve-fibres.  Many  of  them 
connect  the  sporadic  and  sympathetic  ganglia  with  one  an- 
other and  with  the  cerebro-spinal  centre,  while  others  place 
different  parts  of  this  latter  in  direct  communication;  as  for 
instance  different  parts  of  the  spinal  cord,  the  brain  and  the 
spinal  cord,  and  the  two  halves  of  the  brain.  The  paths  taken 
by  some  of  these  commissural  fibres  will  be  stated  in  connec- 
tion with  the  physiology  of  the  brain  and  spinal  cord. 

General  Table.  We  may  physiologically  classify  nerve- 
fibres  as  in  the  following  tabular  form,  which  is  founded  upon 
the  facts  above  stated. 


Nerve-fibres. 


Peripheral. 


Afferent. 


KllVrenf. 


Intercentral.       I  i:x,ili"- 
(  Inhibitory. 


Sb'isory. 
Keflex. 

EAcito-motor. 
Inhibitory. 

f  Motor. 
j  V;i so-motor. 
<  Secretory. 
I  Trophic. 
I  Inhibitory. 


The  Electrical  Phenomena  of  Nerves.      Under  certain 
conditions  electrical  currents  can  be  led  off  from  living  nerve- 


11)4  THE  HUMAN  BODY. 

trunks  and  studied  by  ;iid  of  a  galvanometer:  in  all  respects 
these  currents  correspond  to  those  of  muscle,  excepl  thai  they 
arc  feebler.  A  perfectly  fresh  uninjured  nerve  at  rest  is 
isolectric,  and  so  is  a  eompletely  dead  nerve.  A  dying  por- 
tion <>f  a  nerve  is  negative  to  a  more  normal  portion,  and  in 
consequence,  if  electrodes  he  placed,  one  on  the  centre  and 
I  he  other  on  the  cut,  end  of  a  freshly-removed  portion  of  nerve, 
a  current  will  he  found  passing  through  the  connecting  wire 
from  the  central  portion  of  the  piece  of  nerve  towards  the 
peripheral.  A  region  of  nerve  in  activity,  that  is  transmit- 
ting a  nervous  impulse,  is  electro-negative  to  a  region  at  rest, 
other  things  being  equal;  accordingly,  an  action-current  or 
negative  variation  can  he  demonstrated  on  nerves  as  on  mus- 
cles; the  electrical  change  starting  from  the  point  of  stimu- 
lation and  travelling  along  the  trunk,  to  be  found  at  a  distant 
part  at  a  time  when  it  has  gone  from  the  place  of  its  first  ap- 
pearance. The  account  of  the  similar  phenomena  in  muscle 
(Chap.  X)  may  be  consulted  for  a  fuller  statement. 

The  Stimuli  of  Nerve-Fibres.  Nerve-fibres,  like  mus- 
cular fibres,  possess  no  automaticity;  acted  upon  by  certain 
external  forces  or  stimuli  they  propagate  a  change,  which  is 
known  as  a  nervous  impulse,  from  the  point  acted  upon  to 
their  ends;  but  they  do  not  generate  nervous  impulses  when 
left  entirely  to  themselves.  Normally,  in  the  living  Body 
the  stimulus  acts  on  a  nerve-fibre  at  one  of  its  ends,  being 
either  some  change  in  a  nerve-centre  with  which  the  fibre  is 
connected  (efferent  nerves)  or  some  change  in  an  organ  at- 
tached to  the  outer  end  of  the  nerve  (afferent  fibres).  Ex- 
periment shows,  however,  that  a  nerve  can  be  stimulated  in 
any  part  oi  its  course;  that  it  is  irritable  all  through  its  ex- 
tent. If,  for  example,  the  sciatic  of  a  frog  be  exposed  in  the 
thigh  and  divided,  it  will  be  found  that  electric  shocks  ap- 
plied at  the  point  of  division  to  the  outer  half  of  the  nerve 
stimulate  the  motor  fibres  in  it,  and  cause  the  muscular  fibres 
of  the  leg  to  contract:  and  similarly  such  shocks  applied  to 
the  cut  end  of  the  central  half  irritate  the  afferent  fibres  in 
it,  as  shown  by  the  signs  of  feeling  exhibited  by  the  animal. 
In  ourselves,  too,  we  often  have  the  opportunity  of  observing 
that  the  sensory  fibres  can  be  stimulated  in  their  course  at 
some  distance  from  their  ends.  A  blow  at  the  back  of  the 
elbow,  at  the  point  commonly  known  as  the  "  funny  bone  "or 
the  "  crazy  bone,"  compresses  the  ulnar  nerve  there  against  the 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     195 

subjacent  bone,  and  in  addition  to  irritating  the  nerves  of 
the  skin  at  the  point  struck,  starts  nervous  impulses  which 
make  themselves  known  by  severe  tingling  pain  referred  to 
the  little  and  ring  fingers  to  which  the  ulnar  nerve  is  dis- 
tributed. This  shows  not  only  that  the  nerve-fibres  can  be 
irritated  in  their  course  as  well  as  at  their  ends,  but  also  that 
sensations  do  not  directly  tell  us  where  a  nerve-fibre  has  been 
excited.  No  matter  where  in  its  course  an  impulse  causing 
sensation  has  been  started,  we  irresistibly  refer  its  origin  to 
the  peripheral  end  of  the  afferent  nerve-fibre  affected. 

General  and  Special  Nerve-stimuli.  Certain  external 
forces  excite  all  nerve-fibres,  and  in  any  part  of  their  course. 
These  are  known  as  general  nerve-stimuli ;  others  act  only 
on  the  end-organs  of  nerve-fibres,  and  often  only  on  one  kind 
of  end-organ,  and  hence  cannot  be  made  to  excite  all  nerves: 
these  latter  are  commonly  known  as  special  nerve-stimuli, 
In  reality  they  are  not  properly  nerve-stimuli  at  all;  but 
only  things  which  so  affect  the  irritable  tissues  attached  to 
the  ends  of  certain  nerve-fibres  as  to  make  these  tissues  in 
turn  excite  the  nerves.  For  example,  light  itself  will  not 
stimulate  any  nerve,  not  even  the  optic:  but  in  the  eye  it 
effects  changes  (perhaps  of  a  chemical  nature)  by  which 
nerve-stimuli  are  produced  and  these  stimulate  the  optic 
nerve-fibres.  The  ends  of  the  nerves  in  the  skin  are  not 
accessible  to  light  nor  are  the  proper  end  organs  on  which 
the  light  acts  present  there,  so  light  does  not  lead  to  the  pro- 
duction of  nervous  impulses  in  them:  but  the  optic  nerve 
without  its  peculiar  end-organs  would  be  just  as  insensible 
to  light  as  these  are.  Similarly  the  aerial  vibrations  which 
affect  us  as  sounds  do  not  stimulate  directly  the  fibres  of  the 
auditory  nerve.  They  act  on  terminal  organs  in  the  ear,  and 
these  then  stimulate  the  fibres  of  the  nerve  of  hearing,  just 
as  they  would  any  other  nerve  which  happened  to  be  con- 
nected with  them. 

General  Nerve  Stimuli.  Those  known  are  :  (1)  Electric 
curre?its.  An  electric  shock  passed  through  any  part  of  any 
nerve-fibre  powerfully  excites  it.  A  steady  current  passing 
through  a  nerve  does  not  stimulate  it,  but  any  sudden 
change  in  this,  whether  an  increase  or  a  decrease,  does.  A 
very  gradual  change  in  the  amount  of  electricity  passing 
through  a  nerve  in  a  unit  of  time  does  not  stimulate  it. 
(:.')  Mechanical  stimuli.    Any  sudden  pressure  or  traction,  as 


196  THE  HUMAN  BODY. 

a  blow  or  a  pull,  will  stimulate  a  nerve-fibre.  On  the  other 
hand  steady  pressure,  or  pressure  very  slowly  increased  from 
a  minimum,  will  not  excite  the  nerve.     (3)  Thermal  stimuli. 

Any  sudden  heating  or  cooling  of  a  nerve,  as  for  instance 
bringing  a  hotwire  close  to  it,  will  stimulate;  slow  changes 
of  temperature  will  not.  (4)  Chemical  stimuli.  Many  sub- 
stances which  chemically  alter  the  nerve-fibre  stimulate 
before  killing  it;  thus  dipping  the  cut  end  of  a  nerve  into 
a  strong  solution  of  common  salt  will  excite  it;  very  slow 
chemical  change  in  a  nerve  fails  to  stimulate. 

In  the  case  of  all  these  general  stimuli  it  will  be  seen  that 
as  one  condition  of  their  efficacy  they  must  act  with  con- 
siderable snddenness.  On  the  other  hand  too  transient  in- 
fluences have  no  effect.  An  electric  shock  sent  for  only 
0.0015  of  a  second  through  a  2ierve  does  not  stimulate  it:  ap- 
parently the  inertia  of  the  nerve  molecules  is  too  great  to  be 
overcome  by  so  brief  an  action.  So,  also,  strong  sulphuric 
acid  and  some  other  liquids  kill  nerves  immediately,  altering 
them  so  rapidly  that  they  die  before  being  stimulated. 

Special  Nerve-stimuli.  These  as  already  explained  act 
only  on  particular  nerves,  not  because  one  nerve  is  essen- 
tially different  from  another,  but  because  their  influence  is 
excited  through  special  end-organs  which  are  attached  to  some 
nerves.  These  stimuli  are:  (1)  Changes  occurring  in  central 
organs,  of  whose  nature  we  know  next  to  nothing,  but  which 
excite  the  efferent  nerve-fibres  connected  with  them.  The 
remaining  special  stimuli  act  on  afferent  fibres  through  the 
sense-organs.  They  are:  (2)  Light,  which  by  the  interven- 
tion of  organs  in  the  eye  excites  the  optic  nerve.  (3)  Sound, 
which  by  the  intervention  of  organs  in  the  ear  excites  the 
auditory  nerve.  (4)  Heat,  which  through  end-organs  in 
the  skin  is  able,  by  very  slight  changes,  to  excite  certain 
nerve-fibres:  such  slight  changes  of  temperature  being 
efficient  as  would  be  quite  incapable  of  acting  as  general 
nerve-stimuli  without  the  proper  end-organs.  (5)  Chemical 
agencies,  which  when  extremely  feeble  and  incapable  of 
acting  as  general  stimuli  can  act  as  special  stimuli  through 
special  end-organs  in  the  mouth  and  nose  (as  in  taste  and 
smell)  and  probably  in  other  parts  of  the  alimentary  tract, 
where  very  feeble  acids  and  alkalies  seem  able  to  excite  cer- 
tain nerves,  and  reflexly  through  them  excite  movements  or 
render  active  the  cells  concerned   in  making  the   digestive 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     197 

liquids.  (6)  Mechanical  stimuli  when  so  feeble  as  to  be  ineffi- 
cient as  general  stimuli.  Pressure  on  the  skin  of  the  fore- 
head or  of  the  back  of  the  hand,  equal  to  .002  gram  (.03  grain) 
can  be  felt  through  the  end-organs  of  the  sensory  fibres  there, 
but  would  be  quite  incapable  of  acting  as  a  general  stimulus 
if  applied  directly  to  the  nerve-fibre. 

It  will  be  noticed  as  regards  the  special  stimuli  of  afferent 
nerves  that  many  of  them  are  merely  less  degrees  of  general 
stimuli  ;  the  end-organs  in  skin,  mouth,  and  nose  are  in  fact 
excited  by  the  same  things  as  nerve-fibres,  but  they  are  far 
more  irritable.  In  the  case  of  the  higher  senses,  seeing  and 
hearing,  however,  the  end-organs  seem  to  differ  entirely  in 
property  from  nerve-fibres,  being  excited  by  sonorous  and 
luminous  vibrations  which,  so  far  as  we  know,  will  in  no 
degree  of  intensity  directly  excite  nerve-fibres.  To  construct 
an  end-organ  capable  of  recognizing  very  slight  pressures  we 
may  imagine  that  all  that  would  be  needed  would  be  to  expose 
directly  a  very  delicate  end-branch  of  the  axis  cylinder  ;  and 
tnis  seems  in  fact  to  be  the  case  in  the  nerves  of  the  transpar- 
ent exposed  part  of  the  eyeball,  if  not  in  some  other  parts  of 
the  external  integument  of  the  Body.  But  as  axis  cylinders 
are  quite  unirritable  by  light  or  sound  a  mere  exposure  of 
them  would  be  useless  in  the  eye  or  ear,  and  in  each  case  we 
find  accordingly  a  very  complex  apparatus  developed,  whose 
function  it  is  to  convert  modes  of  motion  which  do  not 
excite  nerves  into  others  which  do.  We  might  compare 
this  apparatus  to  a  fuse  with  a  detonating  cap  attached  ;  the 
stimulus  of  a  blow  from  a  hammer  which  would  not  itself 
ignite  the  fuse,  acting  on  the  detonating  material  (repre- 
senting an  "end-organ")  causes  it  to  give  off  a  spark,  and 
this  in  turn  ignites  the  fuse  which  answers  to  the  nerve-fibre. 

Specific  Nerve-energies.  We  have  already  seen  that  a 
nervous  impulse  propagated  along  a  nerve-fibre  will  give  rise 
to  different  results  according  as  different  nerve-fibres  are 
concerned.  Travelling  along  one  fibre  it  will  arouse  a  sensa- 
tion, along  anol  her  a  movement,  along  a  third  a  secretion.  In 
addition  we  may  observe  readily  that  these  different  results 
may  be  produced  by  the  same  physical  force  when  it  acts 
upon  different  nerves.  Radiant  energy,  for  example,  falling 
into  the  eye  causes  a  sensation  of  sight,  but  falling  upon  the 
skin  one  of  heat,  if  any.  The  different  results  which  follow 
the  stimulation  of  different  nerves  do  not  then  depend  upon 


198  THE  HUMAN  BODY. 

differences  in  the  physical  forces  exciting  them.  This  is 
still  further  shown  by  the  fact  that  different  physical  forces 
acting  upon  the  same  nerve  arouse  the  same  kind  of  sensa- 
tion. Light  reaching  the  eye  causes  a  sight  sensation,  but  if 
the  optic  nerve  be  irritated  by  a  blow  on  the  eyeball  a  sensa- 
tion of  light  is  felt  just  as  if  actual  light  had  stimulated  the 
nerve-ends;  and  a  similar  result  follows  if  an  electric  shock  be 
sent  through  the  eyeball  and  optic  nerve.  Different  nerves 
excited  by  the  same  stimulus  produce  different  results,  and 
the  same  nerve  excited  by  different  stimuli  gives  the  same 
result.     How  are  these  facts  to  be  explained  ? 

The  first  explanation  which  suggests  itself  is  that  the 
various  nerves  differ  in  their  properties  :  that  electricity  ap- 
plied to  a  motor  nerve  causes  a  muscle  to  contract,  and 
to  the  optic  nerve  a  visual  sensation,  and  to  the  lingual 
nerve  a  sensation  of  taste,  because  nervous  impulses  in 
the  motor,  optic,  and  lingual  nerves  differ  from  one  an- 
other. This  was  the  view  held  by  the  older  physiologists; 
and  that  supposed  peculiarity  of  a  muscular  nerve  by  which 
its  irritation  caused  a  muscular  contraction,  and  that  of 
of  the  oj)tic  nerve  in  consecpience  of  which  its  excitation 
caused  a  sensation  of  sight,  and  so  on.  they  called  the  specific 
energy  of  the  nerve.  Seeing  further  that  when  a  pure  motor 
nerve  was  cut  and  its  peripheral  stump  pinched  the  muscles 
connected  with  it  contracted,  but  that  when  its  central  end 
was  pinched  no  sensation  or  other  recognizable  change  fol- 
lowed, while  exactly  the  reverse  was  true  of  a  sensory  nerve, 
they  believed  that  afferent  nerves  differed  essentially  from 
efferent  nerves,  inasmuch  as  the  latter  could  only  propagate 
impulses  centrifugally  and  the  former  only  centripetally. 
Now,  however,  we  have  much  reason  to  believe  that  this  view 
is  wrong,  and  that  ad  nerve-fibres,  though  perhaps  exhibiting 
some  minor  differences,  are  essentially  alike  in  their  physio- 
logical properties,  and  can  carry  nervous  impulses  either  way. 
The  differences  observed  depend  in  fact  not  on  any  differ- 
ences in  the  nerve-fibres,  but  on  the  different  parts  connected 
with  their  ends;  that  is  to  say,  on  the  different  terminal 
organs  excited  by  the  impulses  conveyed  by  the  fibre.  A 
motor  fibre  is  one  which  has  at  its  peripheral  end  a  muscular 
fibre,  and  a  centrifugally  travelling  impulse  reaching  this  will 
cause  it  to  contract:  but  the  cells  connected  with  its  central 
end  are  not  of  such  a  nature  as  to  give  rise  to  sensations 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     199 

when  centripetally  travelling  impulses  reach  them,  or  to 
transmit  these  to  other  efferent  fibres  and  so  cause  reflex 
movements;  and  therefore  when  a  motor  fibre  is  stimulated 
in  the  middle  of  its  course  the  out  ward -going  impulse  causes 
a  movement,  while  the  centrally  travelling  impulse,  starting 
at  the  same  time,  gives  no  direct  sign  of  its  existence.  Simi- 
larly for  a  sensory  nerve  such  as  the  ulnar,  already  referred 
to:  if  it  be  stimulated  at  the  elbow  the  centrally  travelling 
impulse  will  cause  a  sensation  of  pain  by  exciting  the  brain- 
centre  connected  with  it,  but  the  outward  travelling  impulse 
not  reaching  muscular  fibres  or  other  parts  which  it  can 
arouse  to  activity,  remains  concealed  from  our  notice.  In 
other  words,  the  so-called  specific  energy  of  a  nerve-fibre  de- 
pends upon  the  terminal  organs  on  which  it  can  act,  and  not 
on  any  peculiarity  of  the  nerve-fibre  itself. 

Proofs  that  all  Nerve-Fibres  are  Physiologically  Alike. 
(1)  The  most  marked  difference  between  nerve-fibres  is  obvi- 
ously that  between  efferent  and  afferent,  and  the  microscope 
fails  entirely  to  show  any  differences  between  the  two.  Some 
motor  and  some  sensory  fibres  may  be  bigger  or  less  than 
others,  some  may  be  white  and  others  may  be  gray;  but  such 
differences  are  secondary,  and  have  no  direct  relation  to  the 
function  of  a  fibre  as  afferent  or  efferent.  We  can  recognize 
no  constant  difference  in  structure  between  the  two.  (".') 
The  physical  properties  and  chemical  composition  of  motor 
and  sensory  nerves  agree  in  all  known  points.  (3)  When  a 
nerve,  say  a  motor  one,  is  stimulated  half-way  between  the 
centre  and  a  muscle,  a  nervous  impulse,  as  we  call  it,  is 
propagated  to  the  muscle,  which  impulse  starts  at  the  point 
stimulated  and  travels  at  a  definite  rate  to  the  muscle,  the 
contraction  of  which  latter  gives  proof  of  its  arrival.  Now 
starting  at  the  same  moment  from  the  same  point,  and 
travelling  at  the  same  rate,  is  that  change  in  the  elec- 
trical condition  of  the  nerve  which  can  be  detected  by  a 
galvanometer,  the  so-called  negative  variation  or  actio//  cur- 
rent. When  a  nerve  is  excited  from  its  end-organ,  as  for 
example  the  optic  nerve  by  light  falling  into  the  eyeball, 
or  a  motor  nerve  by  a  stimulus  arising  in  a  centre,  an  action 
current  exactly  like  thai  observed  with  artificial  stimulation 
travel.-  along  it.  Since  this  negative  variation  always  accom- 
panies a  nervous  impulse,  appearing  when  it  appears  and  dis- 
appearing when  it  disappears,  we  conclude  that  it  is  a  change 


200  THE  HUMAN  BODY. 

in  the  electrical  properties  of  the  nerve  dependent  on  that 
internal  movement  of  its  molecules  which  constitutes  a  ner- 
vous impulse.  It  is  an  externally  recognizable  physical  sign, 
and  the  only  known  one,  of  the  existence  of  the  nervous  im- 
pulse while  it  is  travelling  along  the  fibre.  If  the  muscle  were 
cui  away  from  the  end  of  the  nerve  we  could  still  detect  that 
a  nervous  impulse  had  travelled  from  the  point  of  stimulation 
to  that  where  the  fibres  were  divided,  by  tracking  the  nega- 
tive variation.  If,  while  stimulating  a  motor  nerve  half-way 
in  its  course,  we  examine  galvanometrically  the  portion  lying 
central  to  the  stimulated  point  we  find  a  well-marked  centripe- 
tally  travelling  action  current;  it  starts  at  the  same  moment  as 
the  efferent  negative  variation  and  travels  in  the  same  manner, 
but  the  nervous  impulse  of  which  it  is  a  sign  produces  no  more 
effect  than  the  efferent  impulse  would  after  the  muscle  had  been 
cut  away;  for  it  does  not  reach  any  muscular  fibre,  or  sen- 
sory or  reflex  centre,  which  it  can  arouse  to  activity.  Hence 
it  is  clear  that  the  motor  nerve  can  conduct  impulses  equally 
well  in  either  direction;  and  similar  experiment  proves  the 
same  thing  for  pure  sensory  nerves. 

While,  however,  by  chemical  or  electrical  stimulation  of 
a  motor  or  a  secretory  nerve  we  can  get  a  muscular  con- 
traction or  a  secretion  apparently  quite  identical  with  that 
produced  by  natural  stimulation,  we  cannot  make  the  same 
assertion  with  regard  to  afferent  nerves.  It  is  possible  by 
gentle  stimulation  of  a  cutaneous  afferent  nerve  through  its 
end-organs  in  the  skin  to  excite  the  centres,  so  that  they  in 
turn  give  rise  to  definitely  combined  reflex  muscular  con- 
tractions, producing,  even  in  the  absence  of  all  consciousness, 
an  useful  movement.  But  if  the  skin  be  removed  and  the 
outer  end  of  its  afferent  nerve  stimulated  directly,  though 
the  centres  can  be  thus  excited  and  caused  to  send  out  im- 
pulses to  muscles,  the  movements  which  result  are  random 
kicks  and  jerks,  very  different  from  the  definite,  orderly 
movements  which  follow  suitable  stimulation  through  the 
skin.  And  as  regards  certain  nerves  of  special  sense  some- 
thing similar  seems  to  be  true.  It  has  indeed  been  stated 
that  mechanical  injury  of  the  optic  nerve,  as  by  cutting  it 
during  a  surgical  operation,  causes  a  sensation  of  light  in 
patients  not  anaesthized,  but  this  has  been  denied;  and 
though  one  positive  observation  counts  for  more  in  such  a 
case   than   many  negative,  we   must  take  into  account  the 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     201 

fact  that  in  no  other  sense-organ  has  the  direct  stimulation 
of  the  proper  nerve-trunk  in  any  other  way  than  through 
the  sense-organ  at  its  outer  end,  been  proved  to  give  rise 
to  true  sensations  of  special  sense.  Stimulation  of  the  nerves 
exposed  in  a  wound  does  not  cause  a  true  touch  sensation, 
but  a  feeling  of  pain;  and  similarly  irritation  of  the  trunks 
of  the  nerves  of  taste  by  diseased  conditions  does  not  seem  to 
ever  cause  true  taste  sensations  unless  the  end-organs  in  the 
mouth  be  also  excited.  Even  if  it  turn  out  to  be  true  that 
a  nerve  of  special  sense  is  only  capable  of  giving  rise  to 
feelings  belonging  to  the  sphere  of  that  sense  when  ex- 
cited through  its  proper  end-organs,  that  does  not  prove 
that  its  nerve-fibres  have  any  unique  faculty  distinguishing 
them  in  property  from  other  nerve-fibres.  It  only  means 
that  the  brain  organ,  the  central  nerve-cell  mechanism,  to  be 
excited  by  the  nerve  is  highly  complex,  and  only  responds 
with  the  proper  sensation  when  stimulated  in  proper  strength 
and  proper  rhythm,  and  the  sense  organs  accomplish  this. 
Even  the  most  delicate  artificial  stimulation  that  we  can 
apply  to  a  naked  nerve-trunk  is  undoubtedly  a  crude  and 
gross  thiug  compared  with  the  stimuli  arising  in  the  retina 
when  light  enters  the  eye,  or  in  certain  skin  nerve  end- 
organs  when  moderate  heat  falls  on  them.  If  we  merely 
wish  to  get  a  noise  out  of  a  piano  it  does  not  matter  how 
we  strike  it,  if  we  strike  hard  enough  ;  and  a  muscular  con- 
traction or  an  irregular  set  of  muscular  contractions  excited 
by  direct  stimulation  of  a  nerve-trunk  may  be  compared  to 
such  a  noise.  If  we  wish  for  a  definite  musical  chord  we 
must  strike  through  the  keyboard  in  a  definite  way;  and  the 
orderly  combined  muscular  movements  and  the  special  sensa- 
tions which  follow  stimulation  through  the  proper  sense- 
organs  may  be  compared  to  such  chords.  In  our  bodies  the 
keyboards  are  different  in  eye,  ear,  and  skin,  and  adapted  to 
be  set  in  action  by  different  external  physical  agencies,  and 
the  strings  in  connection  with  each  keyboard  are  different 
and  give  different  results;  but  the  connecting  apparatus,  the 
nerve-fibre,  lying  between  the  keys  in  the  sense-organ-  ami 
the  strings  respectively  responding  to  them  in  the  centres, 
is  essentially  the  same  in  all  cases. 

To  put  the  case  more  definitely:  Light  outside  the  eye 
exists  as  ethereal  vibrations,  sound  outside  the  ear  as  vibra- 
tions of  the  air  (commonly).     Each  kind  of  vibration  acta   m 


202  THE  HUMAN   BODY. 

a  particular  end-organ  in  eye  or  ear  which  is  adapted  to  be 
acted  upon  by  it,  and  in  turn  these  end-organs  excite  the 
optic  and  auditory  nerve-fibres;  these  in  consequence  trans- 
mit impulses,  which  reaching  different  parts  of  the  brain 
excite  them ;  the  excitement  of  one  of  these  brain-centres  is 
associated  with  sonorous  and  of  the  other  with  visual  sensa- 
tions. The  nervous  impulse  in  the  two  cases  is  quite  alike, 
at  least  as  to  quality  (though  it  may  differ  in  quantity  and 
rhythm)  and  the  resulting  difference  in  quality  of  the  sensa- 
tions cannot  depend  on  it.  The  quality  differences  in  these 
cases  must  be  products  of  the  central  nervous  system.  If  we 
had  a  set  of  copper  wires  we  might  by  sending  precisely 
similar  electric  currents  through  them  produce  very  different 
results  if  different  things  were  interposed  in  their  course. 
In  one  case  the  current  might  be  sent  through  water  and 
decompose  it,  doing  chemical  work;  in  another,  through  the 
coil  of  an  electro-magnet  and  raise  a  weight;  in  a  third, 
through  a  thin  platinum  wire  and  develop  light  and  heat; 
and  so  on,  the  result  depending  on  the  terminal  organs,  as  we 
may  call  them,  of  each  wire.  Or,  on  the  other  hand,  we 
might  generate  the  current  in  each  wire  differently,  in  one 
by  a  Daniell's  cell,  in  a  second  by  a  thermo-electric  machine, 
\w  a  third  by  the  rotation  of  a  magnet  inside  a  coil,  but  the 
currents  in  the  wires  would  be  essentially  the  same,  as  the 
nervous  impulses  are  in  a  nerve-fibre.  No  matter  how  they 
have  been  started,  provided  their  amount  is  the  same, 
whether  they  shall  produce  similar  or  dissimilar  results,  de- 
pends only  on  whether  they  are  connected  with  similar  or 
dissimilar  end-organs. 

To  sum  up:  Afferent  and  efferent  nerve-fibres  differ  in  no 
fundamental  physiological  property;  they  are  simple  trans- 
mitters, everywhere  alike  in  faculty.  We  may  extend  this 
statement  to  the  subdivisions  of  each  kind,  and  say  that 
motor,  vasomotor  and  secretory  efferent  fibres,  and  tactile, 
auditory  and  visual  afferent  fibres  are  in  all  essentials  like  one 
another;  and  that  a  nervous  impulse  is  alike  in  every  nerve, 
varying  it  may  be  in  intensity  and  in  the  rate  at  which 
others  succeed  it,  in  different  cases,  but  the  same  in  kind. 
Just  as  all  muscles  are  alike  in  general  physiological  proper- 
ties, and  differ  in  special  function  according  to  the  parts  on 
which  they  act,  so  are  all  nerve-fibres  alike  in  general  physio- 
logical properties,  and  differ  in  special  function  only  because 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     203 

they  are  attached  to  special  things.  The  special  physiology 
of  various  nerves  will  hereafter  be  considered  in  connection 
with  the  working  of  various  mechanisms  in  the  Body. 

The  Nature  of  a  Nervous  Impulse.  Since  between 
sense-organs  and  sensory  centres,  and  these  latter  and  the 
muscles,  nervous  impulses  are  the  only  means  of  communi- 
cation, it  is  through  them  that  we  arrive  at  our  opinions  con- 
cerning the  external  universe  and  through  them  that  we  are 
able  to  act  upon  it;  their  ultimate  nature  is  therefore  a 
matter  of  great  interest,  but  one  about  which  we  unfortu- 
nately know  very  little.  We  cannot  well  imagine  it  any- 
thing but  a  mode  of  motion  of  the  molecules  of  the  nerve- 
fibres,  but  beyond  this  hypothesis  we  cannot  go  far.  In 
many  points  the  phenomena  rjresented  by  nerve-fibres  as 
transmitters  of  disturbances  are  like  the  phenomena  of  wires 
as  transmitters  of  electricity,  and  when  the  phenomena  of  cur- 
rent electricity  were  first  observed  there  was  a  great  ten- 
dency, explaining  one  unknown  by  another,  to  consider  ner- 
vous impulses  merely  as  electrical  currents.  The  increase  of 
our  knowledge  concerning  both  nerves  and  electric  currents, 
however,  has  made  such  an  hypothesis  almost,  if  not  quite, 
untenable.  In  the  first  place  nerve-fibres  are  extremely  bad 
conductors  of  electricity — so  bad  that  it  is  impossible  to  sup- 
pose them  used  in  the  Body  for  that  purpose;  and  in  the 
second  place,  merely  physical  continuity  of  a  nerve-fibre, 
such  as  would  not  interfere  with  the  passage  of  an  electric 
current,  will  not  suffice  for  the  transmission  of  a  nervous  im- 
pnlse.  For  instance  if  a  damp  string  be  tied  around  a  nerve, 
or  if  it  be  cut  and  its  two  moist  ends  placed  in  contact,  no 
nervou-  impulse  will  be  transmitted  across  the  constricted  or 
divided  point  although  an  electrical  current  would  pass 
readily.  A?i  electrical  shock  may  be  used  like  many  other 
stimuli  to  upset  the  equilibrium  of  the  nerve-molecules  and 
-tint  a  nervous  impulse,  which  then  travels  along  the  fibre, 
but  is  just  as  different  from  the  stimulus  exciting  it  as  a 
muscular  contraction  is  from  the  stimulus  which  calls  it 
forth. 

Careful  study  of  the  action-current  gives,  perhaps,  some 
information  regarding  the  nature  of  nervous  impulses.  That 
local  negativity  which  causes  the  current  begins  at  the  stimu- 
lated point  of  a  nerve  at  t  lie  -nine  time  as  the  nervous  impulse 
ami  travels  along  the  nerve  at  the  same  rate.     Hence  we  con- 


204  THE   II UMAX  BODY. 

elude  that  the  new  internal  molecular  arrangement  in  a  nerve- 
fibre  which  constitutes  its  active  as  compared  with  its  resting 
slate,  is  one  which  changes  also  the  electrical  properties  of  the 
fibre.  Now  it  is  found  that  the  negative  variation  travel-  along 
the  nerve  (in  the  frog)  at  the  rate  of  28  metres  (92.00  feet)  in 
a  second,  and  takes  .000T  second  to  pass  by  a  given  point  : 
accordingly  at  any  one  moment  it  extends  over  about  L8  mm. 
(0.720  inch)  of  the  nerve-fibre.  Moreover,  when  first  reach- 
ing a  point  it  is  very  feeble,  then  rises  to  a  maximum,  and 
gradually  fades  away  again,  'faking  it  as  an  indication  of 
what  is  going  on  in  the  nerve,  we  may  assume  that  the  nerv- 
ous impulse  is  a  progressive  molecular  change  of  a  wavelike 
character,  rising  from  a  minimum  to  a  maximum,  then  grad- 
ually ceasing,  and  about  18  millimetres  in  wave-length. 

A  nervous  impulse  does  not  appear  to  exhaust  a  fibre  when 
transmitted  along  it.  As  a  ray  of  light  traversing  the  ether 
sets  up  a  transient  change  in  it  but  does  not  in  any  way  use  it 
up  or  leave  it  less  fit  to  transmit  a  succeeding  ray,  so  it  is 
with  the  nervous  impulse  in  its  transmission.  It  is  true  that 
when  a  motor  nerve  attached  to  a  muscle  is  continuously 
stimulated  the  muscular  contractions  cease  after  a  certain 
time,  though  the  muscle  still  responds  to  electrical  stimula- 
tion directly  applied,  and  it  has  been  argued  that  we  thus  get 
evidence  of  the  exhaustion  of  the  nerve:  but  it  must  be 
borne  in  mind  that  an  electrical  shock  directly  applied  is  un- 
doubtedly a  much  more  powerful  stimulus  to  the  muscle  than 
any  nervous  impulse,  and  the  muscle  may  have  been  so 
fatigued  by  its  previous  work  as  to  have  become  irresponsive 
to  stimulation  through  its  nerve,  though  still  reacting  to  the 
grosser  excitation.  And  we  have  direct  evidence  that  stimu- 
lation of  a  nerve  may  be  continued  for  a  very  long  time  with 
out  causing  loss  of  activity.  As  an  instance,  we  may  take  the 
nerve  already  mentioned  which  stops  the  beat  of  the  heart : 
when  it  is  stimulated  continuously  for  a  feAV  seconds  the  heart 
breaks  beyond  its  control  and  begins  to  beat  again,  though  the 
stimulation  of  the  nerve  be  kept  up.  This,  however,  is  due  t<> 
fatigue  of  the  endings  of  the  nerve  in  the  heart,  and  not  of 
the  nerve  fibres,  as  may  be  proved  in  this  way:  the  nerve 
(pneumo-gastric)  being  carefully  exposed  in  the  neck  is  arti- 
ficially cooled  in  one  region  to  below  the  temperature  at 
which  it  can  conduct  a  nervous  impulse;  it  is  then  stimu- 
lated at  a  point  nearer  the  head  than  the  cooled  portion:  the 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     205 

resulting  impulses  being  blocked  on  their  way  to  the  heart  it 
goes  on  beating  regularly.  After  stimulation  of  the  nerve 
has  been  continued  for  several  minutes  the  cooled  tract  of  the 
nerve  is  allowed  to  warm  again  until  it  becomes  capable  of 
transmitting  a  nervous  impulse;  then  the  heart-beat  is  found 
to  be  promptly  stopped  or  slowed.  This  shows  that  if  the 
cardiac  endings  of  the  nerve  be  protected  from  fatigue,  pro- 
longed stimulation  of  the  nerve-trunk  does  not  interfere  with 
its  functional  capacity:  the  stimulation  still  starts  nervous 
impulses  in  it,  which  as  soon  as  they  can  pass  on  produce  their 
normal  effect  on  the  heart.  "When  long-continued  sensations 
become  dulled  the  explanation  is  no  doubt  similar:  it  is  the 
end-organs,  central  or  peripheral,  or  both,  which  are  ex- 
hausted, not  the  nerve-fibres  themselves.  It  has,  however, 
been  observed  that  when  artificial  stimulation  is  long  applied 
to  one  point  on  a  nerve-trunk  that  point  sometimes  becomes 
unexcitable,  though  the  nerve  in  general  is  still  epiite  func- 
tional and  acts  perfectly  when  the  point  of  application  of  the 
stimulus  is  shifted  a  little:  this  is  especially  the  case  with 
gray  nerve-fibres  and  white  fibres  having  a  thin  medullary 
sheath. 

The  very  sparse  blood-supply  of  nerve-trunks  is  in  great 
contrast  to  the  rich  supply  of  those  parts  of  the  nervous  system 
containing  nerve-cells  and  to  the  abundant  supply  of  muscles, 
and  is  an  evidence  that  the  chemical  changes  taking  place  in 
them  during  both  rest  and  activity  are  but  small.  Seeing 
that  functional  activity  leads  to  little  or  no  using  up  of  the 
conductive  substance  of  a  nerve-fibre  any  more  than  the 
transmission  of  a  galvanic  current  uses  up  a  copper  wire,  the 
term  irritable  is  not  properly  applicable  to  nerve-fibres.  Ir- 
ritability in  its  physiological  sense  we  have  defined  as  a  con- 
dition of  a  living  tissue  such  that  a  very  small  extraneous 
force  acting  on  it  may  cause  it  to  set  free  a  disproportionately 
large  amount  of  energy,  and  in  that  sense  muscle-fibres  and 
nerve-cells  are  truly  irritable,  and  they  both  use  up  their  ma- 
terial when  at  work  and  are  subject  to  exhaustion.  Nerve- 
fibres  are  excitable  and  conductive,  but  not  really  irritable, 
though  on  account  of  their  great  excitability  they  are  very 
generally  spoken  of  as  irritable. 

The  Rate  of  Transmission  of  a  Nervous  Impulse. 
This  can  be  measured  in  several  ways.  One  of  the  simplest 
is  a  modification  of  the    simple  nerve-muscle  experiment  il- 


206  THE  HUMAN  BODY. 

Lustrated  in  Fig.  02.  The  muscle  M  is  dissected  oul  with  its 
motor  nerve  attached,  and  the  stimulus  applied  to  the  nerve 
and  not  directly  to  the  muscle.  First  the  stimulus  is  given 
to  the  nerve  close  to  the  muscle:  it  is  then  found  that  the 
period  of  latent  excitation,  as  shown  by  the  greater  length  of 
tu,  is  a  very  little  longer  than  when  the  muscle  is  directly 
stimulated.  Next  the  stimulus  is  applied  to  the  nerve,  say  t  wo 
inches  from  the  muscle,  and  it  is  found  that  tu  is  consider- 
ably longer,  the  increase  in  its  length  being  due  to  the  time 
taken  by  the  nervous  impulse  in  travelling  along  two  inches 
of  nerve.  As  we  know  the  rate  of  movement  of  the  surface 
S,  we  can  readily  calculate  the  amount  of  the  time  increase. 
The  rate  of  travel  of  the  nervous  impulse  as  thus  ascertained 
is  almost  incomparably  slower  than  that  of  an  electric  cur- 
rent, being  28  metres  (92.00  feet)  per  L".  In  the  motor  nerves 
of  warm-blooded  animals  the  rate  of  transmission  is  somewhat 
faster.  Considerable  difficulties  are  met  with  in  making  cor- 
responding measurements  on  afferent  nerves,  and  the  rates 
obtained  by  different  observers  differ  widely:  probably  the 
impulse  travels  at  about  the  same  speed  as  in  the  motor  nerves 
of  the  same  animal. 

Functions  of  the  Spinal  Nerve-Roots.  The  great  ma- 
jority of  the  larger  nerve-trunks  of  the  Body  contain  both 
afferent  and  efferent  nerve-fibres.  If  one  be  exposed  in  its 
course  and  divided  in  a  living  animal,  it  will  be  found  that 
irritating  its  peripheral  stump  causes  muscular  contractions, 
and  pinching  its  central  stump  causes  signs  of  sensation, 
showing  that  the  trunk  contained  both  motor  and  sensory 
fibres.  If  the  trunk  be  followed  away  from  the  centre,  as 
it  breaks  up  into  smaller  and  smaller  branches,  it  will  be 
found  that  the&e  too  are  mixed  until  very  near  their  endings, 
where  the  very  finest  terminal  branches  close  to  the  end- 
organs,  whether  muscular  fibres,  secretory  cells,  or  sensory 
apparatuses,  are  only  afferent  or  efferent.  If  the  nerve- 
trunk  be  one  that  arises  from  the  spinal  cord  and  be  ex- 
amined progressively  back  to  its  origin,  it  will  still  be  found 
mixed,  up  to  the  point  where  its  fibres  separate  to  enter 
either  a  ventral  or  a  dorsal  nerve-root.  Each  of  these  latter, 
however,  is  pure,  all  the  efferent  fibres  leaving  the  cord  by 
the  ventral  or  anterior  roots,  and  all  the  afferent  entering  it  by 
the  posterior  or  dorsal.  This  of  course  could  not  be  learned 
from  examination  of  the  dead  nerves,  since  the  best  micro- 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     207 

scope  fails  to  distinguish  au  afferent  from  an  efferent  fibre, 
but  is  readily  proved  by  a  simple  experiment.  If  an  anterior 
root  be  cut  and  its  outer  end  stimulated,  the  muscles  of  the 
parts  to  which  the  trunk  which  it  helps  to  form  is  distributed 
will  be  made  to  contract,  and  the  skin  will  be  made  to  sweat 
also  if  the  root  happen  to  be  one  that  contains  secretory 
fibres  for  the  sweat-glands.  On  the  other  hand,  if  the  cen- 
tral end  of  the  root  (that  part  of  it  attached  to  the  cord)  be 
stimulated  no  result  will  follow,  showing  that  the  root  con- 
tains no  sensory,  reflex,  or  excito-motor  fibres.  With  the 
posterior  roots  the  reverse  is  the  case:  if  one  of  them  be 
divided  and  its  outer  end  stimulated,  no  observed  result  fol- 
lows, showing  the  absence  of  all  efferent  fibres;  but  stimula- 
tion of  its  central  end  will  cause  either  signs  of  feeling,  or 
reflex  actions,  or  both.  We  might  compare  a  spinal  nerve- 
trunk  to  a  rope  made  up  of  green  and  red  threads  with  at 
one  end  all  the  green  threads  collected  into  one  skein  and 
the  red  into  another,  which  would  represent  the  roots.  At 
its  farthest  end  we  may  suppose  the  rope  divided  into  finer 
cords,  each  of  these  containing  both  red  and  green  threads, 
down  to  the  very  finest  branches  consisting  of  only  a  few 
threads,  and  those  all  of  one  kind,  either  red  or  green,  one 
representing  efferent,  the  other  afferent,  fibres. 

The  Cranial  Nerves.  Most  of  these  are  mixed  also,  but 
with  one  exception  (the  fifth  pair,  the  small  root  of  which  is 
efferent  and  the  large  gangliated  one  afferent)  they  do  not 
present  distinct  motor  and  sensory  roots,  like  those  of  the 
spinal  nerves.  At  their  origin  from  the  brain  most  of  them 
are  purely  afferent  or  purely  efferent,  and  the  mixed  character 
which  their  trunks  exhibit  is  due  to  cross-branches  with 
neighboring  nerves,  in  which  afferent  and  efferent  fibres  are 
interchanged.  The  olfactory,  optic,  and  auditory  nerves  re- 
main, however,  purely  afferent  in  all  their  course,  and'  others, 
though  not  quite  pure,  contain  mainly  efferent  fibres  (as  the 
facial)  or  mainly  afferent  (as  the  glosso-pharyngeal). 

The  Intercommunication  of  Nerve-Centres.  From  the 
anatomical  arrangement  of  the  nervous  system  it  is  clear  that 
it  forms  one  continuous  whole.  No  subdivision  of  it  is 
isolated  from  the  rest,  but  nerve-trunks  proceeding  from  the 
centres  in  one  direction  bind  them  to  various  tissues  and, 
proceeding  in  another,  to  other  nerve-centres,  which  in  turn 
an-   united  with   other   tissues  and   other  centres.     Since  the 


208  THE  HUMAN  BODY. 

physiological  character  of  a  nerve-fibre  is  its  conductivity — 
its  power  of  propagating  a  disturbance  when  once  its  mo- 
lecular equilibrium  has  been  upset  at  any  one  point — it  is 
obvious  that  through  the  nervous  system  any  one  part  of  the 
Body,  supplied  with  nerves,  may  react  on  all  other  parts 
(with  the  exception  of  such  as  hairs  and  nails  and  cartilages, 
which  are  not  known  to  possess  nerves)  and  excite  changes  in 
them.  Pre-eminently  the  nervous  system  forms  a  uniting 
anatomical  and  physiological  bond  through  the  agency  of 
which  unity  and  order  are  produced  in  the  activities  of  differ- 
ent and  distant  parts.  We  may  compare  it  to  the  Western 
Union  Telegraph,  the  head  office  of  which  in  New  York 
would  represent  the  brain  and  spinal  cord,  the  more  impor- 
tant central  offices  in  other  large  cities  the  sympathetic 
ganglia,  and  the  minor  offices  in  country  stations  the  sporadic 
ganglia:  while  the  telegrajm-wires,  directly  or  indirectly 
uniting  all,  would  correspond  to  the  nerve-trunks.  Just  as 
information  started  along  some  outlying  wire  may  be  trans- 
lnitted  to  a  central  office,  and  from  it  to  others,  and  then, 
according  to  what  happens  to  it  in  the  centre,  be  stopped 
there,  or  spread  in  all  directions,  or  in  one  or  two  only,  so 
may  a  nervous  disturbance  reaching  a  centre  by  one  nerve- 
trunk  merely  excite  changes  in  it  or  be  radiated  from  it 
through  other  trunks  more  or  less  widely  over  the  Body  and 
arouse  various  activities  in  its  other  component  tissues.  In 
common  life  the  very  frequency  of  this  uniting  activity  of  the 
nervous  system  is  such  that  we  are  apt  to  entirely  overlook 
it.  We  do  not  Avonder  how  the  sight  of  pleasant  food  will 
make  the  mouth  water  and  the  hand  reach  out  for  it;  it 
seems,  as  we  say,  "natural,"  and  to  need  no  explanation. 
But  the  eye  itself  can  excite  no  desire,  cause  the  secretion  of 
no  saliva,  and  the  movement  of  no  limb.  The  whole  com- 
plex result  depends  on  the  fact  that  the  eye  is  united  by  the 
optic  nerve  with  the  brain,  and  that  again  by  other  nerves 
with  saliva-forming  cells,  and  with  muscular  fibres  of  the 
arm;  and  through  these  a  change  excited  by  light  falling 
into  the  eye  is  enabled  to  produce  changes  in  far-removed 
organs,  and  excite  desire,  secretion,  and  movement.  In  cases 
of  disease  this  action  exerted  at  a  distance  is  more  apt  to  ex- 
cite our  attention:  vomiting  is  a  ,rery  common  symptom  of 
certain  brain  diseases,  and  niost  people  know  that  a  disordered 
stomach  will  produce  a  headache;  while  the  pain  consequent 


GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM.     209 

upon  the  hip-disease  of  children  is  usually  felt,  not  at  the  hip- 
joint,  but  at  the  knee. 

The  Degeneration  of  Nerve-Fibres  separated  from  their 
Centre.  A  nerve-fibre  may  in  its  course  be  connected  with 
more  than  one  nerve-cell,  but  one  cell  always  has  a  special 
influence  in  maintaining  its  normal  structure  and  functional 
activity.  If  cut  off  from  this  cell  the  separated  portion 
undergoes  degenerative  changes,  easily  recognized  in  medul- 
lated  fibres  by  a  breaking  up  and,  later,  a  disappearance  of 
the  medullary  sheath.  If,  for  example,  the  sciatic  nerve  of 
a  warm-blooded  animal  be  completely  cut  across,  all  of  the 
nerve  and  its  branches  beyond  the  point  of  section  will  show 
marked  changes  in  three  days  or  less:  the  medullary  sheath 
separates  into  small  cuboidal  pieces,  these  in  a  day  or  two 
more  round  off  at  their  corners  and  then  are  gradually  ab- 
sorbed, so  that  at  the  end  of  ten  days  or  a  fortnight  they  have 
entirely  disappeared.  Meantime  the  nuclei  of  the  internodes 
multiply  and  the  usually  sparse  protoplasm  around  them  in- 
creases, and  encroaches  upon  and  causes  the  absorption  of  the 
axis  cylinder,  so  that  after  some  weeks  little  or  no  trace  of 
true  nervous  elements  can  be  found.  Some  three  or  four  days 
after  making  the  section  the  peripheral  portion  of  the  nerve 
ceases  to  be  excitable.  If  the  part  of  the  nerve  above  the  sec- 
tion be  examined,  its  fibres  will  be  found  to  have  undergone 
no  degeneration  except  close  to  the  place  of  section,  and  it  re- 
mains excitable;  pinching  it  causes  pain,  and  if  any  muscle 
branch  arising  from  it  be  irritated,  the  muscles  contract.  If 
instead  of  cutting  a  whole  mixed  nerve-trunk,  such  as  the  sci- 
atic, we  divide  only  a  ventral  spinal  root  (as  5,  c,  Fig.  71),  it  is 
found  that  all  the  fibres  in  that  part  of  the  root  which  is  cut 
off  from  the  spinal  cord  degenerate  and  become  unirritable, 
and  degenerated  fibres  can  be  found  in  the  mixed  trunk  into 
which  the  cut  root  is  continued;  while  the  fibres  of  the  part 
of  the  root  still  attached  to  the  cord  do  not  degenerate. 
The  nutritional  integrity  of  the  anterior  root-fibres  depends 
therefore  on  anatomical  continuity  with  the  spinal  cord,  and 
probably  with  cells  there,  of  the  type  shown  in  Fig.  81.  On 
the  other  hand,  if  the  dorsal  root  only  be  cut  across,  the  por- 
tion of  it  attached  to  the  cord  degenerates,  while  that  still 
connected  to  the  spinal  ganglion  and  the  fibres  beyond  the 
ganglion  remain  unaltered  :  the  nutritive  centres  for  the  dor- 
sal root-fibres  are  the  cells  of  the  corresponding  root-ganglion. 


210  T11E  11  UMAX  BODY. 

After  complete  section  of  the  nerve-trunk  supplying  a 
region  of  the  Body  that  region  is  for  a  time  paralyzed,  but 
feeling  and  the  power  of  movement  may  return  to  it.  It 
used  to  be  thought  that  in  such  cases  the  divided  nerve-fibres 

grew  together  again.  Such  is  not  the  case:  all  those  parts  of 
the  fibres  which  have  been  cut  off  from  their  centres  com- 
pletely disappear,  and  when  function  is  restored  it  is  by  the 
formation  of  new  nerve-fibres  around  outgrowths  from  tin- 
cut  ends  of  those  parts  of  the  fibres  still  united  to  their  cen- 
tres, whether  these  be  in  brain,  spinal  cord,  spinal  ganglia,  or 
elsewhere. 

Nerves,  as  we  have  seen,  often  give  fibres  to  one  another 
by  means  of  uniting  branches,  as  in  various  plexuses  and 
elsewhere:  and  when  a  nerve-branch  may  contain  fibres  de- 
rived from  some  one  of  two  or  more  original  trunks  which 
have  communicating  branches,  it  is  often  of  importance  to 
determine  in  which  original  trunk  its  fibres  left  the  brain  or 
spinal  cord.  In  such  cases  the  determination  may  often  be 
made  by  dividing  one  of  the  possible  sources  of  origin  and 
after  a  few  days  examining  the  branch  for  degenerated 
fibres,  which  are  easily  recognized  by  the  microscope.  If 
such  are  found,  then  they  left  the  centre  in  the  divided  trunk; 
if  not,  the  branch  gets  no  fibres  from  that  trunk.  This 
method  of  tracking  the  nerve-fibres  of  a  given  original  trunk 
to  their  final  distribution  in  one  or  more  of  many  possible 
branches  is  known  as  the  Wallerian  method.  Instances  of 
its  application  will  be  given  in  later  chapters. 


CHAPTER   XIV. 


THE  ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS. 


General  Statement.  During  life  the  blood  is  kept  flow- 
ing with  great  rapidity  through  all  parts  of  the  Body  (except 
the  few  non-vascular  tissues  already  mentioned)  in  definite 
paths  prescribed  for  it  by  the  heart  and  blood-vessels. 
These  paths,  which  under  normal  circum- 
stances it  never  leaves,  constitute  a  con- 
tinuous set  of  closed  tubes  (Fig.  87) 
beginning  at  and  ending  again  in  the 
heart,  and  simple  only  close  to  that  organ. 
Elsewhere  it  is  greatly  branched,  the  most 
numerous  and  finest  branches  (I  and  a) 
being  the  capillaries.  The  heart  is  essen- 
tially a  bag  with  muscular  walls,  internally 
divided  into  four  chambers  (d,  g,  e,f). 
Those  at  one  end  (d  and  e)  receive  blood 
from  vessels  opening  into  them  and  known 
as  the  veins.  From  there  the  blood  passes 
on  to  the  remaining  chambers  {g  and  f) 
which  have  very  powerful  Avails  and,  for- 
cibly contracting,  drive  the  blood  out  into 
vessels  (m  and  b)  which  communicate  with 
them  and  are  known  as  the  arteries.  The 
big  arteries  divide  into  smaller;  these  into  smaller  again 
(Fig.  88)  until  the  branches  become  too  small  to  be  traced  by 
the  unaided  eye,  and  these  smallest  branches  end  in  the 
capillaries,  through  which  the  blood  flows  and  enters  the 
commencements  of  the  veins;  and  these  convey  it  again  to 
the  heart.  At  certain  points  in  the  course  of  the  blood-paths 
valves  are  placed,  which  prevent  a  back-flow.  This  alternat- 
ing reception  of  blood  at  one  end  by  the  heart  and  its  ejec- 
tion from  the  other  go  on  during  life  steadily  about  seventy 
times  in  a  minute,  and  so  keep  the  liquid  constantly  in 
motion. 

211 


Fio.  87.— The  heart 
arnl  blood-Teasels  dia- 
gram matically  repre- 
sented. 


212 


THE  HUMAN  BODY. 


The  vascular  system  is  completely  closed  except  at  two 
points  in  the  neck  where  lymph-vessels  open  into  the  veins; 
there  some  lymph  is  poured  in  and  mixed  directly  with  the 
blood.  Accordingly  everything  which  leaves  the  blood  must 
do  so  by  oozing  through  the  walls  of  the  blood-vessels,  and 
everything  which  enters  it  must  do  the  same,  except  matters 


Fig.  88. — The  arteries  of  the  hand,  showing  the  communications  or  anastomose, 
of  different  arteries  and  the  fine  terminal  twigs  given  off  from  the  larger  trunks; 
these  twigs  end  in  the  capillaries  which  would  only  become  visible  if  magnified.  R, 
the  radial  artery  on  which  the  pulse  is  usually  felt  at  the  wrist ;  U,  the  ulnar  ar- 
tery. 

conveyed  in  by  the  lymph  at  the  points  above  mentioned. 
This  interchange  through  the  walls  of  the  vessels  takes  place 
only  in  the  capillaries,  which  form  a  sort  of  irrigation  system 
all  through  the  Body.  The  heart,  arteries,  and  veins  are  all 
merely  arrangements  for  keeping  the  capillaries  full  and 
renewing  the  blood  within  them.  Jt  is  in  the  capillaries 
alone  that  the  blood  does  its  uhvsiologieal  work. 


ANATOMY  OF  THE  I1EART  AND  BLOOD-VESSELS.      213 

The  Position  of  the  Heart.  The  heart  (h,  Fig.  1)  lies 
in  the  chest  immediately  above  the  diaphragm  and  opposite 
the  lower  two  thirds  of  the  breast-bone.  It  is  conical  in 
form  with  its  base  or  broader  end  turned  upwards  and  pro- 
jecting a  little  on  the  right  of  the  sternum,  while  its  narrow 
end  or  apex,  turned  downwards,  projects  to  the  left  of  that 
bone,  where  it  may  be  felt  beating  between  the  cartilages  of 
the  fifth  and  sixth  ribs.  The  position  of  the  organ  in  the 
Body  is  therefore  oblique  with  reference  to  its  long  axis.  It 
does  not,  however,  lie  on  the  left  side  as  is  so  commonly  sup- 
posed but  very  nearly  in  the  middle  line,  with  the  upper  part 
inclined  to  the  right,  and  the  lower  (which  may  be  more 
easily  felt  beating — hence  the  common  belief)  to  the  left. 

The  Membranes  of  the  Heart.  The  heart  does  not  lie 
bare  in  the  chest  but  is  surrounded  by  a  loose  bag  composed 
of  connective  tissue  and  called  the  'pericardium.  This  bag, 
like  the  heart,  is  conical  but  turned  the  other  way,  its  broad 
part  being  lowest  and  attached  to  the  upper  surface  of  the 
diaphragm.  Internally  it  is  lined  by  a  smooth  serous  mem- 
brane like  that  lining  the  abdominal  cavity,  and  a  similar 
layer  (the  visceral  layer  of  the  pericardium)  covers  the  out- 
side of  the  heart  itself,  adhering  closely  to  it.  Each  of  the 
nerous  layers  is  covered  by  a  stratum  of  flat  cells,  and  in  the 
space  between  them  is  found  a  small  quantity  of  liquid 
which  moistens  the  contiguous  surfaces,  and  diminishes  the 
friction  which  would  otherwise  occur  during  the  movements 
of  the  heart. 

Internally  the  heart  is  also  lined  by  a  fibrous  membrane, 
covered  with  a  single  layer  of  flattened  cells,  and  called  the 
endocardium.  Between  the  endocardium  and  the  visceral 
layer  of  the  pericardium  the  bulk  of  the  wall  of  the  heart 
lies  and  is  made  up  mainly  of  striped  muscular  tissue  (myocar- 
dium) differing  from  that  of  the  skeletal  muscles;  but  con- 
nective tissues,  blood-vessels,  nerve-cells,  and  nerve-fibres  are 
also  abundant  in  it. 

Xole. — Sometimes  the  pericardium  becomes  inflamed,  this 
affection  being  known  as  pericarditis.  It  is  extremely  apt  to 
occur  in  acute  rheumatism,  and  great  care  .should  be  taken 
never,  even  for  a  moment,  except  under  medical  advice,  to 
expose  a  patient  to  cold  daring  that  disease,  since  any  chill 
is  then  especially  apl  to  set  up  pericarditis.  In  the  earlier 
stages   of  pericardiac  inflammation  the  rubbing  surfaces  on 


214 


1  TIE  HUMAN  BODY. 


Fig.  89.— Diagram   representinsr    a    section 
through  the  heart  from  base  to  apex. 


the  outside  of  the  heart  and  the  inside  of  the  pericardium 
hecome  roughened,  and  their  friction  produces  a  sound 
which  can  be  recognized  through  the  stethoscope.  Jn  later 
stages  great  quantities  of  liquid  may  accumulate  in  the  peri- 
cardium so  as  to  seriously  impede  the  heart's  beat. 

The  Cavities  of  the  Heart.     On  opening  the   heart  (see 
diagram   Fig.  89)  it  is  found  to  be  subdivided  by  a  longi- 
.       p  tudinal    partition    or   sep- 

tum into  completely  sepa- 
rated right  and  left  halves, 
the  partition  running  from 
about  the  middle  of  the 
base  to  a  point  a  little  on 
the  right  of  the  apex. 
Each  of  the  chambers  on 
the  sides  of  the  septum  is 
again  incompletely  divided 
transversely,  into  a  thinner 
basal  portion  into  which 
veins  open,  known  as  the 
auricle,  and  a  thicker  apical  portion  from  which  arteries 
arise,  called  the  ventricle.  The  heart  thus  consists  of  a  right 
auricle  and  ventricle  and  a  left  auricle  and  ventricle,  each 
auricle  communicating  by  an  auriculo-venlricular  orifice 
with  the  ventricle  on  its  own  side,  and  there  is  no  direct 
communication  whatever  through  the  septum  between  the 
opposite  sides  of  the  heart.  To  get  from  one  side  to  the 
other  the  blood  must  leave  the  heart  and  pass  through  a  set 
of  capillaries,  as  may  readily  be  seen  by  tracing  the  course  of 
the  vessels  in  Fig.  87. 

The  Heart  as  seen  from  its  Exterior.  When  the  heart 
is  viewed  from  the  side  turned  towards  the  sternum  (Fig.  90) 
the  two  auricles,  Aid  and  As,  are  seen  to  be  separated  by  a 
deep  groove  from  the  ventricles,  Vd  and  Vs.  A  more 
shallow  furrow  runs  between  the  ventricles  and  indicates  the 
position  of  the  internal  longitudinal  septum.  On  the  dorsal 
aspect  of  the  heart  (Fig.  91)  similar  furrows  may  be  noted, 
and  on  one  or  other  of  the  two  figures  the  great  vessels 
opening  into  the  cavities  of  the  heart  may  be  seen.  The 
pulmonary  artery,  P,  arises  from  the  right  ventricle,  and 
very  soon  divides  into  the  right  and  left  pulmonary  arteries, 
Pd  and  Ps,  which  break  up  into  smaller  branches  and  enter 


ANA  T0M7  OF  THE  HEART  AND  BLOOD  -  VESSELS.      215 

the  corresponding  lungs.  Opening  into  the  right  auricle  are 
two  great  veins  (see  also  Fig.  S9),  cs  and  ci,  known  re- 
spectively as  the  upper  and  lower  vence  cavce,  or  "  hollow " 
veins;  so  called  by  the  older  anatomists  because  they  are 
frequently  found  empty  after  death.  Into  the  back  of  the 
right  auricle  opens  also  another  vein,  Vc,  called  the  coronary 


Ad^  ,9s     ?*     aA 


Fig.  90.— The  heart  and  the  great  blood-vessel  attached  to  it.  seen  from  the 
Bide  towards  the  sternum.  The  left  cavities  and  the  vessels  connected  with  them 
are  colored  red;  the  right  black.  Atd,  right  auricle;  Adx  and  As,  the  right  and 
lefl  auricular  appendages;  Ptf,  right  ventricle;  Vs,  left  ventricle;  An,  aorta:  Ah, 
innominate  artery;  Cs,  left  common  carotid  artery;  8ai.  left  subclavian  artery; 
/',  main  trunk  of  the  pulmonary  artery,  ami  I'd  and  I's,  its  branches  to  the  right 
and  left  lungs;  cs,  superior  vena  cava;  Arte  and  Asi,  the  right  and  left  innominate 
veins;  prt  and  7).s,  the  light  and  left  pulmonary  veins;  crd  and  crs,  the  right  and 
left  coronary  arteries. 

vein  or  sinus,  which  brings  back  blood  that  has  circulated 
in  the  walls  of  the  heart  itself.  Springing  from  the  left  ven- 
tricle, and  appearing  from  beneath  the  pulmonary  artery 
when  the  heart  is  looked  ;it  from  the  ventral  side,  is  ;i  great 

artery,  the  aorta,  Ati.  It  forms  an  arch  over  the  bast;  of  the 
heart  and  then  rims  down  behind  it  at  the  back  of  the  chest. 
From  tin;  eonvexity  of   the  arch  of  the  aorta  several  great 


216 


THE  HUMAN  BODY. 


branches  arc  given  off,  Ssi,  Cs,  Ah;  but  before  that,  close  to 
the  heart,  the  aorta  gives  off  two  coronary  arteries,  branches 
of  which  are  seen  at  crd  and  cr&  lying  in  the  groove  over  the 
partition  between  the  ventricles,  and  which  carry  to  the  sub- 
stance of  the  organ  that  blood  which  comes  back  through  the 


Fig.  91.— The  heart  viewed  from  its  dorsal  aspect.  Atd,  right  auricle;  ci,  in- 
ferior vena  cava;  Vc,  coronary  vein.  The  remaining  letters  of  reference  have 
vhe  same  signification  as  in  Fig.  90. 

coronary  sinus.  Into  the  left  auricle  open  two  right  and  two 
left  pulmonary  veins,  ps  and  pd,  which  are  formed  by  the 
union  of  smaller  veins  proceeding  from  the  lungs. 

In  the  diagram  Fig.  8!)  from  which  the  branches  of  the 
great  vessels  near  the  heart  have  been  omitted  for  the  sake 
of  clearness,  the  connection  of  the  various  vessels  with  the 


ANATOMY  OF  THE  HEART  AND  BLOOD  -  VESSELS.      217 

chambers  of  the  heart  can  be  better  seen.  Opening  into  the 
right  auricle  are  the  superior  and  inferior  veuse  cavae  (cs  and 
ci)  and  proceeding  from  the  right  ventricle  the  pulmonary 
artery,  P.  Opening  into  the  left  auricle  are  the  right  and 
left  pulmonary  veins  (pel  and  ps)  and  springing  from  the  left 
ventricle  the  aorta,  A. 

The  Interior  of  the  Heart.  The  communication  of  each 
auricle  with  its  ventricle  is  also  represented  in  the  diagram 
Fig.  89,  and  the  valves  which  are  present  at  those  points 
and  at  the  origin  of  the  pulmonary  artery  and  that  of  the 
aorta.  Internally  the  auricles  are  for  the  most  part  smooth, 
but  from  each  a  hollow  pouch,  the  auricular  appendage,  pro- 
jects over  the  base  of  the  corresponding  ventricle  as  seen  at 
Adx  and  As  in  Figs.  90  and  91.  These  pouches  have  some- 
what the  shape  of  a  dog's  ear  and  have  given  their  name  to 
the  whole  auricle.  Their  interior  is  roughened  by  muscular 
elevations,  covered  by  endocardium,  known  as  the  fleshy  col- 
umns (columnar  carnce).  On  the  inside  of  the  ventricles  (Fig. 
92)  similar  fleshy  columns  are  very  prominent. 

The  Auriculo-Ventricular  Valves.  These  are  known  as 
right  atid  left,  or  as  the  tricuspid  and  mitral  valves  respec- 
tively. The  mitral  valve  (Fig.  92)  consists  of  two  flaps  of  the 
endocardium  fixed  by  their  bases  to  the  margins  of  the  auric- 
ulo-ventricular  aperture  and  with  their  edges  hanging  down 
into  the  ventricle  when  the  heart  is  empty.  These  unattached 
edges  are  not  however  free,  but  have  fixed  to  them  a  number 
of  stout  connective-tissue  cords,  the  cordes  tendinece,  which 
are  fixed  below  to  muscular  elevations,  the  papillary  muscles, 
Mpm  and  Mpl,  on  the  interior  of  the  ventricle.  The  cords 
are  long  enough  to  let  the  valve  flaps  rise  into  a  horizontal 
position  and  so  close  the  opening  between  auricle  and  ven- 
tricle which  lies  between  them,  and  passes  up  behind  the 
opened  aorta,  Sp,  represented  in  the  figure.  The  tricuspid 
valve  is  like  the  mitral,  but  with  three  flaps  instead  of  two. 

Semilunar  Valves.  These  are  six  in  number:  three  at 
the  mouth  of  the  aorta,  Fig.  92,  and  three,  quite  like  them, 
at  the  mouth  of  the  pulmonary  artery.  Each  is  a  strong 
crescentic  pouch  fixed  by  its  more  curved  border,  and  with 
its  free  edge  turned  away  from  the  heart.  When  the  valves 
arc  in  action  these  free  edges  meet  across  the  vessel  and  pre- 
vent blood  from  flowing  baci  into  the  ventricle.  In  the 
middle  of  the  free  border  of  each  valve   is  a  little  cartilagi- 


218 


THE  HUMAN   BODY. 


nous  nodule,  the  corpus  Arantii,  and  on  each  side  of  tin's  the 
edge  of  the  valve  is  very  thin  and  when  it  meets  its  neighbor 
turns  up  against  it  and  bo  secures  the  closure. 

The  Arterial  System.  All  the  arteries  of  the  Body  arise 
either  directly  or  indirectly  from  the  aorta  or  pulmonary 
artery,  and  the  great  majority  of  them  from  the  former  vessel. 


Fig.  02.— The  left  ventricle  and  the  commencement  of  the  aorta  laid  open. 
Mpm,  Dipt,  the  papillary  muscles.  From  tlieir  upper  ends  arc  seen  the  cm-dee 
tendinece  proceeding  t"  the  edges  of  the  flaps  of  the  mitral  valve.  The  opening 
into  t be  auricle  lies  Between  these  flaps.  At  the  beginning  of  the  aorta  are  seen  its 
three  pouch-like  semilunar  valves. 

The  pulmonary  artery  only  carries  blood  to  the  lungs,  to  un- 
dergo exchangee  with  the  air  in  them  after  it  has  circulated 
through  the  Body  generally. 

After  making  its  arch  the  aorta  continues  back  through 
the  chest,  giving  off  many  branches  on  its  way.     Piercing  the 


ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS.      219 

diaphragm  it  enters  the  abdomen  and  after  supplying  the 
parts  in  and  around  that  cavity  with  branches,  it  ends  oppo- 
site the  last  lumbar  vertebra  by  dividing  into  the  right  and 
left  common  iliac  arteries,  which  carry  blood  to  the  lower 
limbs.  We  have  then  to  consider  the  branches  of  the  arch  of 
tbe  aorta,  and  those  of  the  descending  aorta.,  which  latter  is 
for  convenience  described  by  anatomists  as  consisting  of  the 
thoracic  aorta,  extending  from  the  end  of  the  arch  to  the 
diaphragm,  and  tbe  abdominal  aorta,  extending  from  the 
diaphragm  to  the  final  subdivision  of  the  vessel. 

Branches  of  the  Arch  of  the  Aorta.  From  this  arise  first 
the  coronary  arteries  (crcl  and  crs,  Figs.  90  and  91)  which 
spring  close  to  the  heart,  just  above  two  of  the  pouches  of  the 
semilunar  valve,  and  carry  blood  into  the  substance  of  that 
organ.  The  remaining  branches  of  the  arch  are  three  in 
number,  and  all  arise  from  its  convexity.  The  first  is  the 
innominate  artery  (Ab,  Fig.  90),  which  is  very  short,  imme- 
diately breaking  up  into  the  right  subclavian  artery,  and  the 
right  common  carotid.  Then  comes  the  left  common  carotid, 
Cs,  and  finally  the  left  subclavian,  Ssi. 

Each  subclavian  artery  runs  out  to  the  arm  on  its  own 
side  and  after  giving  off  a  vertebral  artery  (which  runs  up 
the  neck  to  the  head  in  the  vertebral  canal  of  the  transverse 
processes  of  the  cervical  vertebrae),  crosses  the  arm-pit  and 
takes  there  the  name  of  the  axillary  artery.  This  contin- 
ues down  the  arm  as  the  brachial  artery,  which,  giving  off 
branches  on  its  way,  runs  to  the  front  of  the  arm,  and  just 
below  the  elbow-joint  divides  into  the  radial  and  ulnar  ar- 
teries, the  lower  ends  of  which  are  seen  at  R  and  U  in  Fig.  88. 
These  supply  the  forearm  and  end  in  the  hand  by  uniting  to 
form  an  arch,  from  which  branches  are  given  off  to  the  fingers. 

The  common  carotid  arteries  pass  out  of  the  chest  into  the 
neck,  along  which  they  ascend  on  the  sides  of  the  windpipe. 
Opposite  the  angle  of  the  lower  jaw  each  divides  into  an 
internal  and  external  carotid  artery,  right  or  left  as  the  case 
may  be.  The  external  ends  mainly  in  branches  for  the  face, 
scalp,  and  salivary  glands,  one  great  subdivision  of  it  with  a 
tortuous  course,  the  temporal  artery,  being  often  seen  in  thin 
persons  beating  on  the  side  of  the  brow.  The  internal  carotid 
artery  enters  the  skull  through  an  aperture  in  its  base  and 
supplies  the  brain,  which  it  will  he  remembered  also  gets 
blood  through  tin;  vertebral  arteries. 


220  THE  HUMAN  BODY. 

Branches  of  the  Thoracic  Aorta.  These  are  numerous 
but  small.  Some,  the  intercostal  arteries,  run  out  between 
the  ribs  and  supply  the  chest-walls;  others,  the  bronchial  ar- 
teries, carry  blood  to  the  lungs  for  their  nourishment,  that 
carried  to  them  by  the  pulmonary  arteries  being  brought 
there  for  another  purpose;  and  a  few  other  small  branches 
are  given  to  other  neighboring  parts. 

Branches  of  the  Abdominal  Aorta.  These  are  both  large 
and  numerous,  supplying  not  only  the  wall  of  the  posterior 
part  of  the  trunk,  but  the  important  organs  in  the  abdominal 
cavity.  The  larger  are:  the  ca&liac  axis  which  supplies  stom- 
ach, spleen,  liver,  and  pancreas;  the  superior  mesenteric 
artery,  which  supplies  a  great  part  of  the  intestine;  the  renal 
arteries,  one  for  each  kidney;  and  finally  the  inferior  mes- 
enteric artery,  which  supplies  the  rest  of  the  intestine.  Be- 
sides these  the  abdominal  aorta  gives  off  very  many  smaller 
brunches. 

Arteries  of  the  Lower  Limbs.  Each  common  iliac  di- 
vides into  an  internal  and  external  iliac  artery.  The  former 
mainly  ends  in  branches  to  parts  lying  in  the  pelvis,  but  the 
latter  passes  into  the  thighs  and  there  takes  the  name  of  the 
femoral  artery.  At  first  this  lies  on  the  ventral  aspect  of  I  he 
limb,  hut  lower  down  passes  to  the  back  of  the  femur,  and 
above  the  knee-joint  (where  it  is  called  the  popliteal  artery) 
divides  into  the  anterior  and  posterior  tibial  arteries,  which 
supply  the  leg  and  foot. 

The  Capillaries.  As  the  arteries  are  followed  from  tin- 
heart,  their  branches  become  smaller  and  smaller,  and  finally 
cannot  he  traced  without  the  aid  of  a  microscope.  Ulti- 
mately they  pass  into  the  capillaries,  the  walls  of  which  are 
simpler  than  those  of  the  arteries,  and  which  form  very  close 
networks  in  nearly  all  parts  of  the  Body;  their  immense  num- 
ber compensating  for  their  smaller  size.  The  average  diame- 
ter of  a  capillary  vessel  is  .016  mm.  (lg»tf0  inch)  so  that  only 
two  or  three  blood-corpuscles  can  pass  through  it  abreast, 
and  in  many  parts  they  arc  so  close  that  a  pin's  point  could 
not  be  inserted  between  two  of  them.  It  is  while  flowing  in 
these  delicate  tubes  that  the  blood  does  its  nutritive  work,  the 
arteries  being  merely  supply-tubes  for  the  capillaries. 

The  Veins.  The  first  veins  arise  from  the  capillary  net- 
works in  various  organs,  and  like  the  last  arteries  are  very 
small.     They  soon   increase  in  size  by  union,  aud  so  form 


ANATOMY  OF  THE  HEART  AND  BLOOD  -  VESSELS.      221 

larger  and  larger  trunks.  These  in  many  places  lie  near  or 
alongside  the  main  artery  pf  the  part,  but  there  are  many 
more  large  veins  just  beneath  the  skin  than  there  are  large 
arteries.  This  is  especially  the  case  in  the  limbs,  the  main 
veins  of  which  are  superficial,  and  can  in  many  persons  be 
seen  as  faint  blue  marks  through  the  skin.  Fig.  94  repre- 
sents the  arm  at  the  front  of  the  elbow- joint  after  the  skin 
and  subcutaneous  areolar  tissue  and  fat  have  been  removed. 


ft 

Via.  03— A  small  portion  of  the  capillary  network  as  seen  in  the  frog's  wet>  when 
magnified  about.25  diameters.  ",  a  Bmall  artery  feeding  the  capillaries;  v,  t\  small 
reiw  carrying  blood  back  from  the  latter. 

The  brachial  artery,  J>,  colored  red,  is  seen  lying  tolerably 
deep,  and    accompanied  by  two   small    veins    (rrinr    COmites) 

which  communicate  by  cross-branches.  The  great  median 
nerve,  I,  a  branch  of  the  brachial  plexus  which  supplies 
several  muscles  of  the  forearm  and  hand,  the  skin  over  n 
greal  part  of  the  palm  and  the  three  inner  fingers,  is  seen 
alongside  the  artery.     Tin;  larger  veins  of  the  part  are  seen 


222 


THE  HUMAN  BODY. 


to  form  a  more  superficial  network,  joined  here  and  there,  as 
for  instance  at  *,  by  brandies  from  deeper  parts.  Several 
small  nerve-branches  which  supply  the  skin  (2,  3,  4)  are  seen 
among   these  veins.     It  is  from  the  vessel,  cep,  called  the 


— rt 


bas 


Fig.  94.— The  superficial  veins  in  front  of  the  elbow  joint.  B',  tendon  of  biceps 
muscle;  Bi,  brachialis  intemus  muscle;  ft.  pronator  It-res  muscle:  1,  median 
nerve;  2,  'i,  4.  nerve-branches  to  the  skin;  B,  brachial  artery,  with  its  small  accom- 
panying veins:  cep,  cephalic  vein;  bus,  basilic  vein;  m',  median  vein;  *,  junction  of 
a  deep-lying  vein  with  the  cephalic. 

cephalic  vein,  just  above  the  point  where  it  crosses  the  median 
nerve,  that  surgeons  usually  bleed  a  patient. 

A  great  part  of  the  blood  of  the  lower  limb  is  brought  back 
by  the  long  saphenous  vein,  which  can  be  seen  in  thin  persons 
running  from  the  inner  side  of  the  ankle  to  the  top  of  the 


ANATOMY  OF  THE  HEART  AND  BLOOD  -  VESSELS.      223 

thigh.  All  the  blood  which  leaves  the  heart  by  the  aorta, 
except  that  flowing  through  the  coronary  arteries,  is  finally 
collected  into  the  stiperior  and  inferior  vence  cavce  (cs  and  ci, 
Figs.  90  and  91),  and  poured  into  the  right  auricle.  The 
jugular  veins  which  run  down  the  neck,  carrying  back  the 
blood  which  went  out  along  the  carotid  arteries,  unite  below 
with  the  arm-vein  (subclavian)  to  form  on  each  side  an  in- 
nominate rein  (Asi  and  Acle,  Fig.  90)  and  the  innominates 
unite  to  form  the  superior  cava.  The  coronary-artery  blood 
after  flowing  through  the  capillaries  of  the  heart  itself  also 
returns  to  this  auricle  by  the  coronary  veins  and  sinus. 

The  Pulmonary  Circulation.  Through  this  the  blood 
gets  back  to  the  left  side  of  the  heart  and  so  into  the  aorta 
again.  The  pulmonary  artery,  dividing  into  branches  for 
each  lung,  ends  in  the  capillaries  of  those  organs.  From 
these  it  is  collected  by  the  pulmonary  veins,  which  carry  it 
back  to  the  left  auricle,  whence  it  passes  to  the  left  ventricle 
to  recommence  its  flow  through  the  Body  generally. 

The  Course  of  the  Blood.  From  what  has  been  said  it  is 
clear  that  the  movement  of  the  blood  is  a  circulation.  Start- 
ing from  any  one  chamber  of  the  heart  it  will  in  time  return 
to  it;  but  to  do  this  it  must  pass  through  at  least  two  sets  of 
capillaries;  one  of  these  is  connected  with  the  aorta  and  the 
other  with  the  pulmonary  artery,  and  in  its  circuit  the  blood 
returns  to  the  heart  twice.  Leaving  the  left  side  it  returns  to 
the  right,  and  leaving  the  right  it  returns  to  the  left:  and 
there  is  no  road  for  it  from  one  side  of  the  heart  to  the  other 
except  throng])  a  capillary  network.  Moreover,  it  always 
leaves  from  a  ventricle  through  an  artery,  and  returns  to  an 
auricle  through  a  vein. 

There  is  then  really  only  one  circulation;  but  it  is  not  un- 
common to  speak  of  two,  the  flow  from  the  left  side  of  the 
heart  to  the  right,  through  the  Body  generally,  being  called 
the  systemic  circulation,  and  from  the  right  to  the  left, 
through  the  lungs,  the  pulmonary  circulation.  But  since 
after  completing  either  of  these  alone  the  blood  is  not  back 
at  the  point  from  which  it  started,  but  is  separated  from  it  by 
the  septum  of  the  heart,  neither  is  a  "circulation"  in  the 
proper  sense  of  the  word. 

The  Portal  Circulation.  A  certain  portion  of  the  blood 
which  leaves  the  left  ventricle  of  the  heart  through  the  aorta 
haa  to  pass  through  I  hree  sets  of  capillaries  before  it  can  again 


224 


THE  HUMAN  BODY. 


return  there.  This  is  the  portion  whirl,  goes  through  the 
stomach,  spleen,  pancreas,  and  intestines.  After  traversing 
the  capillaries  of  those  organs  it  is  collected  into  the  portal 
vein  which  enters  the  liver,  and  breaking  up  in  it  into  finer 
and  finer  branches  like  an  artery,  ends  in  the  capillaries  of 
that  organ,  forming  the  second  set  which  this  blood  passes 
through  on  its  course.  From  these  it  is  collected  by  the  he- 
patic veins,  which  pour  it  into  the  inferior  vena  cava,  which 
carries  it  to  the  light  auricle,  so  that 
it  has  still  to  pass  through  the  pulmo- 
nary capillaries  to  get  back  to  the  left 
side  of  the  heart.  The  portal  vein  is 
the  only  one  in  the  human  Body  which 
like  an  artery  feeds  a  capillary  net- 
work, and  the  flow  from  the  stomach 
and  intestines  through  the  liver  to  the 
vena  cava  is  often  sjioken  of  as  the 
portal  circulation. 

Diagram  of  the  Circulation.  Since 
the  two  halves  of  the  heart  are  actu- 
ally completely  separated  from  one 
another  by  an  impervious  partition, 
although  placed  in  proximity  in  the 
Body,  we  may  conveniently  represent 
the  course  of  the  blood  as  in  the  accom- 
panying diagram  (Fig.  95),  in  which 
the' 


Fig.  05. --Diagram  of  the 
blood  vascular  system,  show- 
ing   that    it    forms    a    single 


right  and  left  halves  of  the  heart 


closed  circuit  with  twt>  pumps  „_„    _„__„,,,._  (.„  A   «*-  ^Ii^F«,.^.,+    „    ;,.,.    ;.. 

in  it  consisting  of  the  right  are  represented  at  uirterent  points  in 

^iclerar5a,r^rein*edhs^:  the  vascular  system.    Such  an  arrange- 

ratein  the  diagram,    ra  and  ment  makes  it  clear  that  the  heart  is 

rv,  right  auricle  and  ventricle; 

\<<  and  iv,  left  auricle  and  ren-  really  two  pumps  working  side  bv  side, 

tncle:  ao.  aorta;  sr,  systemic  "  *  -J  °  ■ 

capillaries:    it.  venae  cavse;  each  engaged  m  forcing  the  blood  to 

f,pc,pul-  .  cu.  £  i  !    £L 


fin.  pulmonary  artery 
monary   capillaries:     /<)• 
monary  veins. 


pul- 


the  other.  Starting  from  the  left  au- 
ricle, la,  and  following  the  flow,  we 
trace  it  through  the  left  ventricle  and  along  the  branches  of 
the  aorta  into  the  systemic  capillaries,  sc;  from  thence  it 
passes  back  through  the  systemic  veins,  vc.  Reaching  tin- 
right  auricle,  ra,  it  is  sent  into  the  right  ventricle,  rv,  and 
thence  through  the  pulmonary  artery,  pa,  to  the  lung  capilla- 
ries, pc,  from  which  the  pulmonary  veins,  pv,  carry  it  to  the 
left  auricle,  which  drives  it  into  the  left  ventricle,  lv,  and  this 
again  into  the  aorta. 


ANATOMY  OF  THE  HEART  AND  BLOOD-VESSELS.      225 

Arterial  and  Venous  Blood.  The  blood  when  flowing  in 
the  pulmonary  capillaries  gives  up  carbon  dioxide  to  the  air 
and  receives  oxygen  from  it;  and  since  its  coloring  matter 
(haemoglobin)  forms  a  scarier  compound  with  oxygen,  it  flows 
to  the  left  auricle  through  the  pulmonary  veins  of  a  bright 
red  color.  This  color  it  maintains  until  it  reaches  the  sys- 
temic capillaries,  but  in  these  it  loses  much  oxygen  to  the 
surrounding  tissues  and  gains  much  carbon  dioxide  from  them. 
But  the  blood  coloring-matter  which  has  lost  its  oxygen  has  a 
dark  purple  color,  and  since  this  unoxidized  or  "'reduced" 
haemoglobin  is  now  in  excess,  the  blood  returns  to  the  heart 
by  the  venae  cavae  of  a  dark  purple-red  color.  This  hue  it 
keeps  until  it  reaches  the  lungs,  when  the  reduced  haemoglo- 
bin becomes  again  oxidized.  The  bright  red  blood,  rich  in 
oxygen  and  poor  in  carbon  dioxide,  is  known  as  "arterial 
blood"  and  the  dark  red  as  "venous  blood:"  and  it  must  be 
borne  in  mind  that  the  terms  have  this  peculiar  technical 
meaning,  and  that  the  pulmonary  veins  contain  arterial  blood, 
and  the  pulmonary  arteries,  venous  blood;  the  change  from 
arterial  to  venous  taking  place  in  the  systemic  capillaries,  and 
from  venous  to  arterial  in  the  pulmonary  capillaries.  The 
chambers  of  the  heart  and  the  great  vessels  containing  arte- 
rial blood  are  shaded  red  in  Figs.  90  and  91. 

The  Structure  of  the  Arteries  A  large  artery  can  by 
careful  dissection  be  separated  into  three  coats:  an  infernal, 
a  middle,  and  an  outer.  The  internal  coat  tears  readily  across 
the  long  axis  of  the  artery  and  consists  of  an  inner  lining  of 
flattened  nucleated  cells,  enveloped  by  a  variable  number  of 
layers  composed  of  membranes  or  networks  of  elastic  tissue. 
The  middle  coat  is  made  up  of  alternating  layers  of  elastic 
fibres  and  plain  muscular  tissue;  the  former  running  for  the 
mosi  part  longitudinally  and  the  latter  across  the  long  axis 
of  the  vessel.     The  outer  coat   is  the  toughest  and  strongest 

1  .         .  ° 

because  it  is  mainly  made  up  of  white  fibrous  connective 
tissue;  it  contain-  a  considerable  amount  of  elastic  tissue  also, 
and  gradually  shades  off  into  a  loose;  areolar  tissue  which 
forms  the  nheath  of  the  artery,  or  the  tunica  adventitia,  and 
packs  it  between  surrounding  parts.  The  smaller  arteries 
have  all  the  elastic  elements  less  developed.  The  internal 
COal  ifl  consequently  thinner,  and  the  middle  coal  [fi  made  up 
mainly  of  involuntary  muscular  fibres.  A.8  a  result  the  large 
arteries  are  highly  elastic,  the  aorta  being  physically  much 
like  a  piece  of  india-rubber  tubing,  while  the  smaller  arte- 


226  TEE  HUMAN  BODY. 

ries  are  highly  contractile,  in  the  physiological  sense  of  the 
word. 

Structure  of  the  Capillaries.  In  the  smaller  arteries  the 
outer  and  middle  coats  gradually  disappear,  and  the  elastic 
layers  of  the  inner  coat  also  go.  Finally,  in  the  capillaries 
the  lining  epithelium  alone  is  left,  with  a  more  or  less  de- 
veloped layer  of  connective-tissue  corpuscles  around  it,  repre- 
senting the  remnant  of  the  tunica  adventitia.  These  vessels 
are  thus  extremely  well  adapted  to  allow  of  filtration  or  dif- 
fusion taking  place  through  their  thin  walls. 

Structure  of  the  Veins.  In  these  the  same  three  primary 
coats  as  in  the  arteries  are  found:  the  inner  and  middle  coats 
are  less  developed,  while  the  outer  one  remains  thick,  and  is 
made  up  almost  entirely  of  white  fihrous  tissue.  Hence  the 
venous  walls  are  much  thinner  than  those  of  the  correspond- 
ing arteries,  and  the  veins  collapse  when  empty  while  the 
stouter  arteries  remain  open.  But  the  toughness  of  their 
outer  coats  gives  the  veins  great  strength. 

Except  the  pulmonary  artery  and  the  aorta,  which  possess 
the  semilunar  valves  at  their  cardiac  orifices,  the  arteries  pos- 
sess no  valves.  Many  veins  on  the  contrary  have  such,  formed 
by  semilunar  pouches  of  the  inner  coat,  attached  by  one 
margin  and  having  the  edge  turned  towards  the  heart  free. 
These  valves,  sometimes  single,  oftener  in  pairs,  and  rarely 
three  at  one  level,  permit  blood  to  flow  only  towards  the 
heart,  for  a  current  in  that  direction  (as  in  the  upper  dia- 
gram, Fig.  9C)  presses  the  valve  close  against  the  side  of  the 
vessel  and  meets  with  no  obstruction 
A  from  it.     Should  any  back-flow  be  at- 

tempted, however,  the  current  closes 
up  the  valve  and  bars  its  own  passage 
as  indicated  in  the  lower  figure.  These 
valves  are  most  numerous  in  super- 
ficial veins  and  those  of  muscular  parts. 
t/J°  ,i!fi  -r,i^ra"1  *°  fflus'  They  are  absent  in  the  venae  cavae  and 

tratf   the    mode   of  action  of 

the  valves  of  the  veins,  a  the  the  portal  and  pulmonarv  veins.    Usu- 

capillary,   and   H,   the    heart  .  ., 

end  of  the  vessel.  ally  the  vein  is  a  little  dilated  opposite 

a  valve,  and  hence  in  parts  where  the 
valves  are  numerous  gets  a  knotted  look.  On  compressing 
the  forearm  so  as  to  stop  the  flow  in  its  subcutaneous  veins 
and  cause  their  dilatation,  the  points  at,  which  valves  are 
placed  can  be  recognized  by  their  swollen  appearance.  They 
are  most  frequently  situated  where  two  veins  communicate. 


CHAPTER  XV. 
THE   WORKING   OF   THE   HEART  AND   BLOOD-VESSELS. 

The  Beat  of  the  Heart.  It  is  possible  with  some  little 
skill  and  care  to  open  the  chest  of  a  living  narcotized  ani- 
mal, such  as  a  rabbit,  and  see  its  heart  at  work,  alternately 
contracting  and  diminishing  the  cavities. within  it,  and  relax- 
ing and  expanding  them.  It  is  then  observed  that  each  beat 
commences  at  the  mouths  of  the  great  veins;  from  there  runs 
over  the  rest  of  the  auricles,  and  then  over  the  ventricles;  the 
auricles  commencing  to  dilate  the  moment  the  ventricles 
commence  to  contract.  Having  finished  their  contraction 
the  ventricles  also  commence  to  dilate,  and  so  for  some  time 
neither  they  nor  the  auricles  are  contracting,  but  the  whole 
heart  is  expanding.  The  contraction  of  any  part  of  the  heart 
is  known  as  its  systole  and  the  relaxation  as  its  diastole,  and 
since  the  two  sides  of  the  heart  work  synchronously,  the  au- 
ricles together  and  the  ventricles  together,  we  may  describe  a 
whole  "cardiac  period  "  or  "heart-beat"  as  made  up  succes- 
sively of  auricular  systole,  ventricular  systole,  and  pause. 
This  cycle  is  repeated  about  seventy  times  a  minute;  and  if 
the  whole  time  occupied  by  it  be  subdivided  into  100  parts, 
about  9  of  these  will  be  occupied  by  the  auricular  systole, 
about  30  by  the  ventricular  systole,  and  61  by  the  pause: 
during  more  than  half  of  life,  therefore,  the  muscle-fibres  of 
the  heart  are  at  rest.  In  the  pause  the  heart  if  taken  be- 
tween the  finger  and  thumb  feels  soft  and  flabby,  but  during 
tin-  systole  it  (especially  its  ventricular  portion)  becomes  hard 
and  rigid. 

Change  of  Form  of  the  Heart.  During  its  systole  the 
In-art  becomes  Bhorter  and  rounder,  mainly  from  a  change  in 
tli<-  Bhape  of  the  ventricles.  A  cross-section  of  the  heart  at 
tin-  base  of  these  latter  during  diastole  would  be  elliptical  in 
outline,  wit li  its  long  diameter  from  right  to  left;  during  the 
ole  it  is  more  circular,  tin;  long  axis  of  the  ellipse  becom- 
ing shortened,  while  the  dorso-ventral  diameter  remains  little 

227 


228  TI1E  HUMAN  BODY. 

altered.  At  the  same  time  the  length  of  the  ventricles  is 
lessened,  the  apex  of  the  heart  approaching  the  base  and  be- 
coming blunter  and  rounder. 

The  Cardiac  Impulse.  The  human  heart  lie.s  with  its 
apex  touching  the  chest-wall  between  the  fifth  and  sixth  ribs 
on  the  left  side  of  the  breast-bone.  At  every  beat  a  sort  of 
tap,  known  as  the  "cardiac  impulse"  or  "apex  beat,"  may  be 
felt  by  the  linger  at  that  point.  There  is,  however,  no  actual 
"  tapping.'"  since  the  heart's  apex  never  leaves  the  chest-wall. 
During  the  diastole  the  soft  ventricles  yield  to  the  chest-wall 
where  they  touch  it,  but  during  the  systole  they  become  hard 
and  tense  and  push  it  out  a  little  between  the  ribs,  and  so 
cause  the  apex  beat..  Since  the  heart  becomes  shorter  during 
the  ventricular  systole,  it  might  be  supposed  that  at  that  time 
the  apex  would  move  up  a  little  in  the  chest.  This,  how- 
ever, is  not  the  case,  the  ascent  of  the  apex  towards  the  base 
of  the  ventricles  being  compensated  for  by  a  movement  of 
the  whole  heart  in  the  opposite  direction.  If  water  be  pumped 
into  an  elastic  tube,  already  moderately  full,  the  tube  will  be 
distended  not  only  transversely  but  longitudinally.  This  is 
what  happens  in  the  aorta:  when  the  left  ventricle  contracts 
and  pumps  blood  forcibly  into  it,  the  elastic  artery  is  elongated 
as  well  as  widened,  and  the  lengthening  of  that  limb  of  its  arch 
attached  to  the  heart  pushes  the  latter  down  towards  the  dia- 
phragm, and  compensates  for  the  upward  movement  of  the 
apex  due  to  the  shortening  of  the  ventricles.  Hence  if  the 
exposed  living  heart  be  watched  it  appears  as  if  during  the 
systole  the  base  of  the  heart  moved  towards  the  tip.  rather 
than  the  reverse. 

■  Events  occurring  within  the  Heart  during  a  Cardiac 
Period.  Let  us  commence  at  the  end  of  the  ventricular 
systole.  At  this  moment  the  semilunar  valves  at  the  orifices 
of  the  aorta  and  the  pulmonary  artery  are  closed,  so  that  no 
blood  can  flow  back  from  those  vessels.  The  whole  heart, 
however,  is  soft  and  distensible  and  yields  readily  to  blood 
flowing  into  it  from  the  pulmonary  veins  and  the  venae  cava^; 
this  passes  on  through  the  open  mitral  and  tricuspid  valves 
and  fills  up  the  dilating  ventricles,  as  well  as  the  auricles.  As 
the  ventricles  fill,  back  currents  are  set  up  along  their  walls 
and  these  carry  up  the  flaps  of  the  valves  so  that  by  the  end 
of  the  pause  they  are  nearly  closed.  At  this  moment  the  au- 
ricles contract,  and  since  this  contraction  commences  at  and 


WORKING  OF  THE  HEART  AND  BLOOD-VESSELS.      229 

narrows  the  mouths  of  the  veins  oj)ening  into  them,  and  at 
the  same  time  the  blood  in  those  vessels  opposes  some  resist- 
ance to  a  back-flow  into  them,  while  the  still  flabby  and 
dilating  ventricles  oppose  much  less  resistance,  the  general 
result  is  that  the  contracting  auricles  send  blood  into  the 
ventricles,  and  not  back  into  the  veins.  At  the  same  time  the 
increased  direct  current  into  the  ventricles  produces  a  greater 
back  current  on  the  sides,  which,  when  the  auricles  cease 
their  contraction  and  the  filled  ventricles  become  tense  and 
press  on  the  blood  inside  them,  completely  closes  the  auriculo- 
ventricular  valves.  That  this  increased  filling  of  the  ventri- 
cles, due  to  auricular  contractions,  will  close  the  valves  may 
be  seen  easily  in  a  sheep's  heart.  If  the  auricles  be  carefully 
cut  away  from  this  so  as  to  expose  the  mitral  and  tricuspid 
valves,  and  water  be  then  poured  from  a  little  height  into  the 
ventricles,  it  will  be  seen  that  as  these  cavities  are  filled  the 
valve-flaps  are  floated  up  and  close  the  orifices. 

The  auricular  contraction  now  ceases  and  the  ventricular 
commences.  The  blood  in  each  ventricle  is  imprisoned  be- 
tween the  auriculo-ventricular  valves  behind  and  the  semi- 
lunar valves  in  front.  The  former  cannot  yield  on  account 
of  the  cordaj  tendineae  fixed  to  their  edges:  the  semilunar 
valves,  on  the  other  hand,  can  open  outwards  from  the  ven- 
tricle and  let  the  blood  pass  on,  but  they  are  kept  tightly  shut 
by  the  pressure  of  the  blood  on  their  other  sides,  just  as  the 
lock-gates  of  a  canal  are  by  the  pressure  of  the  water  on 
them.  In  order  to  open  the  canal-gates  water  is  let  in  or  out 
of  the  lock  until  it  stands  at  the  same  level  on  each  side  of 
them;  but  of  course  they  might  be  forced  open  without  this 
by  applying  sufficient  power  to  overcome  the  higher  water 
pressure  on  one  side.  It  is  in  this  latter  way  that  the  semi- 
lunar valves'are  opened.  The  contracting  ventricle  tightens 
its  grip  on  the  blood  inside  it  and  becomes  rigid  to  the  touch. 
As  it  squeezes  harder  and  harder,  at  last  the  pressure  on  the 
blood  within  it  becomes  greater  than  the  pressure  exerted  on 
the  other  Bide  of  the  valves  by  the  blood  in  the  arteries,  the  flaps 
are  forced  open  and  the  blood  begins  to  pass  out:  the  ventri- 
cle continues  its  contraction  until  it  has  obliterated  its  cavity 
and  completely  emptied  itself;  this  total  emptying  appears, 
at  least,  to  oocnr  in  the  normally  boating  heart,  but  in  some 
pathological  conditions  and  under  the  influence  of  certain 
drugs  the  emptying  of  the  ventricles  is  incomplete.     After 


230  THE   //r.V.I.V  BODY. 

the  systole  the  ventricle  commences  to  relax  and  blood  imme- 
diately to  How  back  towards  it  from  the  highly  stretched  ar- 
teries. This  return  current,  however,  catches  the  pockets  of 
the  semilunar  valves,  drives  them  back  and  closes  the  valve 
so  as  to  form  an  impassable  barrier;  and  so  the  blood  which 
has  been  forced  out  of  either  ventricle  cannot  How  directly 
back  into  it. 

Use  of  the  Papillary  Muscles.  In  order  that  the  con- 
tracting ventricles  may  not  force  blood  back  into  the  auricles 
it  is  essential  that  the  flaps  of  the  mitral  and  tricuspid  valves 
be  maintained  in  position  across  the  openings  which  they 
close,  and  be  not  pushed  back  into  the  auricles.  At  the  com- 
mencement of  the  ventricular  systole  this  is  provided  for  by 
the  cordae  tendineae,  which  are  of  such  a  length  as  to  keep 
the  edges  of  the  flaps  in  apposition,  a  position  which  is 
further  secured  by  the  fact  that  each  set  of  cordae  tendineas 
(Fig.  92)  radiating  from  a  point  in  the  ventricle,  is  not  at- 
tached around  the  edges  of  one  flap  but  on  the  contiguous 
edges  of  two  flaps,  and  so  tends  to  pull  them  together.  But 
as  the  contracting  ventricles  shorten,  the  cordae  tendineae,  if 
directly  fixed  to  their  interior,  would  be  slackened  and  the 
valve-flaps  pushed  up  into  the  auricle.  The  little  papillary 
muscles  prevent  this.  Shortening  as  the  ventricular  systole 
proceeds,  they  keep  the  cordae  taut  and  the  valves  closed. 

The  mechanism  is  indeed  even  better  working  than  this. 
The  area  of  the  valve-flaps  is  greater  than  is  sufficient  to 
stretch  across  the  auriculo-ventricular  orifice,  so  that  when 
their  edges  are  in  apposition  they  form  a  cone  projecting  into 
the  ventricle.  Towards  the  ends  of  the  systole  the  papillary 
muscles  pull  this  cone  down  into  the  ventricular  cavity  so  as 
to  practically  obliterate  it  and  force  out  from  it  nearly  every 
drop  of  blood. 

Sounds  of  the  Heart.  If  the  ear  be  placed  on  the  chest 
over  the  region  of  the  heart  during  life,  two  distinguishable 
sounds  will  be  heard  during  each  cardiac  cycle.  They  are 
known  respectively  as  the  first  and  second  sounds  of  the 
heart.  The  first  is  of  lower  pitch  and  lasts  longer  than  the 
second  and  sharper  sound:  vocally  their  character  may  be 
tolerably  imitated  by  the  words  lubb,  dup.  The  cause  of  the 
second  sound  is  the  closure,  or,  as  one  might  say,  the  "click- 
ing up,"  of  the  semilunar  valves,  since  it  occurs  at  the 
moment  of  their  closure  and  ceases  if  they  be  hooked  back  in 


WORKING  OF  THE  HEART  AND  BLOOD-VESSELS.      231 

a  living  animal.  The  origin  of  the  first  sound  is  still  uncer- 
tain: it  takes  place  during  the  ventricular  systole  and  is 
probably  due  to  vibrations  of  the  tense  ventricular  wall  at 
that  time.  It  is  not  due,  at  least  not  entirely,  to  the  auriculo- 
ventricular  valves,  since  it  may  still  be  heard  in  a  beating 
heart  empty  of  blood,  and  in  which  there  could  be  no  closure 
or  tension  of  those  valves.  In  various  forms  of  heart  disease 
these  sounds  are  modified  or  cloaked  by  additional  '•  mur- 
murs "  which  arise  when  the  cardiac  orifices  are  roughened 
or  narrowed  or  dilated,  or  the  valves  inefficient.  By  paying 
attention  to  the  character  of  the  new  sound  then  heard,  the 
exact  period  in  the  cardiac  cycle  at  which  it  occurs,  and  the 
region  of  the  chest-wall  at  which  it  is  heard  most  distinctly, 
the  physician  can  often  get  important  information  as  to  its 
cause. 

Diagram  of  the  Events  of  a  Cardiac  Cycle.  In  the  fol- 
lowing table  the  phenomena  of  the  heart's  beat  are  repre- 
sented with  reference  to  the  changes  of  form  which  are  seen 
on  an  exposed  working  heart.  Events  in  the  same  vertical 
column  occur  simultaneously;  on  the  same  horizontal  line, 
from  left  to  right,  successively. 


Auricular 
Systole. 

Commence- 
ment of 

Ventricular 
Systole. 

Ventricular 
Systole. 

Cessation 
of  Ven- 
tricular 
Systole. 

Pause. 

Auricles 

Contracting 

and 

emptying. 

Dilating   and 

filling. 

Dilating    and 
filling. 

Contracting. 

Apex  beat. 

Closed. 

Closed . 

First  sound. 

Dilating   and 
filling. 

Contracting" 

and 
emptying. 

Closed. 
Open. 

Dilating 

and  filling. 

Dilating. 

Opening. 
Closing. 
Second 
Bound. 

Dilating 
and  filling. 

Dilating 
and  filling. 

Auriculo-v.  utric- 
ular valves 
Semilunar  valves 

Closing. 
Closed. 

Open. 
Closed. 

Function  of  the  Auricles.  The  ventricles  have  to  do  the 
work  of  pumping  the  blood  through  the  blood-vessels.  Ac- 
cordingly their  walls  arc  far  thicker  and  more  muscular  than 
those  of  the  auricles;  and  the  left  ventricle,  which  has  to 
force  the  blood  over  the  Body  generally,  is  stonier  than  the 
right,  which  has  only  to  send  blood  around  the  comparatively 
Bhoii  pulmonary  circuit.  The  circulation  of  the  blood  is  in 
fact  maintained  by  the  ventricles,  and  we  have  to  inquire 
what  ie  the  ase  of  the  auricles.     Not  [infrequently  the  heart's 


232  THE  HUMAN  BODY. 

action  is  described  us  if  the  auricles  first  filled  with  blood  and 
then  contracted  and  filled  the  ventricles;  and  then  the  latter 
contracted  and  drove  the  blood  into  the  arteries.  From  the 
account  given  above,  however,  it  will  be  seen  that  the  events 
are  not  accurately  so  represented,  but  that  during  all  the 
pause  blood  flows  on  through  the  auricles  into  the  ventricles, 
which  latter  are  already  nearly  full  when  the  auricles  con- 
tract; this  contraction  merely  completing  their  filling  and 
finishing  the  closure  of  the  auriculo-ventricular  valves.  The 
real  use  of  the  auricles  is  to  afford  a  reservoir  into  which 
the  veins  may  empty  while  the  comparatively  long-lasting 
ventricular  eontraction  is  taking  place:  they  also  largely 
control  the  amount  of  work  done  by  the  heart. 

If  the  heart  consisted  of  the  ventricles  only,  with  valves 
at  the  points  of  entry  and  exit  of  the  blood,  the  circulation 
could  be  maintained.  During  diastole  the  ventricle  would 
fill  from  the  veins,  and  during  systole  empty  into  the  arteries. 
But  in  order  to  accomplish  this,  during  the  systole  the  valves 
at  the  point  of  entry  must  be  closed,  or  the  ventricle  would 
empty  itself  into  the  veins  as  well  as  into  the  arteries;  and 
this  closure  would  necessitate  a  great  loss  of  time  which 
might  be  utilized  for  feeding  the  pump.  This  is  avoided  by 
the  auricles,  which  are  really  reservoirs  at  the  end  of  the 
venous  system,  collecting  blood  when  the  ventricular  pump  is 
at  work.  When  the  ventricles  relax,  the  blood  entering  the 
auricles  flows  on  into  them:  but  previously,  during  the  y3^ 
of  the  cardiac  cycle  occupied  by  the  ventricular  systole,  the 
auricles  have  accumulated  blood,  and  when  they  at  last  con- 
tract they  send  on  into  the  ventricles  this  accumulation. 
Even  were  the  flow  from  the  veins  stopped  during  the  auric- 
ular contraction  this  would  be  of  comparatively  little  conse- 
quence, since  that  event  occupies  so  brief  a  time.  But,  al- 
though no  doubt  somewhat  lessened,  the  emptying  of  the 
veins  into  the  heart  does  not  seem  to  be,  in  health,  stopped 
while  the  auricle  is  contracting.  For  at  that  moment  the 
ventricle  is  relaxing  and  receives  the  blood  from  the  auricles 
under  a  less  pressure  than  it  enters  the  latter  from  the  veins. 
The  heart  in  fact  consists  of  a  couple  of  "  feed-pumps  " — the 
auricles — and  a  couple  of  "force-pumps" — the  ventricles; 
and  so  wonderfully  perfect  is  the  mechanism  that  the  supply 
to  the  feed-pumps  is  never  stopped.  The  auricles  are  never 
empty,  being  supplied  all  the  time  of  their  contraction,  which 


WORKING  OF  THE  HEART  AND  BLOOD  -  VESSELS.       233 

is  never  so  great  as  to  obliterate  their  cavities;  while  the  ven- 
tricles contain  no  blood  at  the  end  of  their  systole. 

The  auricles  also  govern  to  a  certain  extent  the  amount 
of  work  done  by  the  ventricles.  These  latter  contract  with 
more  than  sufficient  force  to  completely  drive  out  all  the 
blood  contained  in  them.  If  the  auricles  contract  more 
powerfully  and  empty  themselves  more  completely  at  any 
given  time,  the  ventricles  will  contain  more  blood  at  the  com- 
mencement of  their  systole,  and  will  have  pumped  out  more 
at  its  end.  Now,  as  we  shall  see  in  Chapter  XVI II,  the  con- 
traction of  the  auricles  is  under  the  control  of  the  nervous 
system,  and  through  the  auricles  the  whole  work  of  the 
heart.  In  fact  the  ventricles  represent  the  brute  force  con- 
cerned in  maintaining  the  circulation,  while  the  auricles  are 
part  of  a  highly-developed  co-ordinating  mechanism,  by 
which  the  rate  of  the  blood-flow  is  governed  according  to 
the  needs  of  the  whole  Body  at  the  time. 

The  "Work  Done  by  the  Heart.  This  can  be  calculated 
with  approximate  correctness.  At  each  systole  each  ven- 
tricle sends  out  the  same  quantity  of  blood — about  180  grams 
(6.3  ounces);  the  pressure  exerted  by  the  blood  in  the  aorta 
against  the  semilunar  valves,  and  which  the  ventricle  has  to 
overcome,  is  about  that  which  would  be  exerted  on  the  same 
surface  by  a  column  of  mercury  200  millimeters  (S  inches) 
high.  The  left  ventricle  therefore  drives  out,  seventy  times 
in  a  minute,  180  grams  (6.3  ounces)  of  blood  against  this 
pressure.  Since  the  specific  gravity  of  mercury  is  12.5  and 
that  of  blood  may  for  practical  purposes  be  taken  as  1,  the 
work  of  each  stroke  of  the  ventricle  is  equivalent  to  raising 
ISO  grams  (6.3  ounces)  of  blood  200  X  12.5  =  2500  millim. 
(8.2  feet);  or  one  gram  450  meters  (one  ounce  51.66  feet); 
or  one  kilogram  0.45  meter  (one  lb.  3.23  feet).  Work  is 
measured  by  the  amount  of  energy  needed  to  raise  a  definite 
weight  a  given  distance  against  gravity  at  the  earth's  surface, 
the  unit,  called  a  kilogram/meter,  being  either  that  necessary 
to  raise  one  kilogram  one  meter,  or,  called  a  foot-pound,  that 
necessary  to  raise  one  pound  one  foot.  Expressed  thus  the 
work  of  the  left  ventricle  in  one  minute,  when  the  heart's 
rate  is  seventy  strokes  in  that  time,  is  0.45  X  TO  —  31  50  kilo- 
grammeters  (3.23  X  70  =  226.1  foot-pounds);  in  one  hour  it 
is  31.50  X  60  =  1890  kilogrammeters  (226.1  X  60  =  13,566 
foot-pounds);  and  in  twenty-four  hours  1890  X  24  =  45,360 


234  THE  HUMAN  BODY. 

kilogrammetera  (325,584  foot-pounds).  The  pressure  in  the 
pulmonary  artery  against  which  the  righl   ventricle  works  is 

about  ^  of  that  in  the  aorta;  hence  this  ventricle  in  twenty- 
four  hours  will  do  one  third  as  much  work  as  the  left,  or 
15,120  kilogrammeters  (108,528  Eoot-pounds),  and  adding 
this  to  the  amount  done  by  the  left,  we  get  as  the  total  work 
of  the  ventricles  in  a  day  the  iiniiicii.se  amount  of  60,480 
kilogrammeters  (4:14,112  foot-pounds).  If  a  man  weighing 
75  kilograms  (165  lbs.)  climbed  up  a  mountain  806  meters 
(2644  feet)  high  his  skeletal  muscles  would  probably  lie 
greatly  fatigued  at  the  end  of  the  ascent,  and  yet  in  lifting 
his  Body  that  height  they  would  only  have  performed  the 
amount  of  work  that  the  ventricles  of  the  heart  do  daily 
without  fatigue. 

The  Plow  of  the  Blood  Outside  the  Heart.  The  blood 
leaves  the  heart  intermittently  and  not  in  a  regular  stream, 
a  quantity  being  forced  out  at  each  systole  of  the  ventricles: 
before  it  reaches  the  capillaries,  however,  this  rhythmic 
movement  is  transformed  into  a  steady  flow,  as  may  readily 
be  seen  by  examining  under  the  microscope  thin  transparent . 
parts  of  various  animals,  as  the  web  of  a  frog's  foot,  a  mouse's 
ear,  or  the  tail  of  a  small  fish,  in  consequence  of  the  steadi- 
ness with  which  the  capillaries  supply  the  veins  the  flow  in 
these  is  also  unaffected,  directly,  by  each  beat  of  the  heart; 
if  a  vein  be  cut  the  blood  wells  out  uniformly,  while  a  cut 
artery  spurts  out  not  only  with  much  greater  force,  but  in  jets 
which  are  much  more  powerful  at  regular  intervals  corre- 
sponding with  the  systoles  of  the  ventricles. 

The  Circulation  of  the  Blood  as  Seen  in  the  Frog's  Web. 
There  is  no  more  fascinating  or  instructive  phenomenon  than 
the  circulation  of  the  blood  as  seen  with  the  microscope  in 
the  thin  membrane  between  the  toes  of  a  frog's  hind  limb. 
Upon  focusing  beneath  the  epidermis  a  network  of  minute 
arteries,  veins,  and  capillaries,  with  the  blood  flowing  through 
them,  comes  into  view  (Fig.  91).  The  arteries,  a,  are 
readily  recognized  by  the  fact  that  the  flow  in  them  is  fastest 
and  from  larger  to  smaller  branches.  The  latter  are  seen 
ending  in  capillaries,  which  form  networks,  the  channels  of 
which  are  all  nearly  equal  in  size.  While  in  the  veins  aris- 
ing from  the  capillaries  the  flow  is  from  smaller  to  larger 
trunks,  and  slower  than  in  the  arteries,  but  faster  than  in  the 
capillaries. 


WORKING  OF  THE  HEART  AND  BLOOD  -  VESSELS.       235 

The  reason  of  the  slower  flow  of  the  capillaries  is  that 
their  united  area  is  considerably  greater  than  that  of  the 
arteries  supplying  them,  so  that  the  same  quantity  of  blood 
flowing  through  them  in  a  given  time  has  a  wider  channel 
to  flow  in  and  moves  more  slowly.  The  area  of  the  veins  is 
smaller  than  that  of  the  capillaries  but  greater  than  that  of 
the  arteries,  and  hence  the  rate  of  movement  in  them  is  also 
intermediate.  Almost  always  when  an  artery  divides,  the 
area  of  its  branches  is  greater  than  that  of  the  main  trunk, 
and  so  the  arterial  current  becomes  slower  and  slower  from 
the  heart  onwards.  In  the  veins,  on  the  other  hand,  the  area, 
of  a  trunk  formed  by  the  union  of  two  or  more  branches  is 
less  than  that  of  the  branches  together,  and  the  flow  becomes 
quicker  and  quicker  towards  the  heart.  But  even  at  the 
heart  the  united  cross-sections  of  the  veins  entering  the  auri- 
cles are  greater  than  those  of  the  arteries  leaving  the  ventri- 
cles, so  that,  since  as  much  blood  returns  to  the  heart  in  a 
given  time  as  leaves  it,  the  rate  of  the  current  in  the  pul- 
monary veins  and  the  vena?  cava?  is  less  than  in  the  pulmonary 
artery  and  aorta.  AYe  may  represent  the  vascular  system  as 
a  double  cone,  widening  from  the  ventricles  to  the  capillaries 
and  narrowing  from  the  latter  to  the  auricles.  Just  as  water 
forced  in  at  a  narrow  end  of  this  would  flow  quickest  there 
and  slowest  at  the  widest  part,  so  the  blood  flows  quickest  in 
the  aorta  and  slowest  in  the  capillaries,  which  taken  together 
form  a  much  wider  channel. 

The  Axial  Current  and.  the  Inert  Layer.  If  a  small 
artery  in  the  frog's  web  be  closely  examined  it  will  be  seen 
that  the  rate  of  flow  is  not  the  same  in  all  parts  of  it.  In  the 
centre  is  a  very  rapid  current  carrying  along  all  the  red  cor- 
] nisr -le.s  and  known  as  the  axial  stream,  while  near  the  wall 
of  the  vessel  the  flow  is  much  slower,  as  indicated  by  the  rate 
at  which  the  pale  blood-corpuscles  are  carried  along  in  jt_ 
This  is  a  purely  physical  phenomenon.  If  any  Liquid  he  for- 
cibly driven  through  a  fine  tube  which  it  wets,  water  for  in- 
stance through  a  glass  tube,  the  outermost  layer  of  the  liquid 
will  remain  motionless  in  contact  with  the  tube:  the  next 
layer  of  nioleeules  will  move  a  little,  the  next  faster  still; 
and  so  on  until  a  rapid  current  is  found  in  the  centre.  If 
solid  bodies,  as  powdered  sealing-wax,  be  suspended  in  the 

water,  these  will  all  be  Carried    on    in    the   central    faster   cur- 
rent or  axial  stream,  ju.-t  as  the  red  corpuscles  are  in  the 


236  THE  HUMAN  BODY. 

artery.  The  white  corpuscles,  partly  because  of  their  less 
specific  gravity,  and  partly  because  of  their  sometimes  irregu- 
lar form,  due  to  amoeboid  movements,  get  frequently  pushed 
out  of  the  axial  current,  so  that  many  of  them  are  found  in 
the  inert  layer. 

Internal  Friction.  It  follows  from  the  above-stated  facts 
that  there  is  no  noticeable  friction  between  the  blood  and 
the  lining  of  the  vessel  through  which  it  flows:  since  the 
outermost  blood-layer  in  contact  with  the  wall  of  the  vessel  is 
changed  only  by  diffusion.  There  is  great  friction  between 
the  different  concentric  layers  of  the  liquid,  since  each  of 
them  is  moving  at  a  different  rate  from  that  in  contact  with 
it  on  each  side.  This  form  of  friction  is  known  in  hydro- 
dynamics as  "internal  friction,"  and  it  is  of  great  importance 
in  the  circulation  of  the  blood.  Internal  friction  increases 
very  fast  as  the  calibre  of  the  tube  through  which  the  liquid 
flows  diminishes:  so  that  with  the  same  rate  of  flow  it  is  dis- 
proportionately much  greater  in  a  small  tube  than  in  a  larger 
one.  Hence  a  given  quantity  of  liquid  forced  in  a  minute 
through  one  large  tube  would  experience  much  less  resistance 
from  internal  friction  than  if  sent  in  the  same  time  through 
four  or  five  smaller  tubes,  the  united  transverse  sections  oi 
which  were  together  equal  to  that  of  the  single  larger  one.  In 
the  blood-vessels  the  increased  total  area,  and  consequently 
slower  flow,  in  the  smaller  channels  partly  counteracts  this 
increase  of  internal  friction,  but  only  to  a  comparatively 
slight  extent;  so  that  the  internal  friction,  and  consequently 
the  resistance  to  the  blood-flow,  is  far  greater  in  the  capil- 
laries than  in  the  small  arteries,  and  in  the  small  arteries  than 
in  the  large  ones.  Practically  we  may  regard  the  arteries  as 
tubes  ending  in  a  sponge:  the  united  areas  of  all  the  channels 
in  the  latter  might  be  considerably  larger  than  that  of  the 
supplying  tubes,  but  the  friction  to  be  overcome  in  the  flow 
through  them  would  be  much  greater. 

The  Conversion  of  the  Intermittent  into  a  Continuous 
Flow.  Since  the  heart  sends  blood  into  the  aorta  intermit- 
tently, we  have  still  to  inquire  how  it  is  that  the  flow  in  the 
capillaries  is  continuous.  In  the  larger  arteries  it  is  not, 
since  we  can  feel  them  dilating  as  the  "pulse,"  on  applying 
the  finger  over  the  radial  artery  at  the  wrist,  or  over  the  tem- 
poral artery  on  the  side  of  the  brow. 

The  first  explanation  which  suggests  itself  is  that  since 


WORKING  OF  THE  HEART  AND  BLOOD-VESSELS.       237 


B 


the  capacity  of  the  blood-vessels  increases  from  the  heart  to 
the  capillaries,  an  acceleration  of  the  flow  during  the  ven- 
tricular contraction  which  might  be  very  manifest  in  the 
vessels  near  the  heart  would  become  less  and  less  obvious  in 
the  more  distant  vessels.  But  if  this  were  so,  then  when  the 
blood  was  collected  again  from  the  wide  capillary  sponge  into 
the  great  veins  near  the  heart,  which  together  are  but  little 
bigger  than  the  aorta,  we  ought  to  find  a  pulse,  but  we  do 
not:  the  venous  pulse  which  sometimes  occurs  having  quite  a 
different  cause,  being  due  to  a  back-flow  from  the  auricles,  or 
a  checking  of  the  on-flow  into  them,  during  the  cardiac  sys- 
tole. The  rhythm  of  the  flow  caused  by  the  heart  is  therefore 
not  merely  cloaked  in  the  small  arteries  and  capillaries,  but 
abolished  in  them. 

We  can,  however,  readily  contrive  conditions  outside  the 
Body  under  which  an  intermittent  supply  is  transformed  into 
a  continuous  flow.  Suppose  we 
have  two  vessels,  A  and  B  (Fig. 
97)-  containing  water  and  con- 
nected below  in  two  ways: 
through  the  tube  a  on  which 
there  is  a  pump  provided  with 
valves  so  that  it  can  only  drive 
liquid  from  A  to  B;  and  through 
b.  which  may  be  left  wide  open 
or  narrowed  by  the  clamp  c,  at 
will.  If  the  apparatus  be  left 
at  rest  the  water  will  lie  at  the 
same  level,  d,  in  each  vessel.  If  now  we  work  the  pump,  at 
each  stroke  a  certain  amount  of  water  will  be  conveyed  from 
A  to  B,  and  as  a  result  of  the  lowering  of  the  level  of  liquid 
in  A  and  its  rise  in  B,  there  will  be  immediately  a  return 
flow  from  B  to  A  through  the  tube  b.  A,  in  these  circum- 
stances, would  represent  the  venous  system,  from  which  the 
heart  constantly  takes  blood  to  pump  it  into  B,  representing 
the  arterial  system;  and  b  would  represent  the  capillary  ves- 
sels through  which  the  return  flow  takes  place;  but,  so  far, 
we  should  have  as  intermittent  a  flow  through  the  capillaries, 
l>.  as  through  the  heart-pump,  a.  Now  imagine  b  to  be  nar- 
rowed at  one  point  so  as  to  oppose  resistance  to  the  back-flow, 
while  the  pump  goes  on  working  steadily.  The  result  will  be 
an  accumulation  of  water  in  B,  and  a  fall  of  its  level  in  A. 


—  cL 


-~d" 


Fig.  97. 


238  11  IF.    HUMAN  BODY. 

But  the  more  the  difference  of  level  in  the  two  vessels  in- 
creases, the  greater  is  the  force  tending  to  drive  water  back 
through  b  to  A,  and  more  will  flow  back,  under  the  greater 
difference  of  pressure,  in  a  given  time,  until  at  last,  when  the 
water  in  B  has  reached  a  certain  level,  d'f  and  that  in  A  has 
correspondingly  fallen  to  d",  the  current  through  h  will  carry 
back  in  one  minute  just  so  much  water  as  the  pump  sends  the 
other  way,  and  this  I  tack-flow  will  be  nearly  constant;  it  will 
not  depend  directly  upon  the  strokes  of  the  pump,  but  upon 
the  head  of  water  accumulated  in  l'>\  which  head  of  water 
will,  it  is  true,  be  slightly  increased  at  each  stroke  of  the 
pump,  but  the  increase  will  be  very  small  compared  with  the 
whole  driving  force,  and  its  influence  will  be  inappreciable. 
We  thus  gain  the  idea  that  an  incomplete  impediment  to  the 
flow  from  the  arteries  to  the  veins  (from  B  to  A  in  the  dia- 
gram), such  as  is  afforded  by  internal  friction  in  the  capil- 
laries, may  bring  about  conditions  which  will  lead  to  a  steady 
flow  along  the  latter  vessels. 

But  in  the  arterial  system  there  can  be  no  accumulation  of 
blood  at  a  higher  level  than  that  in  the  veins,  such  as  is  sup- 
posed in  the  above  apparatus;  and  we  must  next  consider  if 
the  "head  of  water"  can  be  replaced  by  some  other  form  of 
driving  force.  It  is  in  fact  replaced  by  the  elasticity  of  the 
large  arteries.  Suppose  an  elastic  bag  instead  of  the  vessel  B 
connected  with  the  pump  "a."  If  there  be  no  resistance  to 
the  back-flow  the  current  through  b  will  be  discontinuous. 
But  if  resistance  be  interposed,  then  the  elastic  bag  will  be- 
come distended,  since  the  pump  sends  in  a  given  time  more 
liquid  into  it  than  it  passes  back  through  b.  But  the  more  it 
becomes  distended  the  more  will  the  bag  squeeze  the  liquid 
inside  and  the  faster  will  it  send  that  back  to  A,  until  at  last 
its  squeeze  is  so  powerful  that  each  minute  or  two  or  five  min- 
utes it  sends  back  into  A  as  much  as  it  receives.  Thenceforth 
the  back-flow  through  b  will  be  practically  constant,  being  im- 
mediately dependent  upon  the  elastic  reaction  of  the  bag,  and 
only  indirectly  upon  the  action  of  the  pump  which  keeps  it 
distended.  Such  a  state  of  things  represents  very  closely  the 
phenomena  occurring  in  the  blood-vessels.  The  highly  elastic 
large  arteries  are  kept  stretched  with  blood  by  the  heart;  and 
the  reaction  of  their  elastic  walls,  steadily  squeezing  on  the 
blood  in  them,  forces  it  continuously  through  the  small  arte- 
ries and  capillaries.     The  steady  flow  in  the  latter  depends 


WORKING  OF  THE  HEART  AND  BLOOD-VESSELS.      239 

thus  on  two  factors:  first,  the  elasticity  of  the  large  arteries; 
and,  secondly,  the  resistance  to  their  emptying,  dependent 
upon  internal  friction  in  the  small  arteries  and  the  capillaries, 
which  calls  into  play  the  elasticity  of  the  large  vessels.  Were 
the  capillary  resistance  or  the  arterial  elasticity  absent  the 
blood-flow  in  the  capillaries  would  be  rhythmic. 


CHAPTER  XVI. 

ARTERIAL  PRESSURE.    THE  PULSE. 

"Weber's  Schema.  It  is  clear  from  the  statements  made 
in  the  last  chapter  that  it  is  the  pressure  exerted  by  the  elas- 
tic arteries  upon  the  blood  inside  them  which  keeps  up  the 
flow  through  the  capillaries,  the  heart  serving  to  keep  the 
big  arteries  tightly  filled  and  so  to  call  the  elastic  reaction  of 
their  walls  into  play.  The  whole  circulation  depends  prima- 
rily, of  course,  upon  the  beat  of  the  heart,  but  this  only  in- 
directly governs  the  capillary  flow,  and  since  the  latter  is  the 
aim  of  the  whole  vascular  apparatus,  it  is  of  great  importance 
to  know  all  about  arterial  pressure;  not  only  how  great  it  is 
on  the  average,  but  how  it  is  altered  in  different  vessels  in 
various  circumstances  so  as  to  make  the  flow  through  the 
capillaries  of  a  given  part  greater  or  less  according  to  circum- 
stances; for,  as  blushing  and  pallor  of  the  face  (which  fre- 
quently occur  without  any  change  in  the  skin  elsewhere) 
prove,  the  quantity  of  blood  flowing  through  a  given  part  is 
not  always  the  same,  nor  is  it  always  increased  or  diminished 
in  all  parts  of  the  Body  at  the  same  time.  Most  of  what  we 
know  about  arterial  pressure  has  been  ascertained  by  experi- 
ments made  upon  the  lower  animals,  from  which  deductions 
are  then  made  concerning  what  happens  in  man,  since  An- 
atomy shows  that  the  circulatory  organs  are  arranged  upon 
the  same  plan  in  all  the  mammalia.  A  great  deal  can,  how- 
ever, be  learnt  by  studying  the  flow  of  liquids  through  ordi- 
nary elastic  tubes.  Suppose  we  have  a  set  of  such  (Fig.  98) 
supplied  at  one  point  with  a  pump,  c,  possessing  valves  of 
entry  and  exit  which  open  only  in  the  direction  indicated  by 
the  arrows,  and  that  the  whole  system  is  slightly  overfilled 
with  liquid  so  that  its  elastic  walls  are  slightly  stretched. 
These  will  in  consequence  press  upon  the  liquid  inside  them 
and  the  amount  of  this  pressure  will  be  indicated  by  the 
gauges;  so  long  as  the  pump  is  at  rest  it  will  be  the  same 
everywhere  (and  therefore  equal  in  the  gauges  on  B  and  A), 

240 


ARTERIAL  PRESSURE.     THE  PULSE. 


241 


since  liquid  in  a  set  of  horizontal  tubes  communicating  freely, 
as  these  do  at  D,  always  distributes  itself  so  that  the  pressure 
upon  it  is  everywhere  the  same.  Let  the  pump  c  now  con- 
tract once,  and  then  dilate :  during  the  contraction  it  will 
empty  itself  into  B  and  during  the  dilatation  fill  itself  from 
A.  Consequently  the  pressure  in  B,  indicated  by  the  gauge 
x,  will  rise  and  that  in  A  will  fall.  But  very  rapidly  the 
liquid  will  redistribute  itself  from  B  to  A  through  D,  until 
it  again  exists  everywhere  under  the  same  pressure.     Every 


Diagram  of  Weber's  Schema. 


time  the  pump  works  there  will  occur  a  similar  series  of 
phenomena,  and  there  will  be  a  disturbance  of  equilibrium 
causing  a  wave  to  flow  round  the  tubing;  but  there  will  be 
no  steady  maintenance  of  a  pressure  on  the  side  B  greater 
than  that  in  A.  Now  let  the  upper  tube  D  be  closed  so  that 
the  liquid  to  get  from  B  to  A  must  flow  through  the  narrow 
lower  tubes  D' ,  which  oppose  considerable  resistance  to  its 
passage  on  account  of  their  frequent  branchings  and  the 
great  internal  friction  in  them;  then  if  the  pump  works  fre- 
quently enough  there  will  be  produced  and  maintained  in  B 
a  pressure  considerably  higher  than  that  in  A,  which  may 
even  become  negative.  If,  for  example,  the  pump  works  60 
times  a  minute  and  at  each  stroke  takes  180  cubic  centi- 
meters of  liquid  (G  ounces)  from  A  and  drives  it  into  B,  the 
quantity  sent  in  at  the  first  stroke  will  not  (on  account  of  the 
resistance  to  its  flow  offered  by  the  small  branched  tubes), 
have  all  got  back  into  A  before  the  next  stroke  takes  place, 
sending  180  more  cubic  centimeters  (6  oz.)  into  B.  Conse- 
quently at  each  stroke  B  will  become  more  and  more  dis- 
tended and  A  more  and  more  emptied,  and  the  gauge  x  will 


242  THE  HUMAN  BODY. 

indicate  a  much  higher  pressure  than  that  on  A.  As  B  is 
more  stretched,  however,  it  squeezes  harder  upon  its  con- 
tents, until  al  lasi  a  time  comes  when  this  squeeze  is  power- 
ful enough  to  force  through  the  small  tubes  ju  si  ISO  cubic 
centimeters  (6  oz.)  in  a  second.  Then  further  accumulation 
in  B  ceases.  The  pump  sends  into  it  10,800  cubic  centi- 
meters (3G0  ounces)  in  a  minute  at  one  end  and  it  squeezes 
out  exactly  that  amount  in  the  same  time  from  its  other  end  : 
and  so  long  as  the  pump  works  steadily  the  pressure  in  B 
will  not  rise,  nor  that  in  A  fall,  any  more.  But  under  such 
circumstances  the  flow  through  the  small  tubes  will  be  nearly 
constant  since  it  depends  upon  the  difference  in  pressure  pre- 
vailing between  B  and  A,  and  only  indirectly  upon  the  pump 
which  serves  simply  to  keep  the  pressure  high  in  B  and  low- 
in  A.  At  each  stroke  of  the  pump  it  is  true  there  will  be 
a  slight  increase  of  pressure  in  B  due  to  the  fresh  180  cub. 
cent.  (G  oz.)  forced  into  it,  but  this  increase  will  be  but  a 
small  fraction  of  the  total  pressure  and  so  have  but  an  in- 
significant influence  upon  the  rate  of  flow  through  the  small 
connecting  tubes. 

Arterial  Pressure.  The  condition  of  things  just  de- 
scribed represents  very  closely  the  phenomena  presented  in 
the  blood-vascular  system,  in  which  the  ventricles  of  the 
heart,  with  their  auriculo-ventricular  atid  semilunar  valves, 
represent  the  pump,  the  smallest  arteries  and  the  capillaries 
the  resistance  at  D',  the  large  arteries  the  elastic  tube  B,  and 
the  veins  the  tube  A.  The  ventricles  constantly  receiving 
blood  through  the  auricles  from  the  veins,  send  it  into  the 
arteries,  which  find  a  difficulty  in  emptying  themselves 
through  the  capillaries,  and  so  blood  accumulates  in  them 
until  the  elastic  reaction  of  the  stretched  arteries  is  able  to 
squeeze  in  a  minute  through  the  capillaries  just  so  much 
blood  as  the  left  ventricle  pumps  into  the  aorta,  and  the  right 
into  the  pulmonary  artery,  in  the  same  time.  Accordingly  in 
a  living  animal  a  pressure-gauge  connected  with  an  artery 
shows  a  much  higher  pressure  than  one  connected  with  a  vein, 
and  this  persisting  difference  of  pressure,  only  increased  by  a 
small  fraction  of  the  whole  at  each  heart-beat,  brings  about 
a  steady  flow  from  the  arteries  to  the  veins.  The  heart  keeps 
the  arteries  stretched  and  the  stretched  arteries  maintain  the 
flow  through  the  capillaries,  and  the  constancy  of  the  current 
in  them  depends  on  two  factors:  (1)  the  resistance  experi- 


ARTERIAL  PRESSURE.     THE  PULSE.  243 

euced  by  the  blood  in  its  flow  from  the  ventricles  to  the 
veins,  and  (2)  the  elasticity  of  the  larger  arteries  which  allows 
the  blood  to  accumulate  in  them  under  a  high  pressure,  iu 
consequence  of  this  resistance. 

The  Arterial  Pressure.  This  cannot  be  directly  meas- 
ured with  accuracy  in  man,  but  from  measurements  made  on 
other  animals  it  is  calculated  that  in  the  human  aorta  its 
average  is  equal  to  that  of  a  column  of  mercury  200  milli- 
meters (8  inches)  high.  During  the  systole  it  rises  about  5 
millimeters  (I  inch)  above  this  and  during  the  pause  falls  the 
same  amount  below  it.  The  pressure  in  the  vena?  cavae  on  the 
other  hand  is  often  negative,  the  blood  being,  to  use  ordinary 
language,  often  "  sucked"  out  of  them  into  the  heart,  and  it 
rarely  rises  above  5  millimeters  (^  inch)  of  mercury  except 
under  conditions  (such  as  powerful  muscular  effort  accom- 
panied by  holding  the  breath)  which  force  blood  on  into  the 
venae  cavae  and,  by  impeding  the  pulmonary  circulation,  in- 
terfere with  the  emptying  of  the  right  auricle.  Hence  to 
maintain  the  flow  from  the  aorta  to  the  vena  cava  we  have 
an  average  difference  of  pressure  equal  to  200  —  5  =  195 
millimeters  (7|  inches)  of  mercury,  rising  to  205  —  5  =  200 
mm.  (8  inches)  during  the  cardiac  systole  and  falling  to 
195  —  5  =  190  mm.  (7|  inches)  during  the  pause;  but  the 
slight  alterations,  only  about  ^  of  the  whole  difference  of 
aortic  and  vena-cava  pressures  which  maintain  the  blood- 
flow,  are  too  small  to  cause  appreciable  changes  in  the  rate  .of 
the  current  in  the  capillaries.  The  pressure  on  the  blood  in 
the  pulmonary  artery  is  about  -J  of  that  in  the  aorta. 

Since  the  blood  flows  from  the  aorta  to  its  branches  and 
from  these  to  the  capillaries  and  thence  to  the  veins,  and 
liquids  in  a  set  of  continuous  tubes  flow  from  points  of 
greater  to  those  of  less  pressure,  it  is  clear  that  the  blood- 
pressure  must  constantly  diminish  from  the  aorta  to  the 
right  auricle;  and  similarly  from  the  pulmonary  artery  to 
the  left  auricle.  At  any  point,  in  fact,  the  pressure  is  pro- 
portionate  to  the  resistance  in  front,  and  since  the  farther 
the  blood  has  gone  the  less  of  this,  due  to  impediments  at 
branchings  and  to  internal  friction,  it  has  to  overcome  in 
finishing  its  round,  the  pressure  on  the  blood  diminishes  as 
we  follow  it  from  the  aorta  to  the  venae  cavae.  In  the  larger 
arteries  the  fall  of  pressure  is  gradual  and  small,  since  (lie 
amount  of  resistance  met,  with  in  the  flow  through  them  is 


244  THE  HUMAN   BODY. 

but  little.  In  the  small  arteries  and  capillaries  the  resistance 
overcome  and  left  behind  is  (on  account  of  the  great  internal 
friction  due  to  their  small  calibre)  very  great,  and  conse- 
quently the  fall  of  pressure  between  the  medium-sized  arteries 
and  the  veins  is  rapid  and  considerable. 

Modifications  of  Arterial  Pressure  by  Changes  in  the 
Rate  of  the  Heart's  Beat.  A  little  consideration  will  make 
it  clear  that  the  pressure  prevailing  at  any  timo  in  a  given 
artery  depends  on  two  things — the  rate  at  which  the  vessel 
is  filled,  i.e. ,  upon  the  amount  of  work  done  by  the  heart; 
and  the  ease  or  difficulty  with  which  it  is  emptied,  that  is, 
upon  the  resistance  in  front.  A  third  factor  has  to  be  taken 
into  account  in  some  cases;  namely,  that  when  the  muscular 
coats  of  the  small  arteries  contract  the  local  capacity  of  the 
vascular  system  is  diminished,  and  has  to  be  compensated  for 
by  greater  distention  elsewhere,  and  vice  versa.  This  would 
of  itself  of  course  bring  about  changes  in  the  pressure  ex- 
erted on  the  contained  liquid,  but  for  the  present  it  may  be 
left  out  of  consideration.  Returning  to  the  system  of  elastic 
tubes  with  a  pump  represented  in  Fig.  98,  let  us  suppose  the 
pump  to  be  driving  as  before  10,800  cub.  cent.  (360  oz.)  per 
minute  into  the  tubes  B,  and  that  these  latter  are  so  dis- 
tended that  they  drive  out  just  that  quantity  in  the  same 
time.  Under  such  conditions  the  pressure  at  any  given 
point  in  B  will  remain  constant,  apart  from  the  small  varia- 
tions dependent  upon  each  stroke  of  the  pump.  Now,  how- 
ever, let  the  latter,  while  still  sending  in  180  cult.  cent. 
(6  oz.)  at  each  stroke,  work  80  instead  of  GO  times  a  minute 
and  so  send  in  that  time  180  X  80  =  14,400  cub.  cent. 
(480  oz.)  instead  of  the  former  quantity.  This  will  lead  to 
an  accumulation  in  B,  since  its  squeeze  is  only  sufficient, 
against  the  resistance  opposed  to  it,  to  send  out  10,800  cub. 
cent.  (3C0  oz.)  in  a  minute.  B  consequently  will  become 
more  stretched  and  the  pressure  in  it  will  rise.  As  this 
takes  place,  however,  it  will  press  more  powerfully  on  its 
contents  until  at  last  its  distention  is  such  that  its  elastic 
reaction  is  able  to  force  out  in  a  minute  through  the  small 
tubes  D,  14,400  cub.  cent.  (480  oz.)  Thenceforth,  so  long 
as  the  pump  beats  with  the  same  force  and  at  the  same  rate 
and  the  peripheral  resistance  remains  the  same,  the  mean 
pressure  in  B  will  neither  rise  nor  fall — B  sending  into  A  in 
a  minute  as  much  as  c  takes  from  it,  and  we  would  have  a 


ARTERIAL  PRESSURE.     THE  PULSE.  245 

steady  condition  of  things  with  a  higher  mean  pressure  in  B 
than  before. 

On  the  other  hand2  if  the  pump  begins  to  work  more 
slowly  while  the  resistance  remains  the  same,  it  is  clear  that 
the  mean  pressure  in  B  must  fall.  If,  for  example,  the 
pump  works  only  forty  times  a  minute  and  so  sends  in  that 
time  180  X  40  =  7200  cub.  cent.  (240  oz.)  into  B,  which  is 
so  stretched  that  it  is  squeezing  out  10,800  cub.  cent.  (360 
oz. ),  in  that  time,  it  is  clear  that  B  will  gradually  empty 
itself  and  its  walls  become  less  stretched  and  the  pressure  in 
it  fall.  As  this  takes  place,  however,  it  will  force  less  liquid 
in  a  minute  through  the  small  tubes,  until  at  last  a  pressure  is 
reached  at  which  the  squeeze  of  B  only  sends  out  7200  cub. 
cent.  (340  oz.)  in  a  minute;  and  then  the  fall  of  pressure  will 
cease  and  a  steady  one  will  be  maintained,  but  lower  than 
before.  , 

Applying  the  same  reasoning  to  the  vascular  system,  we 
see  that  (the  j:>eripheral  resistance  remaining  unaltered),  if  the 
heart's  force  remains  the  same  but  its  rate  increases,  arterial 
pressure  will  rise  to  a  new  level,  while  a  slowing  of  the  heart's 
beat  will  bring  about  a  fall  of  pressure. 

Modifications  of  Arterial  Pressure  Dependent  on 
Changes  in  the  Force  of  the  Heart's  Beat.  Returning 
again  to  Fig.  98:  suppose  that,  while  the  rate  of  the  pump 
remains  the  same,  its  power  alters  so  that  each  time  it  sends 
200  cub.  cent.  (G.6  oz.)  instead  of  180  (6  oz.)  and  so  in  a 
minute  12,000  cub.  cent  (396  oz.)  instead  of  10,800  (360  oz.) 
— the  quantity  which  B  is  stretched  enough  to  squeeze  out 
in  that  time.  Water  will  in  consequence  accumulate  in  B 
until  it  becomes  stretched  enough  to  squeeze  out  12,000  cub. 
cent.  (396  oz.)  in  a  minute,  and  then  a  steady  pressure  at  a 
new  and  higher  level  will  be  maintained.  On  the  other 
hand  if  the  pump,  still  beating  sixty  times  a  minute,  works 
more  feebly  so  as  to  send  out  only  160  cub.  cent.  (5.6  oz.)  at 
each  stroke,  then  B,  squeezing  out  at  first  more  than  it 
receives  in  a  given  time,  will  gradually  empty  itself  until  it 
only  presses  hard  enough  upon  its  contents  to  force  160  X  60 
=  9600  cub.  cent.  (336  oz.)  out  in  a  minute. 

Similarly,  if  while  the  resistance  in  the  small  arteries  and 
capillaries  remains  the  same  and  the  heart's  rate  unchanged 
the  stroke  of  the  latter  alters,  so  that  at  each  beat  it  sends 


246  THE  HUM  A  A   no/)  v. 

more  blood  out  than  it  did  previously,  then  arterial  pressure 

will  rise;  while  if  the  heart  beats  more  feebly  it  will  fall. 

Modifications  of  Arterial  Pressure  by  Changes  in  the 
Peripheral  Resistance.  Let  the  pump  c  in  Fig.  98  .still 
work  steadily  sending  10,800  cub.  cent.  (360  oz.)  per  minute 
into  B  and  the  resistance  increase,  it  is  clear  arterial  pressure 
must  rise.  For  l!  is  only  stretched  enough  to  squeeze  out  in 
a  minute  the  above  quantity  of  liquid  againsl  the  original  re- 
sistance, and  cannot  at  first  send  out  that  quantity  against  the 
greater.  Liquid  will  consequently  accumulate  in  it  until  at 
last  it  becomes  stretched  enough  to  send  out  10,800  cub. 
cent.  (360  cubic  oz.)  in  a  minute  through  the  small  tubes,  in 
spite  of  the  greater  resistance  to  be  overcome.  A  new  mean 
pressure  at  a  higher  level  will  then  be  established.  If,  on  the 
contrary,  the  resistance  diminishes  while  the  pump's  work 
remains  the  same,  then  B  will  at  first  squeeze  out  in  a  minute 
more  than  it  receives,  until  finally  its  elastic  pressure  is 
reduced  to  the  point  at  which  its  receipts  and  losses  balance, 
and  a  new  and  lower  mean  pressure  will  be  established  in  B. 

Similarly  in  the  vascular  system,  increase  of  the  peripheral 
resistance  by  narrowing  of  the  small  arteries  will  increase 
arterial  pressure  in  all  parts  nearer  the  heart,  while  dilata- 
tion of  the  small  arteries  will  have  the  contrary  effect. 

Summary.  We  find  then  that  arterial  pressure  at  any 
moment  is  dependent  upon — (1)  the  rate  of  the  heart's  beat; 
(2)  the  quantity  of  blood  forced  into  the  arteries  at  each 
heat :  ('■>)  the  calibre  of  the  smaller  vessels.  All  of  these, 
and  consequently  the  capillary  circulation  which  depends 
upon  arterial  pressure,  are  under  the  control  of  the  nervous 
system  (see  Chap.  XVIL). 

The  Pulse.  When  the  left  ventricle  contracts  it  forces  a 
certain  amount  of  blood  into  the  aorta,  which  is  already  dis- 
tended and  on  account  of  the  resistance  in  front  cannot 
empty  itself  as  fast  as  the  contracting  ventricle  fills  it.  As 
a  consequence  its  elastic  walls  yield  still  more — it  enlarges 
both  transversely  and  longitudinally  and  if  exposed  in  a 
living  animal  can  be  seen  and  felt  to  pulsate,  swelling  out  at 
each  systole  of  the  heart,  and  shrinking  and  getting  rid  of 
the  excess  during  the  pause.  A  similar  phenomenon  can  be 
observed  in  all  the  other  large  arteries,  for  just  as  the  con- 
tracting ventricle  tills  the  aorta  faster  than  the  latter  empties 
(the  whole  period  of  diastole  and  systole  being  required  by 


ARTERIAL  PRESSURE.     THE  PULSE.  247 

the  aorta  to  pass  on  the  blood  sent  in  during  systole),  so  the 
increased  tension  in  the  aorta  immediately  after  the  cardiac 
•contraction  drives  on  some  of  its  contents  into  its  branches, 
and  fills  these  faster  than  they  are  emptying,  and  so  causes 
a  dilatation  of  them  also,  which  only  gradually  disappears  as 
the  aortic  tension  falls  before  the  next  systole.  Hence  after 
each  beat  of  the  heart  there  is  a  sensible  dilatation  of 
all  the  larger  arteries,  known  as  the  pulse,  which  becomes 
less  and  less  marked  at  points  on  the  smaller  branches 
farther  from  the  heart,  but  which  in  health  can  readily  be 
recognized  on  any  artery  large  enough  to  be  felt  by  the 
finger  through  the  skin,  etc.  The  radial  artery  near  the 
wrist,  for  example,  will  always  be  felt  tense  by  the  finger, 
since  it  is  kept  overfilled  by  the  heart  in  the  way  already  de- 
scribed. But  after  each  heart-beat  it  becomes  more  rigid 
and  dilates  a  little,  the  increased  distension  and  rigidity 
gradually  disappearing  as  the  artery  pas*s  on  the  excess  of 
blood  before  the  next  heart-beat.  The  pulse  is  then  a  wave 
of  increased  pressure  started  by  the  ventricular  systole,  ra- 
diating from  the  semilunar  valves  over  the  arterial  system, 
and  gradually  disappearing  in  the  smaller  branches.  In  the 
aorta  the  pulse  is  most  marked,  for  the  resistance  there  to 
the  transmission  onwards  of  the  blood  sent  in  by  the  heart  is 
greatest,  and  the  elastic  tube  in  which  it  consequently  accu- 
mulates is  shortest,  and  so  the  increase  of  pressure  and  the 
dilatation  caused  are  considerable.  The  aorta,  however, 
gradually  squeezes  out  the  excess  blood  into  its  branches,  and 
so  this  becomes  distributed  over  a  wider  area,,  and  these 
branches  having  less  resistance  in  front  find  less  and  less  diffi- 
culty in  passing  it  on ;  consequently  the  pulse-wave  becomes 
less  and  less  conspicuous  and  finally  altogether  disappears  be- 
fore the  capillaries  are  reached,  the  excess  of  liquid  in  the 
whole  arterial  system  after  a  ventricular  systole  being  too 
small  to  sensibly  raise  the  mean  pressure  once  it  has  been 
widely  distributed  over  the  elastic  vessels,  which  is  the  case 
by  the  time  the  wave  has  reached  the  small  branches  which 
supply  the  capillaries. 

The  pulse- wave  travels  over  the  arterial  system  at  the  rate 
of  about  '.)  metres  (29.5  feet)  in  a  second,  commencing  at  the 
wrist  0.15S  Becond,  and  in  the  posterior  tibial  artery  at  the 
ankle  0. 19:;  second,  after  the  ventricular  systole.  The  blood 
itself  doefl  not  of  course  travel  as  fast    as   the   pulse-wave,  for 


248  THE  HUMAN  BODY. 

that  quantity  sent  into  the  aorta  at  eacli  heart- beat  does  not 
immediately  rush  on  over  the  whole  arterial  system,  but  by 
raising  the  local  pressure  causes  the  vessel  to  squeeze  out 
faster  than  before  some  of  the  blood  it  already  contains,  and 
this  entering  its  branches  raises  the  pressure  in  them  and 
causes  them  to  more  quickly  fill  their  branches  and  raise  the 
pressure  in  them;  the  pulse-wave  or  wave  of  increased  press- 
ure is  transmitted  in  this  way  much  faster  than  any  given 
portion  of  the  blood.  How  the  wave  of  increased  pressure 
and  the  liquid  travel  at  different  rates  may  be  made  clearer 
perhaps  by  picturing  what  would  happen  if  liquid  were 
pumped  into  one  end  of  an  already  full  elastic  tube,  closed  at 
the  other  end.  At  the  closed  end  of  the  tube  a  dilatation  and 
increased  tension  would  be  felt  immediately  after  each  stroke 
of  the  pump,  although  the  liquid  pumped  in  at  the  other  end 
would  have  remained  about  its  point  of  entry;  it  would  cause 
the  pulsation  not  by  flowing  along  the  tube  itself,  but  by  giv- 
ing a  push  to  the  liquid  already  in  it.  If  instead  of  absolutely 
closing  the  distal  end  of  the  tube  one  brought  about  a  state 
of  things  more  nearly  resembling  that  found  in  the  arteries 
by  allowing  it  to  empty  itself  against  a  resistance,  say  through 
a  narrow  opening,  the  phenomena  observed  would  not  be  es- 
sentially altered;  the  increase  of  pressure  would  travel  along 
the  distended  tube  far  faster  than  the  liquid  itself. 

The  pulse  being  dependent  on  the  heart's  systole,  "  feeling 
the  pulse"  of  course  primarily  gives  a  convenient  means  of 
counting  the  rate  of  beat  of  that  organ.  To  the  skilled  touch,' 
however,  it  may  tell  a  great  deal  more,  as  for  example  whether 
it  is  a  readily  compressible  or  "soft  pulse"  showing  a  low  ar- 
terial pressure,  or  tense  and  rigid  ("a  hard  pulse")  indicative 
of  high  arterial  pressure,  and  so  on.  In  adults  the  normal 
pulse  rate  may  vary  from  sixty-five  to  seventy-five,  the  most 
common  number  being  seventy-two.  In  the  same  individual 
it  is  faster  when  standing  than  when  sitting,  and  when  sitting 
than  when  lying  down.  Any  exercise  increases  its  rate  tem- 
porarily, and  so  does  excitement;  a  sick  person's  pulse  should 
not  therefore  be  felt  when  he  is  nervous  or  excited  (as  the 
physician  knows  when  he  tries  first  to  get  his  patient  calm 
and.  confident),  as  it  is  then  difficult  to  draw  correct  conclu- 
sions from  it.  In  children  the  pulse  is  quicker  than  in  adults, 
and  in  old  age  slower  than  in  middle  life. 

The  Rate  of  the  Blood-Flow.     As  the   vascular  system 


ARTERIAL  PRESSURE.     THE  PULSE.  249 

becomes  more  capacious  from  the  aorta  to  the  capillaries  the 
rate  of  flow  in  it  becomes  proportionately  slower,  and  as  the 
total  area  of  the  channels  diminishes  again  from  the  capilla- 
ries to  the  vense  cava?,  so  does  the  rate  of  flow  quicken,  just 
as  a  river  current  slackens  when  it  spreads  out,  and  flows 
faster  where  it  is  confined  to  a  narrower  channel ;  a  fact  taken 
advantage  of  in  the  construction  of  Eads'  jetties  at  the  mouth 
of  the  Mississippi,  the  object  of  which  is  to  make  the  water 
flow  in  a  narrower  channel  and  so  with  a  more  rapid  current 
in  that  part  of  the  river.  Actual  measurements  as  to  the  rate 
of  flow  in  the  arteries  cannot  be  made  on  man,  but  from  ex- 
periments on  lower  animals  it  is  calculated  that  in  the  human 
carotid  the  blood  flows  about  400  millimetres  (16  inches)  in  a 
second.  In  the  capillaries  the  current  travels  only  from  0.5 
to  0.75  mm.  (j\  to  ^  inch)  in  a  second.  The  total  time 
taken  by  a  portion  of  blood  in  making  a  complete  circulation 
has  been  measured  by  injecting  some  easily  detected  sub- 
stance into  an  artery  on  one  side  of  the  body  and  noting  the 
time  which  elapses  before  it  can  be  found  in  a  corresponding 
vein  on  the  oppos'  e  side.  In  dogs  this  time  is  15  seconds, 
and  it  is  calculated  for  man  at  about  23  seconds.  Of  this 
total  time  about  half  a  second  is  spent  in  the  systemic  and 
another  half  second  in  the  pulmonary  capillaries,  as  each  jior- 
tion  of  blood  on  its  course  from  the  last  artery  to  the  first 
vein  passes  through  a  length  of  capillary  which  on  the  aver- 
age is  0.5  mm.  (-£$  inch).  The  rate  of  flow  in  the  great  veins 
is  about  100  mm.  (4  inches)  in  a  second,  but  is  subject  to  con- 
siderable variations  dependent  on  the  respiratory  and  other 
movements  of  the  Body;  in  the  small  veins  it  is  much  slower. 
Secondary  Causes  of  the  Circulation.  While  the  heart's 
beat  is  the  great  driving  force  of  the  circulation,  certain  other 
things  help  more  or  less — viz.,  gravity,  compression  of  the 
veins,  and  aspiration  of  the  thorax.  All  of  them  are,  how- 
ever, quite  subsidiary;  experiment  on  the  dead  Body  shows 
that  the  injection  of  whipped  blood  into  the  aorta  under  a 
Less  force  than  that  exerted  by  the  left  ventricle  during  life 
is  more  than  sufficient  to  drive  it  round  and  back  by  the  vena? 
cavae.  Not  infrequently  the  statement  is  made  in  books  that, 
probably,  the  systemic  capillaries  have  an  attractive  force  for 
arterial  blood  and  the  pulmonary  capillaries  for  venous  blood, 
but  there  is  not  the  slightest  evidence  of  the  correctness  of 
such  a  supposition,  nor  any  necessity  for  making  it. 


250  TllF.   ill  i/.i.\    BODY. 

The  Influence  of  Gravity.  Under  ordinary  circum- 
stances this  may  be  neglected,  since  in   parts  of  the   Body 

below  the  level  of  the  hear!  it  will  assist  the  How  in  the  ar< 
teries  and  impede  it  equally  in  t  he  veins,  while  the  reverse 
is  the  case  in  the  upper  parts  of  the  Body.  In  certain  ci 
however,  it  is  well  to  bear  these  points  in  mind.  A  part 
"congested"  or  gorged  with  blood  should  if  possible  be  raised 
so  as  to  make  the  back-flow  in  its  veins  easier;  ami  somel  imes 
when  the  heart  is  acting  feebly  it  may  be  able  to  drive  Mood 
along  arteries  in  which  gravity  helps,  but  not  otherwise.  Ac- 
cordingly in  a  tendency  to  fainting  it  is  best  to  lie  down,  and 
make  it  easier  for  the  heart  to  .-end  blood  up  to  the  brain, 
bloodlessness  of  which  is  the  cause  of  the  loss  of  consciousness 
in  a  fainting-fit.  In  fact,  so  long  as  the  breathing  continues, 
the  aspiration  of  the  thorax  will  keep  up  the  venous  How  (see 
below),  while,  in  the  circumstances  supposed,  a  slight  dimi- 
nution in  the  resistance  opposed  to  the  arterial  How  may  he 
of  importance.  The  head  of  a  person  who  has  fainted  should 
accordingly  never  he  raised  until  he  has  undoubtedly  recov- 
ered, a  fact  rarely  borne  in  mind  by  spectators,  who  commonly 
rush  at  once  to  lift  any  one  whom  they  see  fall  in  the  street 
or  elsewhere. 

The  Influence  of  Transient  Compression  of  the  Vein?', 
The  valves  of  the  veins  being  so  disposed  as  to  permit  only 
a  flow  towards  the  heart,  when  external  pressure  empties  a 
vein  it  assists  the  circulation.  Continuous  pressure,  as  by 
a  tight  garter,  is  of  course  bad,  since  it  checks  all  subsequent 
flow  through  the  vessel;  but  intermittent  pressure,  such  as  is 
exerted  on  many  veins  by  muscles  in  the  ordinary  move- 
ments of  the  Body,  acts  as  a  pump  to  force  on  the  blood  in 
them. 

The  valves  of  the  veins  have  another  use  in  diminishing 
the  pressure  on  the  lower  part  of  those  vessels  in  many 
regions.  If,  for  instance,  there  wrere  no  valves  in  the  long 
saphenous  vein  of  the  leg  the  considerable  weight  of  the 
column  of  blood  in  it,  which  in  the  erect  position  would  be 
about  a  metre  (39  inches)  high,  would  press  on  the  lower  part 
of  the  vessel.  But  each  set  of  valves  in  it  carries  the  weight 
of  the  column  of  blood  between  it  and  the  next  set  of  valves 
above,  and  relieves  parts  below,  and  so  the  weight  of  the  col- 
umn of  blood  is  distrihuted  and  does  not  all  hear  on  any  one 
point. 


ARTERIAL  PRESSURE.     THE  PULSE.  251 

Aspiration  of  the  Thorax.  "Whenever  a  breath  is  drawn 
the  pressure  of  the  air  on  the  vessels  inside  the  chest  is  di- 
minished, while  that  on  the  other  vessels  of  the  Body  is  un- 
affected. In  consequence  blood  tends  to  flow  into  the  chest. 
It  cannot,  however,  flow  back  from  the  arteries  on  account  of 
the  semilunar  valves  of  the  aorta,  but  it  can  readily  be  pressed, 
or  in  common  language  "sucked,"  into  the  great  veins  close 
to  the  heart  and  into  the  right  auricle  of  the  latter.  The 
details  of  this  action  must  be  omitted  until  the  respiratory 
mechanism  has  been  considered.  All  parts  of  the  pulmonary 
circuit  being  within  the  thorax,  the  respiratory  movements  do 
not  directly  influence  it,  except  in  so  far  as  the  distention  or 
collapse  of  the  lungs  alters  the  calibre  of  their  vessels. 

The  considerable  influence  of  the  respiratory  movements 
upon  the  venous  circulation  can  be  readily  observed.  In 
thin  persons  the  jugular  vein  in  the  neck  can  often  be  seen 
to  empty  rapidly  and  collapse  during  inspiration,  and  fill  up 
in  a  very  noticeable  way  during  expiration,  exhibiting  a 
sort  of  venous  pulse.  Every  one,  too,  knows  that  by  making 
a  violent  and  prolonged  expiration,  as  exhibited  for  example 
by  a  child  with  whooping-cough,  the  flow  in  all  the  veins  of 
the  head  and  neck  may  be  checked,  causing  them  to  swell  up 
and  hinder  the  capillary  circulation  until  the  person  becomes 
"  black  in  the  face,"  from  the  engorgement  of  the  small  ves- 
sels with  dark-colored  venous  blood. 

In  diseases  of  the  tricusjaid  valve  another  form  of  venous 
pulse  is  often  seen  in  the  superficial  veins  of  the  neck,  since 
at  each  contraction  of  the  right  ventricle  some  blood  is  driven 
back  through  the  right  auricle  into  the  veins. 

Proofs  of  the  Circulation  of  the  Blood.  The  ancient 
physiologists  believed  that  the  movement  of  the  blood  was  an 
ebb  and  flow,  to  and  from  each  side  of  the  heart,  and  out  and  in 
by  both  arteries  and  veins.  They  had  no  idea  of  a  circulation, 
but  thought  pure  blood  was  formed  in  the  lungs  and  impure 
in  the  liver,  and  that  these  partially  mixed  in  the  heart 
through  minute  pores  supposed  to  exist  in  the  septum. 
■  tii-.  who  was  Ijii iiit  alive  by  Calvin  in  L553,  first  stated 
thai  there  was  a  continuous  passage  through  the  lungs  from 
the  pulmonary  artery  to  the  pulmonary  veins,  but  the  great 
Englishman  Harvey  first,  in  lectures  delivered  in  the  College 
of  Physicians  of  Loudon  abonl  L6 16,  demonstrated  thai  the 
movement   of  the   blood  was  a  continuous  circulation  as  we 


2f)2  THE  HUMAN  BODY. 

now  know  it,  and  so  laid  the  foundation  of  modern  Physi- 
ology. In  his  time,  however,  the  capillary  vessels  had  not 
been  discovered,  so  that  although  lie  was  quite  certain  that 
the  hlood  got  somehow  from  the  final  branches  of  the  aorta  to 
the  radicles  of  the  venous  system,  he  did  not  exactly  know 
how. 

The  proofs  of  the  course  of  the  circulation  are  at  present 
quite  conclusive,  and  may  be  summed  up  as  follows:  (1) 
Blood  injected  into  an  artery  in  the  dead  Body  will  return  by 
a  vein;  but  injected  into  a  vein  will  not  pass  back  by  an 
artery.  (2)  The  anatomical  arrangement  of  the  valves  of  the 
heart  and  of  the  veins  shows  that  the  blood  can  only  flow 
from  the  heart,  through  the  arteries  and  back  to  the  heart  by 
the  veins.  (3)  A  cut  artery  spurts  from  the  end  next  the 
heart,  a  cut  vein  bleeds  most  from  the  end  farthest  from  the 
heart.  (4)  A  portion  of  a  vein  when  emptied  fills  only  from 
the  end  farthest  from  the  heart.  This  observation  can  be 
made  on  the  veins  on  the  back  of  the  hand  of  any  thin  per- 
son, especially  if  the  vessels  be  first  gorged  by  holding  the 
hand  in  a  dependent  position  for  a  few  seconds.  Select  then 
a  vein  which  runs  for  an  inch  or  so  without  branching,  place 
a  finger  on  its  distal  end.  and  then  empty  it  up  to  its  next 
branch  (where  valves  usually  exist)  by  compressing  it  from 
below  up.  The  vessel  will  then  be  found  to  remain  empty  as 
long  as  the  finger  is  kept  on  its  lower  end,  but  to  fill  im- 
mediately when  it  is  removed;  which  proves  that  the  valves 
prevent  any  filling  of  the  vein  from  its  heart-end  backwards. 
(5)  If  a  bandage  be  placed  around  the  arm,  so  as  to  close  the 
superficial  veins,  but  not  tight  enough  to  occlude  the  deeper- 
seated  arteries,  the  veins  on  the  distal  side  of  the  bandage 
will  become  gorged  and  those  on  its  proximal  side  empty, 
showing  again  that  the  veins  only  receive  blood  from  their 
ends  turned  towards  the  capillaries.  (0)  In  the  lower  animals 
direct  observation  with  the  microscope  shows  the  steady  flow 
of  blood  from  the  arteries  through  the  capillaries  to  the  veins; 
but  never  in  the  opposite  direction. 


CHAPTER  XVII. 

THE   NERVES   OF   THE   HEART,    AND   SOME   PHYSIOLOGI- 
CAL PECULIARITIES   OF   CARDIAC   MUSCLE. 

The  Co-ordination  of  Heart  and  Arteries.  We  have 
hitherto  considered  the  working  of  the  vascular  system  as  if 
it  were  a  mere  mechanical  hydraulic  apparatus;  and  such  in 
a  certain  sense  it  is,  and  by  so  regarding  it  many  of  the  phe- 
nomena of  the  blood-flow  can  be  explained.  But  life  is  a 
constant  adjustment  to  constantly  varying  conditions,  and 
the  higher  the  organism  the  more  numerous  the  conditions 
which  influence  it  and  the  greater  its  power  of  adapting  itself 
to  them ;  and  this  adaptability,  this  continuous  self-adjust- 
ment, is  nowhere  better  exhibited  than  in  the  heart  and 
blood-vessels. 

The  object  to  be  attained  is  the  maintenance  of  an  orderly 
current  in  the  capillaries  in  accordance  with  the  needs  of  the 
whole  Body  and  of  each  of  its  organs  at  the  time.  This 
clearly  calls  for  some  means  of  interaction  between  heart  ami 
blood-vessels:  should  the  heart  beat  and  the  arteries  relax  or 
contract,  eacli  without  reference  to  the  other,  no  steady  capil- 
lary flow  could  result.  To  secure  such  a  flow  the  work  done 
by  the  heart  and  the  resistance  offered  in  the  vessels  must 
at  all  times  be  correlated;  so  that  the  heart  shall  not 
by  too  powerful  action  over-distend  or  perhaps  burst  the 
small  arteries,  nor  the  latter  contract  too  much  and  so,  by 
increasing  the  peripheral  resistance,  raise  aortic  pressure  to  a 
greal  heigh  I  and  increase  unduly  the  work  to  be  done  by  the 
left  ventricle  in  forcing  open  the  semilunar  valves. 

Kurt  her.  t  In-  total  amount  of  blood  in  the  Body  is  not  suffi- 
cient to  keep  all  its  organs  simultaneously  supplied  with  the 
amount  needful  for  the  full  exercise  of  their  activity;  in  fact 
the  blood-vessels  of  the  spleen,  liver,  and  alimentary  canal,  if 
all  fully  distended,  can  themselves  contain  almost  the  whole 
blood  of  the  Body,  so  that  by  paralyzing  their  coats  in  an  animal 
it  can   be  caused  to  faint,  or  even  be  killed,  by  what  has  been 

2r>:j 


254  THE  HUMAN  BODY. 

called  an  •'  internal  bleeding/'  due  to  the  accumulation  of  so 
much  blood  in  the  vessels  of  the  abdomen  thai  doI  enough  is 
left  over  Cor  the  supply  of  the  brain  and  other  parts.  In  the 
Body,  accordingly,  we  never  find  all  its  parts  hard  at  work  at 
the  same  moment.  If  when  one  group  of  muscles  was  set  at 
work  and  needed  an  extra  blood-supply,  this  should  be  pro- 
vided merely  by  increasing  the  heart's  activity  and  keeping 
up  a  faster  blood-flow  everywhere  throughoul  the  Body, 
there  would  be  a  clear  waste — much  as  if  the  chandeliers 
in  a  house  were  so  arranged  thai  when  a  larger  flame  was 
wanted  at  one  burner  it  could  only  be  obtained  by  turn- 
in-'  more  gas  on  at  all  the  rest  at  the  same  time;  besides  the 
big  tap  at  the  gas-meter  regulating  the  general  supply  of  the 
house,  local  taps  at  each  burner  are  required  which  regulate 
the  gas-supply  to  each  flame  independently  of  the  others.  A 
corresponding  arrangement  is  found  in  the  Body.  Certain 
nerves  control  the  calibre  of  the  arteries  supplying  different 
organs  and,  when  the  latter  are  set  at  work,  cause  their  arte- 
ries to  dilate  aud  so  increase  the  amount  of  blood  flowing 
through  them,  while  the  general  circulation  elsewhere  re- 
mains practically  unaffected.  The  resting  parts  at  any  mo- 
ment thus  get  enough  blood  to  maintain  their  healthy  nutri- 
tion and  the  working  parts  get  the  larger  quantity  required  to 
make  good  used-up  material  and  to  wash  out  wastes :  as  certain 
organs  come  to  rest  and  others  are  set  in  activity,  the  arteries 
of  the  former  narrow  and  of  the  latter  dilate;  in  this  way  the 
distribution  of  the  blood  in  the  Body  is  undergoing  constant 
changes,  parts  which  at  one  time  contain  much  blood  at  an- 
other having  but  little.  In  addition,  then,  to  nervous  organs 
resulatino;  the  work  of  the  heart  and  the  arteries  with  refer- 
ence  to  one  another,  we  have  to  consider  another  set  of  vascu- 
lar nerves  which  govern  the  local  blood-supply  of  different 
regions  of  the  Body.  How  important  this  is  may  be  illus- 
trated by  considering  what  happens  when  the  surface  of  the 
Body  is  exposed  for  some  time  to  cold.  The  skin  normally 
contains  much  blood,  brought  to  it  in  part  to  be  cooled;  but 
under  the  supposed  conditions  the  loss  of  heat  would  soon  be 
so  great  as  to  be  harmful  did  not  small  arteries  of  the  skin 
contract,  as  is  indicated  by  its  pallor,  and  thus  lessen  the 
blood-flow  through  it.  This  contraction  is  not  chiefly,  if  at 
all,  due  to  direct  action  of  the  cold  on  the  vessels,  but  to  the 
stimulation  of  cutaneous  afferent  nerves  which  excite  a  nerve- 


NERVES  OF  THE  HEART.     CARDIAC  MUSCLE.      255 

centre  from  which  efferent  impulses  are  in  turn  sent  to  the 
muscular  coat  of  the  cutaneous  arteries.  The  blood  driven 
from  the  skin  must  find  a  place  elsewhere  in  the  circulatory 
system,  and  so  internal  organs  teud  to  become  over-full  and 
at  the  same  time  general  arterial  pressure  is  raised.  This, 
again  through  nerves,  acts  upon  the  heart,  and  alters  its  rate 
of  beat  for  a  time.  But  in  health  some  internal  arteries  soon 
dilate  sufficiently  to  compensate  for  the  constriction  of  the 
surface  vessels,  and  arterial  pressure  and  the  pulse  again  be- 
come normal,  though  with  a  less  proportion  of  the  total  blood 
flowing  through  the  skin  than 'before:  this  readjustment  is 
brought  about  entirely  through  nerves  and  nerve-centres 
placing  all  the  arteries  in  connection  with  one  another  and 
with  the  heart,  so  that  they  exert  a  mutual  control.  If  the 
cold  be  not  too  prolonged  its  cessation  is  followed  by  a  return 
of  the  blood-flow  to  its  original  condition,  this  action  being 
brought  about  by  cardiac  and  vascular  nerve  apparatuses.  We 
have  to  mainly  consider  in  this  and  the  succeeding  chapter 
the  nerves  which  regulate  the  heart-beat  and  those  which  in- 
fluence the  calibre  of  arteries;  but  it  is  necessary  first  to  study 
the  muscular  tissue  of  the  heart  more  thoroughly  than  we  have 

hitherto  done. 

Some  Physiological  Peculiarities  of  Cardiac  Muscle. 
We  have  already  seen  that  the  muscular  tissue  of  the  heart, 
though  striped,  differs  considerably  in  structure  from  the 
tissue  of  the  skeletal  muscles:  it  differs  also  somewhat  in 
properties,  and  as  the  latter  differences  can  be  most  readily 
studied  on  the  heart  of  the  frog,  which  will  beat  for  a  long 
time  after  excision,  it  will  be  best  to  commence  with  that. 
The  frog's  heart  consists  of  four  contractile  chambers  through 
which  the  blood  flows  successively,  as  is  indicated  in  the  dia- 
,.  Pig.  99,  in  which  no  attempt  has  been  made  to  indicate 
the  actual  appearance  of  the  organ,  which  is  in  fact  curved 
on  itself  somewhat  in  the  form  of  a  capitals:  (see  Z,  Fig.  1)9), 
and  this  is  also  the  shape  of  the  mammalian  heart  in  an  early 
stage  of  embryonic  development.  The  main  chambers  are 
incompletely  separated  by  constrictions,  at  some  of  which 
valve-  arc   placed,  and   are  in  order — the  venous  sinus.   A, 

eiring  blood  from  the  systemic  veins;  the  atrium,  consist- 
in-  ,,[  two  auricles,  B,C,  of  which  the  right  is  much  the 
larger  and  is  i  applied  from  the  sinus,  while  the  left  gets  blood 
from  the  small  pulmonary  veins,  pv;  the  ventricle,  I),  sup- 


256 


THE  HUMAN  B()l>  ). 


plied  from  both  auricles  and  having  projecting  into  it  the  two 
flaps  of  the  auriculo-ventricnlar  valve,  which  are  continued 

from  the  end  of  the  septum 
or    partition    lying    between 
the  auricles;    the  bulbus  ar 
teriosus,  E,  from  which  the 
A  systemic  and  pulmonary  arte- 

ries arc  supplied.  To  describe 
((Jpy  the  very  interesting  mechan- 
ism by  which  the  arterial  and 
venous  blood  supplied  to  the 
single  ventricle  are  kept  sep- 
arate and  sent  from  the  arte- 
rial bulb  through  different 
channels  would  take  us  be- 
yond the  limits  of  this  book, 
but  it  is  well  worth  study  in 
some  treatise  on  comparative 
physiology. 

The  muscujar  tissue  of  the 
frog's  heart  consists  of  cells 
which  are  in  form  somewhat 
like  those  of  involuntary  mus- 
cle, but  they  are  frequently 
forked  at  their  ends,  and  they 
are  obscurely  cross  -  striped 
like  human  cardiac  muscle 
(Fig.  123).  The  main  thick- 
ness of  the  walls  of  all  the 
chambers  of  the  heart  consists 
of  this  muscle,  and  is  known 
as  the  myocardium.  It  com- 
mences on    the  ends  of   the 

Jig.  99. -Diagram  of  the  frog's  heart,  0.rp«lr  vp;n(5  nPiv  wIipvp  tIipv 
4,  venous  sinus;  .B,  C,  right  and  left  au-  6reax  veins  neai  Wlieie  tney 
ricles,  together  forming  the  atrium;  p.  v.,  -:n;n  riip  l1P.,vr  .irirl  ;<,  flipiipp 
pulmonary  veins;  a,  a,  constriction  be-  J0111  Ule  ne'11 1>  dna  1S  inence 
tween  sinus  and  atrium;  D,  ventricle;  continued  to  the  roots  of  the 
</,  ft,    constriction  between    auricles    and 

ventricles;  i,  auricuio-ventrieuiar  valve;  great  arteries  arising  from  the 

E,   arterial    bulb;    l\    pneumogastric    or  ~  .      .  . 

vagus   nerve;    N,  sympathetic   nerve;    A',    blllb;     DUt  it  IS  thinner   at  the 
cardiac  nerve  containing  fibres  from  both                   .      .  ,  .    .        ..         . 

vagus    and    sympathetic.     Z   shows    the   COUStriCtlOllS      Which      lie      De- 
natural    relative    positions   of  the    chief    .  .-,  •  .,.        ,, 

chambers  of  the  heart:  d,  vena  cava;  c,  tween  the  main    Cavities  than 

venous  sinus:  /, /,  auricles;  h,  ventricle;      i i _,  j      j.i.»__     :„     „„ 

k,  arterial  bulb.  elsewhere,   and    there    is   ar- 

ranged   in  rings   around  the  openings. 


NERVES  OF  THE  HEART.     CARDIAC  MUSCLE.      257 

A  single  nerve,  N,  goes  to  the  heart  from  each  side  (only 
that  of  the  right  side  is  represented  in  the  diagram).  This 
nerve  is  usually  spoken  of  as  the  cardiac  branch  of  the  vagus 
or  pneumogastric,  P,  but  it  is  partly  made  up  of  fibres  from 
the  sympathetic  nerve,  S,  which  join  the  pneumogastric  close 
to  the  skull  and  run  on  with  its  cardiac  branch,  the  two  form- 
ing the  apparently  siugle  nerve-trunk,  N,  which  runs  to  the 
venous  sinus,  breakiug  up  near  it  into  several  twigs.  On  these 
twigs  and  in  the  plexus  which  they  form  in  the  wall  of  the  sinus 
are  numerous  nerve-cells,  forming  the  sinus  ganglion  or 
ganglion  of  RemaTc.  From  the  sinus  nerves  run  down  the 
walls  of  the  auricles  to  the  auriculo-ventricular  groove,  #,  and 
two  comparatively  large  twigs  pass  down  the  auricular  septum 
to  the  region  of  the  valve,  i,  and  there  enter  a  collection  of  nerve 
cells  which,  with  other  cells  lying  in  the  groove,  constitute  the 
auriculo-ventricular  or  Bidder's  ganglion.  From  that  gan- 
glion nerves  are  continued  to  the  wall  of  the  ventricle,  and 
near  its  base  have  nerve-cells  mixed  with  them.  A  few 
nerve-cells  are  also  found  among  the  fibres  running  down  the 
auricular  septum:  in  the  apex  of  the  ventricle,  however,  and 
in  the  bulb  there  are  no  ganglion -cells,  though  nerve-fibres 
are  present.  We  find  then  a  considerable  collection  of  nerve- 
cells  in  the  walls  of  the  venous  sinus,  a  few  cells  in  the  au- 
ricular septum,  a  considerable  collection  at  the  junction  of 
atrium  with  ventricle,  and  a  few  scattered  cells  in  the  neigh- 
boring portions  of  the  ventricle.  The  cells  of  the  ganglion 
of  Eemak  and  some  of  those  in  the  septum  belong  to  a  type 
differing  somewhat  from  those  hitherto  described.  Each  is 
pear-shaped,  and  has  a  conspicuous  nucleus  with  a  nucle- 
olus; from  the  narrow  end  of  the  cell  proceeds  a  branch 
which  ultimately  becomes  the  axis  cylinder  of  a  medullated 
nerve-fibre.  Another  branch  arises  by  two  or  more  roots 
which  coil  spirally  around  the  straight  branch,  and  finally 
unite  and  proceed  as  a  non-medullated  fibre.  Most  of  the 
remaining  nerve-cells  of  the  frog's  heart  are  spindle-shaped, 
and  receive  a  nerve- fibre  at  one  end  and  give  one  off  at  the 
other.  They  are  known  as  bipolar  cells.  The  cardiac  nerve, 
N,  Fig.  99,  contains  both  gray  and  medullated  fibres,  the 
latter  coming  entirely  or  almost  entirely  from  its  vagus  root; 
as  the  fibres  passing  on  from  the  sinus  ganglion  to  the  gan- 
glion of  Bidder  contain  very  few  medullated  fibres,  it  is  prob- 
able   that  many  of  the    vagus  fibres  end  in  the  pear-shaped 


258  THE  HUMAN  BODY. 

cells  from  which  gray  fibres  are  given  off  to  the  rest  of  the 
heart,  mingled  with  the  original  gray  fibres  derived  from  the 
sympathetic:  in  the  ventricle  and  bulb  only  non-medullated 

fibres  are  found. 

The  Beat  of  the  Frog's  Heart.  When  both  cardiac 
nerves  are  cut  in  a  frog  the  heart  continues  its  regular  rhyth- 
mic beat,  as  it  does  also  when  carefully  removed  from  the 
body  of  the  animal:  this  makes  it  clear  that  whatever  initi- 
ates the  beat  lies  in  the  heart  itself,  which  must  therefore  be 
regarded  as  an  automatic  organ;  but  leaves  it  still  uncertain 
whether  the  exciting  cause  of  each  beat  is  to  be  sought  in  the 
nervous  elements  of  the  heart  or  in  the  cardiac  muscle  itself. 
Arguing  from  the  analogy  of  ordinary  striped  muscle,  which 
is  not  automatic,  one  would  be  inclined  to  ascribe  to  the 
nerve-cells  of  the  isolated  heart  the  origination  of  nervous 
impulses  for  the  myocardium,  and  certain  experiments  tend 
to  support  this  view;  but  cardiac  muscle  differs  considerably 
from  the  skeletal  muscles  in  its  histology,  so  it  is  unsafe  to 
argue  from  one  to  the  other,  and  some  experiments  show  that 
we  must  ascribe  to  it,  in  addition  to  contractility,  a  certain 
amount  of  automaticity  and  of  conductivity  and  co-ordinating 
power.  In  physiological  properties  it  combines  the  character- 
istic properties  of  fully  differentiated  nerve-cell  and  nerve- 
fibre  with  those  of  muscle-fibre. 

Each  beat  of  the  heart  of  the  frog  can  be  seen  to  com- 
mence where  the  great  veins  enter  the  venous  sinus,  and  from 
there  to  spread  rapidly  over  the  whole  sinus;  then  there  is  a 
brief  check,  and  the  atrium  beats;  then  another  check,  fol- 
lowed by  the  beat  of  the  ventricle;  finally,  again  after  a  very 
short  pause,  comes  the  contraction  of  the  arterial  bulb:  then 
the  series  of  phenomena  is  repeated  in  the  same  unvarying 
order  as  long  as  the  heart  is  in  good  condition  and  is  left  to 
itself.  The  fact  that  each  cycle  of  contractions  begins  at 
the  mouths  of  the  venm  caves  and  the  sinus,  where  nerve-cells 
are  very  numerous,  and  passes  on  to  the  ventricle,  where  they 
are  few,  and  to  the  bulb,  where  there  are  none,  has  been  taken 
as  an  evidence  of  the  origination  of  each  beat  through  stimuli 
developed  in  cardiac  nerve-cells;  and  this  opinion  gains  sup- 
port from  what  is  usually  seen  on  an  excised  heart  Avhen  it  is 
gradually  dying.  The  bulb  and  ventricle  cease  to  beat  first, 
then  the  auricles,  last  the  sinus,  and  this  although  the  ven- 
tricle may  still  be  contractile  and  able  to  give  a  good  beat  or 


NERVES   OF  THE  HEART.     CARDIAC  MUSCLE.      259 


a  set  of  several  beats  when  directly  stimulated,  as  by  pricking 
or  by  induction  shocks.  The  loss  of  irritability  as  the 
heart  dies  also  usually  appears  in  the  same  order:  when  the 
ventricle  and  auricle  have  both  ceased  to  beat,  it  is  frequently 
possible  to  excite  the  auricle  by  a  direct  stimulation  which  is 
powerless  when  applied  to  the  ventricle:  and  when  the  whole 
heart  has  ceased  to  pulsate  the  venous  sinus  will  sometimes  re- 
spond to  direct  stimulation  when  auricle  and  ventricle  will  not. 
Still  further,  if  the  heart  be  carefully  divided  at  the  level  aa, 
Fig.  99,  so  as  to  separate  the  sinus  from  the 
rest,  the  usual  result  is  that  the  sinus  goes  on 
beating,  but  the  rest  of  the  heart  lies  for  a 
time  at  rest:  soon  it  begins  to  beat  quite 
rhythmically,  but  at  a  slower  rate  than  the 
separated  sinus.  If  the  cross-section  be  made 
at  the  level  gg  so  as  to  separate  the  sinus  and 
auricle  from  the  rest,  they  go  on  beating,  but 
the  ventricle  and  bulb  usually  lie  quiescent 
for  a  considerable  time,  and  then  commence. 
On  account  of  the  anatomical  relations  of  the 
parts  (Z,  Fig.  99)  it  is  not  possible  to  com- 
pletely separate  the  ventricle  from  the  sinus 
without  doing  injury  to  the  former;  but  if 
the  lower  third  of  the  ventricle  (which  con- 
tains no  nerve-cells)  be  cut  off  from  the  rest  PUIPose .f  *rcu,a*- 

'  ing  liquids    through 

of  the  heart  alono-  the  line  oo,  this  separated  j.t:  «M"flow;  &  out- 

°M  '  r  flow,  division  of  can- 

portion  never  begins  to  beat  spontaneously,  »uIa- 

though  the  remainder  of  the  heart  continues  its  pulsations. 
So  far  the  case  for  the  view  that  the  nerve-cells  take  the  in- 
itiative in  the  changes  which  result  in  a  normal  beat,  and 
that  cardiac  muscle  is  not  automatic,  is  a  strong  one;  but 
other  facts  show  that  it  cannot  be  accepted  without  modifi- 
cation. 

Although  the  separated  apex  of  the  ventricle  of  the  frog, 
left  to  itself,  does  not  beat,  yet  it  can  be  made  to  beat  without 
the  application  to  it  of  anything  that  we  are  justified  in  call- 
ing a  stimulus:  it  does  under  certain  conditions  exhibit  auto- 
maticity.  If  it  be  tied  on  the  end  of  a  tube  divided  by  a 
partition  (Fig.  100),  and  some  blood  or  blood-serum  be  circu- 
lated through  it,  in  from  a  and  out  by  />.  under  a  slight  press- 
ure, this  bit  of  ventricle,  devoid  of  nerve-cells,  after  a  time 
begins  to  beat  rhythmically.     It  has  been  suggested  that  in 


Fig.  100.— Diagram 
of  a  perfusion  can- 
nula lied  into  the 
separated  apex  of 
the  ventricle  of  a 
frog's    heart  for  the 


260  THE  HUMAN  BODY. 

this  case  the  distension  of  the  muscle  or  some  chemical  con- 
stituent of  the  liquid  acts  as  a  stimulus;  but  in  no  other 
muscle  do  we  find  blood-supply  or  mere  stretching  act  as  a 
stimulus,  and  if  they  are  to  be  assumed  as  so  acting  in  this 
case  their  action  is  uniform,  while  the  resulting  contractions 
are  interrupted  and  rhythmic:  moreover,  they  are  co-ordi- 
nated; they  are  not  irregular  twitches  first  of  one  bundle  of 
the  myocardiac  fibres  and  then  of  auother,  but  duly  combined, 
so  as  by  their  mutual  action  to  empty  the  cavity  they  surround. 
The  evidence  thus  obtained  as  to  the  possession  of  some  auto- 
matic and  some  co-ordinative  properties  by  the  frog's  cardiac 
muscle  is  strengthened  by  experiments  on  the  hearts  of  tor- 
toises and  terrapins.  In  those  animals  the  apical  portions  of 
the  ventricle  are  devoid  of  nerve-cells,  yet  narrow  strips  of 
them  hung  up  and  slightly  loaded  will  usually  begin  to  beat 
after  a  time.  If  they  do  not,  all  that  is  necessary  is  to  stimu- 
late them  rhythmically  for  a  short  time;  then  on  ceasing  the 
stimulation  the  rhythmic  contractions  continue.  Here,  no 
doubt,  the  loading  is  a  favoring  condition,  but  so  it  is  for  the 
activity  of  ordinary  muscles,  on  which,  nevertheless,  it  does 
not  act  as  a  stimulus. 

The  conclusion  to  which  we  are  led  is  that  the  muscle- 
cells  of  the  frog's  heart  have  retained  to  some  extent  those 
automatic  and  co-ordinating  faculties  of  undifferentiated 
protoplasm  which  the  more  highly  evolved  fibre  of  skeletal 
muscle  has  lost.  AVe  find  in  the  presence  of  certain  of  the 
nerve-cells  of  the  heart  a  highly  favorable  condition  for  the 
exhibition  of  those  powers:  the  nerve-elements  perhaps  influ- 
ence the  nutrition,  perhaps  in  some  other  mode  affect  the 
molecular  structure  of  the  muscle-cells  connected  with  them 
so  as  to  favor  spontaneous  contraction,  but,  like  stretching 
the  isolated  strip  of  ventricle,  they  merely  bring  about  a  state 
of  things  promoting  the  exercise  of  powers  inherent  in  the 
cardiac  muscle  tissue  itself. 

The  evidence  as  to  the  automaticity  of  the  muscle  of  the 
mammalian  heart  is  not  quite  as  full  as  in  the  case  of  the  frog. 
In  it  also  there  are  collections  of  ganglion-cells  where  the 
great  veins  join  the  auricles  and  near  the  base  of  the  ventri- 
cles; but  there  are  others  in  the  apical  region  of  the  ventricles, 
so  it  is  not  possible  to  examine  an  isolated  apex  free  from 
ganglion-cells  as  it  is  in  the  frog.  The  musculature  of  the 
auricles  is  prolonged  for  some  little  way  on  the  ends  of  the 


NERVES  OF  THE  HEART.     CARDIAC  MUSCLE.      261 

vence  cava  and  the  pulmonary  veins,  and  there  each  normal 
beat  commences,  the  contraction  spreading  rapidly  over  the 
whole  auricle  and  thence  to  the  ventricle  without  the  brief  in- 
termediate pause  observable  in  the  frog.  In  the  mammal,  also, 
the  ventricles  if  supplied  with  blood  from  the  auricles  go  on 
beating  although  all  nerve  and  muscular  continuity  between 
auricle  and  ventricle  has  been  destroyed,  by  passing  rigid  tubes 
through  the  auriculo-ventricular  openings  and  then  tying  a 
ligature  tight  on  the  outside  of  the  heart  along  the  auriculo- 
ventricular  groove,  so  as  to  crush  the  tissues  between  the 
string  and  the  tubes.  If  the  ligatures  be  so  placed  as  not  to 
impede  the  flow  in  the  coronary  vessels  the  ventricles  beat 
long  and  powerfully,  but  with  a  rhythm  independent  of  that 
of  the  auricles  and  usually  slower.  Also  when  the  mammalian 
heart  is  dying  slowly,  as  in  a  suffocated  animal,  the  auricles 
usually  continue  to  beat  after  the  ventricle  has  ceased,  the 
small  dog's-ear-shaped  projection  of  the  auricles  (which  it 
may  be  noted  has  given  its  name  to  the  whole  auricle)  usually 
being  the  last  portion  to  come  to  rest,  especially  that  on  the 
right  side,  which  was  accordingly  named  ultima  moriens  by 
the  old  physiologists.  On  the  whole  we  are  perhaps  justified 
in  assuming  that  the  myocardium  of  the  mammal  is  automatic, 
like  that  of  the  frog,  and  that  in  it  also  the  presence  and 
influence  of  ganglion-cells  favor  the  production  of  a  beat, 
but  do  not  initiate  it. 

The  muscle  of  the  frog's  heart  is,  we  have  seen,  co-ordi- 
native:  the  isolated  ventricular  apex  can  perform  a  regular 
beat.  It  is  probable  that  this  is  not  the  case  in  the  mammal. 
When  a  dog's  heart  is  injured  the  ventricles  sometimes  cease 
to  give  true  beats  though  the  muscle  bundles  constituting 
them  go  on  contracting,  but  it  is  with  no  combined  action 
such  as  would  empty  the  ventricle.  Irregular  and  useless 
contractions  travel  simultaneously  over  the  myocardium  in 
various  directions,  so  that  the  whole  mass  seems  trembling. 
Such  a  state  (known  as  "fibrillar  contraction")  is  especially 
apt  to  follow  wounds  in  the  region  of  the  main  nerve-trunks 
running  dowD  the  ventricles  alongside  the  larger  branches  of 
the  coronary  arteries,  and  is  probably  due  to  the  injury  of  some 
nervous  apparatus  concerned  in  securing  the  proper  co-ordi- 
nated contractions  of  the  normal  beat.  In  many  other 
regions  wounds  may  be  inflicted  on  the  ventricle  with  con- 
siderable impunity. 


262  nil-;  ii r max  BODY. 

The  Heart-beat  is  not  a  Tetanic  Contraction.  We  have 
seen  thai  it  is  possible  by  rapidly  succeeding  stimuli  to  throw 
the  skeletal  muscles  into  a  prolonged  and  apparently  contin- 
uous contraction,  and  that  there  is  good  reason,  afforded  by 
the  phenomena  of  "  secondary  tetanus,"  for  the  belief  that 
all  normal  contractions  of  the  voluntary  muscles  are  compound 
or  tetanic  contractions.  This  is  not  the  case  with  the  heart. 
It  is  possible  by  repeated  stimuli  to  hurry  the  beat  of  a  frog's 
heart,  bul  not  to  fuse  two  or  more  beats  into  a  Bingle  longer 
uninterrupted  contraction.  And  as  regards  the  normal  beal 
of  the  heart,  experiments  as  to  secondary  tetanus  prove  the 
same  thing.  If  the  heart  of  an  anaesthetized  dog  or  other 
mammal  be  carefully  laid  bare  and  the  nerve  of  a  nerve- 
muscle  preparation  be  laid  on  it,  we  get  for  each  beat  a  single 
twitch  of  the  signal  muscle,  and  not  a  short  tetanus  lasting 
as  lorn:  as  the  ventricular  contraction,  such  as  must  arise 
were  this  contraction  tetanic. 

The  Ventricular  Contraction  is  always  Maximal.  It 
has  been  pointed  out  with  reference  to  the  skeletal  muscles 
that  within  limits  the  extent  of  a  contraction  varies  with  the 
stimulus  used:  a  feeble  stimulus  giving  a  small  contraction, 
a  stronger  a  greater.  This  is  not  the  case  with  cardiac  mus- 
cle. A  quiescent  ventricle  or  strip  of  ventricle  taken  from 
the  heart  of  a  frog  or  turtle  can  often  be  made  to  contract  by 
stimulation;  but  provided  the  stimulus  is  powerful  enough 
to  cause  a  beat  at  all,  it  always  causes  the  fullest  contraction 
the  piece  of  heart  is  capable  of  at  the  time.  Increase  of 
stimulus  causes  no  increase  of  contraction.  There  is  good 
reason  to  believe  that  in  the  physiological  working  of  the 
ventricles  of  the  mammalian  heart  each  completely  expels 
during  its  contraction  all  the  blood  contained  in  it:  the 
papillary  muscles  pulling  dowu  the  flaps  of  the  auriculo- 
ventricular  valves  so  that  they  finally  form  a  cone  on  which 
the  rest  of  the  ventricular  boundaries  can  fit  closely  so  as  to 
obliterate  the  cavity  they  enclose.  This  being  so,  the  quantity 
of  blood  driven  into  the  arteries  by  each  contraction  of  the 
ventricles  depends  on  the  amount 'in  the  latter  when  their 
beat  commences.  This  amount  depends  partly  upon  the 
quantity  of  blood  returned  from  the  great  veins  during  the 
preceding  diastole  and  partly  upon  the  force  with  which 
the  auricles  contract,  for  they,  although  each  contraction  is 
probably  maximal  for  their  condition  at  the  time  being,  do 


NERVES  OF  THE  HEART.     CARDIAC  MUSCLE.      263 

not  completely  empty  themselves  at  each  stroke;  they  some- 
times do  so  more  completely  arid  sometimes  less.  In  this 
manner  the  auricles  can  to  a  great  extent  control  the  work 
done  by  the  ventricles,  through  influencing  the  amount  of 
blood  in  the  latter  at  the  commencement  of  the  ventricular 
.systole:  more  complete  relaxation  of  the  auricles  during 
diastole  promotes  inflow  from  the  great  veins,  more  extensive 
contraction  during  auricular  systole  more  completely  fills  the 
ventricles.  As  Ave  shall  see,  the  force  and  rate  of  the  auricular 
beat  is  much  more  under  the  control  of  nerves  reaching  the 
heart  from  other  parts  than  is  that  of  the  ventricles.  The 
auricles  are  a  feed-pump  adjusting  their  work,  and  through 
it  the  work  of  the  whole  heart,  to  the  general  condition  of 
the  Body;  the  ventricles  are  a  grosser  force-pump  driving  on 
whatever  blood  is  supplied  to  them,  be  it  much  or  be  it  little. 
The  Extrinsic  Nerves  of  the  Mammalian  Heart.  As  in 
the  frog,  these  come  from  two  sources,  at  least  so  far  as  indi- 
cated by  gross  anatomy.  Their  exact  anatomical  arrangement 
differs  in  various  mammals,  as  the  rabbit,  dog,  and  man,  and 
even  somewhat  in  different  individuals  of  these  species,  but  in 
the  main  is  the  same.  The  pneumogastric  gives  off  from  its 
main  stem  in  the  neck  several  cardiac  branches;  so  do  the 
lower  cervical  and  the  upper  thoracic  ganglia  of  the  sympa- 
thetic chain.  Both  sets  intermingle,  and  near  the  heart  end 
in  plexuses  containing  nerve-cells;  from  these  plexuses  nerves 
are  distributed  to  that  organ.  In  the  heart  itself,  as  already 
stated,  are  collections  of  ganglion-cells  in  the  auricles  near  the 
ends  of  the  great  veins,  near  the  base  of  the  ventricles,  and  a 
few  cells  scattered  over  the  ventricles  even  in  their  apical  re- 
gion-. The  nerve-fibres  coming  through  the  pneumogastrics 
are  medullated  and  consist  of  a  set  of  small  fibres  and  a  group 
of  large:  the  smaller  lose  their  medulla  in  ganglion-cells  in  or 
near  the  heart;  the  larger  retain  the  medullary  sheath,  and  may 
be  traced  even  over  the  ventricles,  which  in  this  respect  differ 
from  that  of  the  frog;  the  fibres  supplied  from  the  sympathetic 
are  non-medullated.  Broadly  speaking,  the  nerve-fibres  fall 
into  three  physiological  sets  corresponding  to  the  three 
anatomical  varieties:  the  small  medullated  fibres  are  effer- 
ent and  inhibitory — when  excited  they  slow  the  heart-beat; 
the  large  medullated  are  in  part  at  least  afferent,  conveying 
to  the  central  nervous  system  impulses  which  originate  in  the 
heart:   the  sympathetic   fibres  are  efferenl  and  excitor,  and 


264  THE  HUMAN  BODY. 

when  stimulated  quicken  or  strengthen  the  heart  beat.  The 
afferent  fibres  will  be  more  conveniently  studied  in  connection 
with  nerves  of  the  blood-vessels  (Chap.  XY1II). 

The  Cardio-inhibitory  Fibres.  These,  though  running 
in  the  neck  in  what  seems  to  be  the  main  pueumogastric 
trunk,  do  not  leave  the  skull  in  that  nerve,  but  in  the  spinal 
accessory  (XI  cranial  nerve),  which,  it  will  be  remembered, 
arises  in  part  from  the  brain  and  in  part  from  the  upper  por- 
tion of  the  spinal  cord.  That  nerve  gives  off  near  the  brain 
a  small  branch  which  joins  the  pueumogastric  and  runs  on  in 
it  to  near  the  heart.  The  fibres  may  be  tracked  in  the  pueu- 
mogastric by  their  small  size,  but  more  satisfactorily  by  the 
Wallerian  method.  It  is  then  found  —  1,  when  the  main 
pueumogastric  trunk  is  divided  in  the  neck  all  the  medullated 
fibres  in  it  distal  to  the  place  of  section  degenerate;  2,  if  only 
the  branch  joining  the  spinal  accessory  to  the  pneumogastric 
be  cut,  then  only  some  fibres  in  the  pueumogastric  stem  do- 
generate,  and  these  fibres  are  the  small  medullated  set;  3,  if 
the  pneumogastric  alone  be  divided  above  the  point  where  the 
branch  from  the  spinal  accessory  joins  it,  then  the  large 
medullated  fibres  of  the  cardiac  branches  of  the  vagus  degen- 
erate, but  the  small  do  not.  Hence  we  conclude  that  the 
small  fibres  come  through  the  accessory.  Physiological  ex- 
periment confirms  this.  Immediately  after  cutting  the  main 
pneumogastric  trunk  stimulation  of  its  peripheral  end  checks 
the  beat  of  the  heart;  but  if  the  stimulation  be  applied 
after  several  days,  it  has  no  effect  on  the  heart.  If  instead 
of  cutting  the  whole  pneumogastric  stem  we  divide  only 
the  branch  going  to  it  from  the  accessory,  we  find  similar 
results:  after  two  or  three  days  (i.e.,  when  the  microscope 
reveals  degeneration  of  the  small  medullated  fibres  in  the 
main  stem,  all  the  rest  being  in  their  normal  condition)  stim- 
ulation of  it  is  as  absolutely  without  direct  effect  on  the  heart 
as  after  complete  degeneration  of  the  whole  nerve-trunk.  In 
the  frog  there  is  no  separate  spinal  accessory  nerve;  the  cardio- 
inhibitory  fibres  pass  from  the  brain  directly  into  the  pneumo- 
gastric; but  in  both  frog  and  mammal  their  centre  lies  in  a 
group  of  nerve-cells  of  the  medulla  oblongata  known  as  the 
cardio-inhibitory  centre 

The  cardiac  nerve  of  the  frog  consists  (Fig.  99)  of  a  pneu- 
mogastric and  a  sympathetic  portion:  if  it  be  stimulated  the 
usual  result  is  that  the  heart  is  slowed  when  the  stimulus  is 


NERVES  OF  THE  HEART.     CARDIAC  MUSCLE.     265 

feeble,  and  is  stopped  when  the  stimulus  is  more  powerful; 
and  in  this  animal  it  is  possible  by  carefully  applied  stimula- 
tion to  keep  the  heart  at  rest  for  a  considerable  time,  during 
which  it  lies  distended  and  flabby;  but  nearly  always  it  ulti- 
mately recommences  its  beat  even  though  the  stimulation  of 
the  nerve  be  continued.  During  its  inhibition  the  heart  is 
irritable  and  contractile,  for  it  beats  if  a  direct  stimulus  be 
applied  to  it:  the  myocardium  is  therefore  not  incapable  of 
action;  but  either  some  influence  normally  proceeding  from 
its  nerve-cells  and  promoting  its  automatic  contraction  is  pre- 
vented, or  the  stimulation  directly  acts  on  the  cardiac  muscle 
and  for  the  time  lowers  or  removes  its  spontaneity.  If  the 
stimulus  applied  to  the  cardiac  nerve  be  not  strong  enough  to 
completely  inhibit  the  heart,  it  is  usually  seen  that  the  pulsa- 
tions are  not  only  fewer,  but  more  feeble;  but  this  is  not  always 
the  case  :  the  beats  may  be  slower  and  not  less  powerful  than 
before,  or  they  may  continue  with  the  same  rhythm,  but  be 
less  powerful;  in  any  case  the  result  is  to  diminish  for  the 
time  the  work  done  by  the  heart. 

In  mammalia  the  phenomena  are  essentially  the  same.  If 
artificial  respiration  be  maintained  in  an  anaesthetized  rabbit 
and  its  heart  laid  bare,  and  then  the  pneumogastric  trunk  be 
divided  on  one  side  of  the  neck  and  its  cardiac  end  stimu- 
lated, the  heart  conies  to  rest,  distended  and  soft  to  the  touch; 
or,  with  more  feeble  stimulation,  the  pulsations  are  slowed ; 
or  they  may  be  both  slower  and  feebler,  or  feebler  and  not 
slower;  but  the  amount  of  blood  driven  out  by  the  ventricles 
in  a  given  time  is  usually  much  less.  When  the  beat  is  only 
weakened  it  often  happens  that  the  effect  shows  itself  much 
more  markedly  on  the  auricles  than  on  the  ventricles,  though 
this  of  course  diminishes  the  work  done  by  the  ventricles,  as 
they  are  then  supplied  with  less  blood  to  pump  on;  and  occa- 
sionally it  may  be  seen  that  the  auricles  miss  a  beat,  giving  only 
one  for  each  two  of  the  ventricles,  quite  contrary  to  the  case 
of  a  dying  heart,  in  which,  as  we  have  seen,  the  auricular  beat 
U  more  prominent.  This  illustrates  the  fact  that  the  auricles 
are  more  sensitive  to  external  nervous  control  than  the  ven- 
tricles,  and  provide,  so  to  speak,  the  "  fine  adjustment"  of  the 
cardiac  apparatus. 

Whether  the  heart  is  stopped  or  slowed  or  its  beats  weak- 
ened, the  result  must  be  a  fall  in  arterial  pressure,  for  the 
stretched  arteries  go  on  driving  blood  through  the  capillaries 


266 


THE  HUMAN  BODY. 


to  the  veins,  while  their  supply  from  the  heart  is  cut  off  or 
lessened.  Hence  a  pressure-gauge  attached  to  an  artery 
shows  readily  the  influence  of  stimulation  of  the  cardio-in- 
hibitory  fibres;  and  in  order  to  avoid  the  serious  operation  of 
opening  the  thorax  to  observe  the  heart  directly,  it  is  usual 
to  study  indirectly  the  cardiac  effect  of  stimulation  of  the 
pnemogastric  by  observing  its  influence  on  arterial  pressure. 


Fig.  101.— Manometer  for  recording  variations  in  arterial  pressure,  ddggg,  trlas- 
U-tnbe  partly  filleii  with  mercury,  o:  its  limb.  gg,  is  open  to  the  air,  and  afloat  bear- 
ing the  light  stem  e  on  which  is  the  pen  /  rests  on  the  mercury:  the  limb  dd  i> 
filled  above  the  mercury  with  magnesium  sulphate  solution  and  connected  water- 
tight by  tubes  and  the  cannula  <i  with  the  heart  end  of  a  divided  artery.  The  pen 
writes  on  a  horizontally  travelling  surface  and  rises  and  falls  with  the  mercury  on 
the  side  gg,  a  rise  indicating  increase  of  arterial  pressure,  a  fall  the  reverse:  the 
pressure  in  the  artery  at  any  moment  is  indicated  by  the  vertical  distance  between 
the  tup  of  the  mercurj'  in  dd  and  that  in  gg.  due  allowance  being  made  for  the 
weight  of  the  magnesium  sulphate  and  some  other  possible  sources  of  error. 

For  this  purpose  a  small  glass  tube  ox  cannula,  a,  filled  with 
solution  of  magnesium  sulphate  (to  check  blood-clotting)  is  in- 
troduced into  the  cardiac  end  of  a  divided  artery,  say  the  fem- 
oral, of  a  living  animal,  the  artery  being  clamped  at  a  place 
nearer  the  heart  than  the  point  where  the  cannula  is  tied  on. 


NERVES  OF  THE  HEART.     CARDIAC  MUSCLE.      2Q7 

The  cannula  is  (Fig.  101)  connected  by  an  inelastic  tube,  c,  of 
convenient  length,  also  filled  with  magnesium  sulphate,  to 
one  end  of  a  U-shaped  glass  pressure-gauge  or  manometer, 
ddgg,  containing  mercury.  On  the  top  of  the  mercury  in  the 
limb  gg  of  the  manometer  floats  a  light  stem  e  carrying  a  pen 
which  writes  on  a  travelling  surface.  Above  the  mercury,  o, 
on  the  side  dd,  the  tube  is  filled  with  magnesium  sulphate 
solution.  When  the  pressure  on  each  side  of  the  manometer 
is  alike  the  mercury  stands  at  the  same  level  in  both  limbs, 
but  when  it  is  increased  on  the  side  dd  by  taking  the  clamp 
off  the  artery  and  throwing  in  the  pressure  of  the  blood  the 
mercury  in  gg  rises,  carrying  the  float  and  pen  with  it  and 
draws  a  line  such  as  that  at  yz,  Fig.  102,  on  the  travelling 


Fig.  102.— Tracine  of  arterial  pressure  during  vagus  inhibition  of  the  heart.  To 
be  read  from  right  to  left:  yzyq,  blood  pressure-line  traced  by  the  manometer  pen; 
o  indicates  i»n  the  tracing  the  instant  at  which  the  nerve  was  stimulated:  p,  the 
instant  at  which  the  stimulation  ended;  ae,  line  traced  by  a  pen  marking  half 
Beconds;  xg,  line  of  no  pressure,  that  is.  level  at  which  the  pen  would  write  were 
there  no  arterial  pressure;  the  distance  between  it  and  the  part  of  the  manometer 
line  directly  above  it  multiplied  by  two  gives  the  actual  pressure  in  mercury  in  the 
artery  at  that  moment.  The  small  variations  of  pressure  seen  on  the  curve  are 
due  to  beats  of  the  heart;  they  are  absent  during  the  inhibition  and  slow  for  a 
short  time  after  it. 


surface,  the  small  curves  (pulse-waves)  on  which  correspond  to 
the  slighl  increases  of  arterial  pressure  following  each  contrac- 
tion of  the  left  ventricle.  The  number  of  these  small  curves  in 
a  given  time  gives  us  therefore  the  pulse-rate.  The  pneumo- 
gastric  ie  meanwhile  exposed  in  the  neck  and  cut  across:  the 
objeel  of  dividing  it  ie  to  prevent  stimuli  travelling  to  the 
brain  by  the  afferent  fibres  in  it,  as  they  would  act  on  the  nerve- 
centres  and  lead  to  complicated  results.  The  peripheral  end 
of  the  cut  nerve  is  then  si  i  inula  ted,  the  excitation  commencing 
at .  say,  the  instant  corresponding  to  the  point  o  on  the  tracing. 


268  THE  II UMAX  BODY. 

It  is  seen  that  the  heart  does  not  stop  at  once  but  gives  a  beat 
or  two  and  then  stops  as  indicated  by  the  sudden  fall  <d'  arte- 
rial pressure  and  the  absence  of  all  pulse-waves  from  the 
tracing.  If  the  stimulation  be  stopped  at  the  instant,  indi- 
cated by  p,  the  heart  dues  not  begin  immediately  to  beat,  but 
when  it  does,  the  beats  are  powerful  and  soon  bring  the  arte- 
rial pressure  back  to  its  former  level,  or  in  many  cases  to  a 
point  above  it  for  some  Lime  before  the  previous  pressure  and 
pulse-rate  are  regained.  Such  a  tracing  shows  among  other 
things  that  a  certain  "latent  period"  elapses  before  the 
stimulation  of  the  inhibitory  fibres  influences  the  heart- 
beat, and  that  the  influence  of  the  stimulus  once  established 
continues  a  short  time  after  the  stimulation  is  stopped;  and 
that  the  first  beats  after  cessation  of  the  inhibition  are  slow 
and  powerful.  Of  course  without  any  manometer  one  can 
detect  the  effect  of  cardio-inhibitory  stimulus  by  a  finger 
placed  over  the  pulse  of  an  animal  or  by  listening  to  the 
heart-sounds,  but  the  graphic  method  above  described  allows 
of  much  more  accurate  study. 

It  has  been  stated  in  a  previous  paragraph  that  stimulation 
of  the  cardiac  nerve  usually  stops  or  slows  the  heart-beat  of  a 
frog.  The  reason  for  the  qualifying  term  is  that  sometimes 
the  stimulation  quickens  the  beat.  This  is  due  to  the  fact 
that  the  nerve  (see  Fig.  99)  is  a  mixed  one  and  that  the 
fibres  it  receives  from  the  sympathetic  are  directly  antago- 
nistic in  action  to  those  derived  from  the  vagus.  In  most 
cases  when  the  whole  trunk  is  stimulated  the  vagus  fibres  get 
the  upper  hand,  but  to  be  sure  of  pure  cardio-inhibitory  results 
the  vagus  must  be  stimulated  before  the  sympathetic  branch 
joins  it.  Then  the  action  is  always  inhibitory;  and  certain 
other  important  phenomena  may  be  observed,  showing  that 
the  vagus  contains  fibres  which  tend  to  throw  the  heart  into  a 
better  working  state.  When  an  exposed  frog's  heart  is  dying 
and  has  ceased  to  beat,  or  when  the  ventricle  has  come  to  rest 
though  the  sinus  and  auricles  still  work,  it  not  unfrequently 
happens  that  a  period  of  vagus  stimulation  is  followed  by  a 
set  of  beats:  or  similarly  that  when  the  whole  heart  is  beating 
feebly  stimulation  of  the  vagus  is  after  a  time  followed 
by  more  forcible  contractions.  Hence  it  has  been  suggested 
that  the  nerve  contains  fibres  which  tend  to  promote  the 
nutrition  of  the  cardiac  muscle,  fibres  which  are  anabolic 
and  favor  constructive  chemical  processes.  Whether  these  fibres 


NERVES  OF  THE  HEART.     CARDIAC  MUSCLE.      269 

are  the  same  as  the  cardio-inhibitory  or  are  a  distinct  set  is 
still  uncertain.  In  mammals,  also,  it  is  frequently  noticeable 
that  vagus  inhibition  of  the  heart  is  followed  by  a  period  of 
unusually  powerful  pulsation. 

The  Cardio-inhibitory  Centre.  This  consists  of  nerve- 
cells  lying  in  the  medulla  oblongata  and  giving  origin  to  the 
cardio-inhibitory  fibres.  In  some  animals  it  seems  to  be  nor- 
mally always  in  a  state  of  slight  activity,  sending  out  feeble 
impulses  which  exert  a  slight  check  on  the  rate  of  pulse. 
This  is  the  case  in  the  dog,  for  in  that  animal  division  of 
both  pneumogastric  nerves  in  the  neck  is  followed  by  a 
quicker  heart-beat:  in  the  rabbit,  on  the  other  hand,  the 
centre  appears  usually  at  rest,  as  section  of  the  pneumogas- 
trics  in  that  animal  has  no  effect  on  the  pulse-rate.  Whether 
normally  in  action  or  not  the  centre  can  readily  be  excited, 
especially  by  afferent  impulses  reaching  it  through  abdominal 
nerves.  If  the  intestines  of  a  frog  (the  brain  of  which  in 
front  of  the  medulla  oblongata  has  been  entirely  removed  so 
as  to  make  consciousness  impossible)  be  exposed  and  sharply 
struck,  the  heart  stops  in  diastole;  but  if  both  cardiac  nerves 
have  been  previously  divided  this  result  does  not  follow. 
The  stoppage  is  clearly  then  a  reflex  inhibition  through  the 
cardio-inhibitory  centre  and  nerves,  and  the  afferent  tract  can 
be  readily  traced.  The  afferent  impulses  from  the  intestine 
pass  through  the  mesenteric  branches  of  the  sympathetic,  for 
if  these  be  cut  no  cardiac  standstill  follows  the  mechanical 
stimulation  of  the  intestine,  although  the  vagi  be  intact.  If 
only  the  communicating  branches  from  the  sympathetic  gan- 
glia to  the  spinal  cord  be  cut  or  only  the  anterior  roots  of  the 
corresponding  spinal  nerves,  or  only  the  spinal  cord  above  the 
place  of  entry  of  these  roots,  or  only  the  medulla  oblongata 
destroyed,  yet,  in  each  case,  the  intestinal  stimulation  causes 
do  stoppage  of  the  heart.  When  the  standstill  does  result  it 
is  therefore  reflex,  the  afferent  path  being — sensory  nerve-end- 
ings in  intestine,  mesenteric  nerves,  sympathetic  ganglion, 
communicating  branches,  anterior  spinal  roots,  spinal  cord 
to  centre  in  medulla;  the  efferent  fibres  are  the  inhibitory 
in  the  vagus.  The  fainting  which  in  man  not  infrequently 
follows  a  severe  blow  on  the  pit  of  the  stomach  is  due  to 
similar  reflex  excitation  of  the  cardio-inhibitory  centre:  and 
the  fainting  seer  during  severe  pain  and  that  which  certain 
odors  cause  in  some  persons  are  due  to  similar  stimulation  of 


270  THE  II UMAX   BODY. 

the  cardio-inhibitory  centre  through  sensory  nerves,  and  Berve 
to  illustrate  the  many  afferent  fibres  from  different  regions  of 
the  Body  which  can  influence  the  heart-beat. 

The    cardio-inhibitory   centre    may   also    be    stimulated 

directly  (as  by  piercing  it  with  a  needle)  and  stop  the  heart. 
But  a  more  interesting  instance  is  its  excitation  by  high 
arterial  pressure.  Nearly  always  a  very  high  pressure  in  the 
aorta  is  accompanied  by  a  slow  pulse  due  to  cardio-inhibitory 
nerve-impulses,  for  if  the  vagi  be  cut  under  .such  circum- 
stances the  heart-rate  immediately  increases.  The  Blower 
heat,  of  course,  by  lessening  the  work  of  the  heart  tends  to 
bring  back  the  high  arterial  pressure  to  a  more  normal  level, 
providing  an  adjustment  of  the  heart's  work  to  the  condition 
of  the  arterial  system  at  the  time.  The  brain,  enclosed  in 
the  rigid  skull-cavity,  is  especially  likely  to  be  affected  by 
increased  arterial  tension,  for  distension  of  the  intra-cranial 
arteries  must  bring  about  greater  pressure  on  all  the  other 
contents  of  the  skull;  and  the  cardio-inhibitory  centre  is  very 
sensitive  to  increased  pressure.  If  a  small  hole  be  bored 
through  the  skull  of  a  dog  and  a  little  innocuous  fluid  in- 
jected so  as  to  cause  pressure  on  the  brain,  the  beat  of  the 
heart  is  promptly  slowed  and  weakened,  but  if  the  pneumo- 
gastrics  have  been  previously  cut  the  heart-beat  is  not 
influenced.  In  man  similar  stimulation  of  the  cardio-inhibi- 
tory centre  is  shown  in  apoplexy,  which  is  due  to  the  bursting 
of  some  vessel  inside  the  skull  and  the  effusion  of  blood,  which 
by  pressure  on  the  brain  causes  the  unconsciousness  and  pa- 
ralysis which  characterize  the  stroke.  During  such  a  fit  the 
pulse  is  almost  invariably  very  slow  from  the  action  of  the 
increased  pressure  on  the  cardio-inhibitory  cells.  This  is 
clearly  a  preservative  action,  for  the  resulting  lower  arterial 
pressure  makes  the  haemorrhage  less,  and  more  likely  to  come 
to  an  end.  Among  conditions  of  the  blood  which  stimulate 
the  cardio-inhibitory  apparatus  may  be  mentioned  deficient 
oxygenation,  which  will  be  referred  to  again  when  the  phe- 
nomena of  suffocation  are  described. 

The  Cardio-accelerator  or  Augmentor  Nerves.  The 
influence  of  these  on  the  heart  is  to  quicken  or  strengthen 
its  beat  or  both:  but  only  for  a  time,  their  final  action  being 
to  hasten  exhaustion;  they  are  essentially  katabolic  in  their 
influence  on  the  nutrition  of  the  organ. 

Both  in  frog  and   mammal  they  pass  to  the  heart  from 


NERVES   OF  THE  HEART.     CARDIAC  MUSCLE.      271 

the  sympathetic,  taking  somewhat  different  paths  in  different 
animals.  In  the  frog  their  course  is  shown  in  Fig.  99;  in 
mammals  most  of  them  come  from  the  upper  thoracic  gan- 
glion of  the  sympathetic  and  the  neighboring  parts  of  the 
main  sympathetic  chain.  If  the  heart  of  a  frog  be  exposed 
and  watched  while  the  branch  s,  Fig.  99,  is  stimulated  its 
beat  is  seen  to  be  quickened,  especially  if  the  previous  rate 
were  slow:  and  quite  similar  phenomena  may  be  observed 
when  the  corresponding  nerves  are  stimulated  in  a  rabbit  or 
dog.  And  the  beat  is  not  merely  made  more  rapid:  it  is  dis- 
tinctly more  powerful  for  the  time,  the  heart  driving  out  znore 
blood  at  each  stroke  (even  though  pressure  in  the  aorta  may 
be  high)  and  thus  doing  increased  work. 

Though  the  augmentor  fibres  reach  the  heart  through  the 
sympathetic  they  have  their  centre  (car ■dio -accelerator  centre) 
in  the  medulla  oblongata,  from  which  in  mammalia  they  pass 
down  the  spinal  cord  to  the  anterior  roots  of  the  upper  tho- 
racic spinal  nerves,  to  the  communicating  branches,  to  the 
sympathetic  ganglia,  and  thence  to  the  cardiac  plexus  and 
the  heart.  Their  centre,  like  the  inhibitory,  may  be  reflexly 
excited  :  powerful  stimulation  of  a  sensory  nerve,  after  section 
of  the  vagi,  usually  quickens  the  pulse  if  the  accelerator  fibres 
passing  from  the  thoracic  ganglia  be  intact,  but  has  no  effect 
if  these  be  previously  divided.  If  the  vagi  are  not  cut  the 
result  is  not  so  certain,  as  the  afferent  impulses  may  also 
excite  the  cardio-iiihibitory  centre  and  cause  a  mixed  action: 
but  speaking  generally  afferent  impulses  which  in  a  conscious 
animal  would  cause  acute  but  not  extreme  pain  cause  increase 
of  the  heart-beat.  This  by  raising  general  arterial  tension 
would  for  the  time  put  the  animal  in  good  condition  to  make 
a  vigorous  effort,  and  so  is  obviously  an  unconscious  adaptation 
of  the  organism  for  the  preservation  of  its  safety.  While  ex- 
treme pain  or  extensive  injury  involving  many  afferent  nerves 
tends  to  cause  fainting  and  loss  of  consciousness,  the  cardio- 
inhibitory  centre  getting  the  upper  hand. 

The  Influence  of  Temperature  Changes  and  of  Calcium 
Salts  on  the  Heart-beat.  If  the  excised  heart  of  a  frog  be 
cooled  it  beats  more  slowly;  if  heated,  more  quickly;  until 
the  temperature  approaches  tint  limit  ;it  which  muscle  passes 
into  rigor.  The  observation  is  more  difficult  with  mammals, 
hut  if  the  heart  of  •■>  dog  be  completely  separated  from  all 
the  rest  of  the  body  except  the  lungs  and  supplied  with  blood 


kiT-2  THE  HUMAN  BODY. 

it  is  possible  to  keep  it  alive  for  some  hours,  beating  regularly 
and  powerfully,  and  on  such  a  heart  it  is  easy  to  observe  that 
cooler  blood  causes  slower  heat  and  vice  versa.  While  the 
quick  pulse  observed  in  fevers  may  therefore  be  in  part  due 
to  paralysis  of  the  cardio-inhibitory  centre  or  stimulation 
of  the  cardio-accelerator,  it  is  in  part  at  least  due  solely  to 
the  hotter  blood  circulating  througb  the  coronary  vessels. 
Whether  the  higher  temperature  in  this  ease  acts  primarily 
on  the  nerve-cells  of  the  heart  or  on  the  muscle  is  not  known. 
If  circulation  be  kepi  up  through  a  frog's  heart  by  the 
perfusion  method  (Fig.  LOO), the  organ  may  be  kept  beating  for 
a  very  long  time  if  the  liquid  supplied  be  blood  or  serum.  If 
only  dilute  solution  (0.75$)  of  sodium  chloride  be  given,  the 
heat  continues  for  some  time,  but  not  so  long  as  if  no  liquid 
be  circulated;  the  salt  apparently  washes  out  something 
which  the  heart  needs.  The  beat  of  such  a  "  washed-out" 
heart  may  he  restored  by  substituting  milk  or  serum  or  de- 
fibrinated  blood  for  the  saline  solution,  or  even  by  adding  to 
the  sodium  chloride  a  very  little  of  a  soluble  calcium  salt. 
Serum,  blood,  and  milk  all  contain  calcium  salts,  and  albu- 
minous solutions  free  from  calcium  (as  paraglobulin)  do  not 
restore  the  beat;  nor  do  serum  or  milk  or  blood  deprived  of 
calcium.  Hence  the  presence  of  some  salt  of  that  metal 
seems  to  have  a  close  relation  to  the  functional  activity  of 
the  heart,  as  indeed  it  has  to  muscular  activity  in  general. 


CHAPTER  XVIII. 
THE  VASO-MOTOR   NERVES   AND  NERVE-CENTRES. 

The  Nerves  of  the  Blood-vessels.  The  arteries,  as 
already  pointed  out,  possess  a  muscular  coat  composed  of 
fibres  arranged  around  them,  so  that  their  contraction  can 
narrow  the  vessels.  This  coat  is  most  prominent  in  the 
smaller  vessels, — those  of  the  size  which  go  to  supply  separate 
organs, — but  disappears  again  in  the  smallest  branches,  which 
are  about  to  divide  into  capillaries  for  the  individual  tissue 
elements  of  an  organ.  These  vascular  muscles  are  under  the 
control  of  certain  special  nerves  called  vaso-motor,  and  these 
latter  can  thus  govern  the  amount  of  blood  reaching  any 
organ  at  a  given  time.  Most  of  the  vascular  nerve-fibres 
have  their  origin  in  the  cerebro-spinal  centre,  though  they 
pass  through  sympathetic  ganglia  on  their  way  to  the  vessels. 
In  a  few  regions  ganglion-cells  are  found  lying  close  to  the 
arteries,  and  some  of  the  vaso-motor  fibres  are  probably  con- 
nected with  them,  but  as  a  rule  they  end  directly  in  the  mus- 
cular coat. 

In  the  heart  we  had  to  consider  a  rhythmically  contract- 
ing organ  the  force  of  whose  contractions  could  be  increased 
or  diminished  by  the  influence  of  extrinsic  nerves;  in  the 
arteries,  speaking  broadly,  we  have  to  deal  with  muscle  in  a 
condition  of  tonic  or  constant  contraction,  which  contraction 
can  be  increased  by  impulses  coming  through  excitor  or  vaso- 
constrictor nerves,  and  diminished  through  the  activity  of 
inhibitory  or  vaso-dilator  nerves.  The  general  tonic  con- 
traction of  the  arterial  muscle  is,  however,  much  more  de- 
pendent on  the  vaso-constrictor  nerve-fibres  than  is  the  beat 
of  the  heart  on  the  cardio-excitor  nerves.  The  inhibitory 
set  of  vaso-motor  nerves  have  a  much  less  extensive  distribu- 
tion over  the  arterial  system  than  the  constrictor. 

The  Vaso-constrictor  Nerves.  If  the  ear  of  a  white  rab- 
bit beheld  up  against  the  light  while  the  animal  is  kept  quiet 
and  not,  alarmed,  the  red  central  artery  can  be  seen  coursing 

27? 


'274  THE  HUMAN  BODY. 

along  the  translucent  organ,  giving  off  brunches  which  by 
subdivision  become  too  small  to  be  separately  visible,  and  the 
whole  ear  has  a  pink  color  and  is  warm  from  the  abundant 

blood  flowing  through  it.  Attentive  observation  will  show 
also  that  the  calibre  of  the  main  artery  is  not  constant;  at 
somewhat  irregular  periods  of  a  minute  or  more  it  dilates  and 
contracts  a  little. 

If  the  sympathetic  trunk  have  been  previously  divided  on 
the  other  side  of  the  neck  of  the  animal,  the  ear  on  that  side 
will  present  a  very  different  appearance.  It  arteries  will  be 
much  dilated  and  the  whole  ear  fuller  of  blood,  redder,  and 
distinctly  warmer;  the  slow  alternating  variations  in  arterial 
diameter  also  have  disajypeared.  AVe  get  thus  evidence  that 
the  normal  mean  calibre  of  the  artery  is  maintained  by  influ- 
ences reaching  its  muscular  coat  through  the  cervical  sym- 
pathetic. Stimulation  of  the  upper  end  of  the  cut  nerve 
contirms  this  opinion.  It  is  then  seen  that  the  arteries  of 
•  the  corresponding  ear  gradually  contract  until  even  the  main 
vessel  can  hardly  be  seen,  and  in  consequence  the  whole  car 
becomes  pale  and  cold.  After  the  stimulation  is  stopped  the 
arteries  again  slowly  dilate  until  thev  have  regained  their  full 
paralytic  size,  and  they  usually  remain  permanently  in  that 
condition.  Sometimes  they  regain  after  some  days  almost 
the  size  of  those  in  the  ear  on  the  uninjured  side,  even  when 
the  nerve  has  not  only  been  cut,  but  the  upper  cervical 
sympathetic  ganglion  extirpated;  this  seems  to  indicate  that 
the  arterial  muscle  has  a  small  automaticity  of  its  own  tending 
to  keep  it  in  a  moderate  state  of  contraction,  but  it  is  less 
marked  than  the  automaticity  of  the  myocardium. 

Quite  similar  phenomena  can  be  observed  in  transparent 
parts  of  other  living  animals,  as  in  the  web  of  a  frog's  foot, 
the  arteries  of  which  dilate  after  section  of  the  sciatic  nerve 
and  constrict  when  the  peripheral  end  of  the  nerve  is  stimu- 
lated. In  the  case  of  other  parts  changes  in  temperature 
may  be  used  to  detect  alterations  in  the  flow  of  blood.  In  a 
dog  or  cat,  for  example,  a  sensitive  thermometer  placed  be- 
tween the  toes  indicates  a  rise  of  temperature,  owing  to  in- 
creased flow  of  warm  blood  through  the  skin,  after  section  of 
the  chief  nerve  of  the  limb,  and  a  fall  of  temperature  (usu- 
ally) during  stimulation  of  the  peripheral  end  of  the  divided 
nerve. 

"When  the  vaso-constrietor  nerves  cut  are  those  controlling  a 


VASO-MOTOR  NERVES  AND  NERVE-CENTRES.      275 

large  number  of  arteries,  the  dilatation  of  the  latter  so  much 
diminishes  peripheral  resistance  to  the  blood-flow  as  to  lead 
to  a  marked  fall  of  general  arterial  pressure;  and,  due  care 
being  taken  to  avoid  or  to  allow  for  concomitant  variations  in 
the  rate  or  force  of  the  heart's  beat,  this  gives  us  another  use- 
ful method  of  studying  the  distribution  of  the  nerves  con- 
cerned. For  example,  tbe  splanchnic  nerves  are  branches 
which  spring  from  the  thoracic  portion  of  the  sympathetic 
chain  and  pass  through  the  diaphragm  to  end  in  the  gan- 
gliated  solar  plexus  from  which  nerves  pass  to  the  arteries  of 
most  of  the  abdominal  viscera.  When  the  splanchnic  nerves 
are  cut  on  both  sides  arterial  pressure  falls  enormously,  from 
say  120  millimetres  of  mercury  in  the  carotid  of  a  dog  to  15 
or  20  millimetres,  most  of  the  blood  of  the  body  lying  almost 
stagnant  in  the  dilated  blood-vessels  of  the  abdomen.  On  the 
other  hand,  stimulation  of  the  splanchnic  nerves  so  diminishes 
the  paths  open  for  the  circulation  of  the  blood  as  to  enor- 
mously increase  general  blood-pressure;  especially  if  the 
cardio-inhibitory  nerves  be  first  divided  so  that  raised  blood- 
pressure  inside  the  skull-chamber  may  not  slow  the  heart- 
beat. 

The  skin  and  the  abdominal  organs  seem  to  be  the  pre- 
dominant localities  of  distribution  of  the  vaso-constrictor 
nerves:  other  parts  have  them,  but  not  in  quantity  sufficient 
to  bring  about  any  great  general  change  in  the  blood-flow. 
In  the  abdomen  is  warmer,  in  the  skin  cooler  blood:  and 
according  to  the  amount  of  heat  produced  in  the  Body  and 
the  temperature  of  the  surrounding  medium,  the  vessels  of 
abdomen  and  skin  contract  or  relax  so  as  to  control  the  pro- 
portion of  blood  sent  to  the  skin  to  lose  heat. 

The  Vaso-constrictor  Centre.  The  constrictor  nerves  of 
the  arteries  do  not  originate  in  the  sympathetic  system.  If 
all  the  branches  of  the  latter  be  left  intact,  the  phenomena  of 
paralytic  dilatation  of  the  blood-vessels  ran  be  fully  brought 
about  by  dividing  the  communicating  branches  between  certain 
spinal  nerves  and  the  corresponding  sympathetic  ganglia, 
or  by  dividing  the  anterior  roots  of  certain  spinal  nerves. 
In  this  way  it  can  be  shown  that  the  fibres  all  proceed 
from  the  thoracic  and  lumbar  regions  of  the  spinal  cord, 
but  have  not  their  origin  in  the  cord.  If  it  be  cut  anywhere 
in  the  cervical  region,  all  arteries  having  a  constrictor  nerve 
supply  an-  paralyzed,  while  stimulation  of  the  posterior  end 


276  THE  HUMAN  BODY. 

of  the  divided  cord  causes  widespread  arterial  constriction. 
The  main  centre  for  the  vaso-constrictors  must  then  lie  as 
far  forward  as  the  medulla:  and  as  all  the  brain  in  front  of 
the  medulla  oblongata  can  be  removed  without  any  con- 
sequent arterial  paralysis,  the  centre  must  lie  in  the  medulla 
itself.  This  centre  is  often  named  the  vaso-motor  centre,  but 
it  is  better  to  distinguish  it  as  the  vaso-constrictor  from  the 
centre  for  the  dilator  efferent  nerves. 

The  Control  of  the  Vaso-constrictor  Centre.  The  vaso- 
constrictor centre  is  automatic;  it  maintains  a  certain  amount 
of  activity  of  its  own,  independently  of  any  stimuli  reaching 
it  through  afferent  nerve-fibres.  Nevertheless,  like  nearly  all 
automatic  nerve-centres,  it  is  under  reflex  control,  so  that  its 
activity  may  be  increased  or  lessened  by  afferent  impulses 
conveyed  to  it.  Nearly  every  sensory  nerve  of  the  Body  is  in 
connection  with  it;  any  stimulus  giving  rise  to  pain,  for 
example,  excites  it,  and  thus  constricting  the  arteries,  in- 
creases the  peripheral  resistance  to  the  blood-flow  and  raises 
arterial  pressure.  On  the  other  hand,  certain  fibres  conveying 
impulses  from  the  heart  inhibit  the  centre  and  dilate  the 
arteries,  lower  blood-pressure,  and  diminish  the  resistance  to 
be  overcome  by  the  heart.  These  afferent  fibres,  which  have 
been  already  referred  to  as  the  large  medullated  fibres  (p.  263) 
of  the  pneurnogastric,  are  known  as  the  depressor  fibres,  or  in 
certain  animals,  for  example  the  rabbit,  where  they  are  all 
collected  into  one  branch,  as  the  depressor  nerve.  If  this 
nerve  be  divided  and  its  cardiac  end  stimulated  no  effect  is 
produced,  but  if  its  central  end  (that  still  connected  with  the 
rest  of  the  pneurnogastric  trunk  and  through  it  with  the 
medulla  oblongata)  be  stimulated,  arterial  pressure  gradually 
falls;  this  result  being  dependent  upon  a  dilatation  of  the 
small  arteries,  and  consequent  diminution  of  the  peripheral 
resistance,  following  an  inhibition  of  the  vaso-constrictor 
centre  brought  about  by  the  depressor  nerve.  Through  the  de- 
pressor nerve  the  heart  can  therefore  influence  the  calibre  of 
the  small  arteries  and,  by  lowering  aortic  pressure,  diminish  its 
own  work  if  need  be.  In  Fig.  103  is  reproduced  a  tracing  of 
the  great  but  slow  fall  of  blood-pressure  which  results  from 
stimulation  of  the  depressor  fibres.  It  shows  the  slow  fall  of 
pressure  and  slightly  changed  pulse-rate  accompanying  the 
slow  dilatation  of  the  arteries,  and  may  be  compared  with  the 
rapid  fall  and  slow  pulse  brought  about  (Fig.  102)  by  excita- 


VASOMOTOR  NERVES  AND  NERVE-CENTRES.      277 

tion  of  the  cardio-inhibitory  nerves.  The  latent  period  is 
also  noticeably  long  and  the  effect  of  the  stimulus  outlasts 
considerably  the  time  of  its  application. 

Blushing.  The  depressor  nerves  control  a  great  part  of  the 
vaso-constrictor  centre  (especially  that  portion  of  it  connected 
with  the  splanchnic  nerves)  and  so  can  bring  about  dilatation 
of  a  large  number  of  arteries — their  influence  is  accordingly 
called  into  play  when  general  arterial  pressure  is  to  be  lowered, 
but  is  useless  for  controlling  local  blood-supply.  This  is 
managed  in  part  by  other  afferent  nerves,  each  of  which 
inhibits  a  small  part  only  of  the  vaso-constrictor  centre,  gov- 
erning the  arteries  of  a  limited  tract  of  the  Body;  the  dilata- 


Fig.  103. — Tracing  of  pressure  from  femoral  artery  of  a  rabbit  showing  the  influ- 
ence of  .stimulation  of  the  central  end  of  the  depressor  nerve;  to  be  read  from  right 
to  left:  abode,  tracing  of  arterial  pressure,  the  small  variations  indicating  heart- 
beats; op,  tracing  of  seconds  pen;  s.  moment  of  commencement  of  stimulation;  t, 
cessation  of  stimulation;  xg,  line  of  no  pressure. 

tion  of  these  increases  the  amount  of  blood  flowing  through  the 
particular  region  to  which  they  are  distributed,  but  does  not 
affect  the  total  resistance  to  the  blood-flow  sufficiently  to 
influence  noticeably  the  general  pressure  in  the  arterial  system. 
In  blushing,  for  example  under  the  influence  of  an  emotion, 
that  part  of  the  vase-motor  centre  which  supplies  constrictor 
nerves  to  the  arteries  of  the  skin  of  the  neck  and  face,  is 
inhibited  by  nerve-fibres  proceeding  from  the  cerebrum  to  the 
medulla  oblongata,  and  the  face  and  neck  consequently  be- 
come full  of  blood  and  flush  up.  Quite  similar  phenomena 
occnr  under  other  conditions  in  many  parts  of  the  Body, 
although  when  not  visible  on  the  surface  we  do  not  usually 
call  them  blushes.  The  mucous  membrane  lining  the  empty 
stomach  i;-'  pallid  and  its  arteries  contracted,  but  as  .soon  as 
food  enters  the  organ  ii  becomes  red  and  full  of  blood;  the 
food  stimulating  afferent   nerve-fibres  there,  which  inhibit 


278  THE  HUMAN  BODY. 

ili.it  part  of  the  vasomotor  centre  which  governs  the  gastric 
arteries. 

Taking  Cold.  This  common  disease  is  nol  uit frequently 
caused  through  undue  reflex  excitement  of  the  vaso-motor 
centre.  Cold  acting  upon  the  skin  stimulates,  through  the 
afferent  nerves,  the  portion  of  the  vaso-motor  centre  governing 
the  skin  arteries,  and  the  latter  become  contracted,  as  shown 
by  the  pallor  of  the  surface.  This  has  a  twofold  influence — 
in  the  first  place,  more  blood  is  thrown  into  internal  parts, 
and  in  the  second,  contraction  of  the  arteries  over  so  much  of 
the  Body  considerably  raises  the  general  blood-pressure. 
Consequently  the  vessels  of  internal  parts  become  overgorged 
or  "  congested,"  a  condition  which  readily  passes  into  inflam- 
mation. The  action  is  of  course  primarily  protective,  to 
prevent  too  great  loss  of  heat  from  the  Body;  but  if  internal 
organs  be  weak  or  diseased  or  if  the  exposure  to  wet  or  cold 
be  prolonged,  it  is  apt  to  be  followed  by  catarrh  or  inflam- 
mation of  more  or  less  of  the  respiratory  tract  causing  bron- 
chitis, or  of  the  intestines  causing  diarrhoea.  In  fact  the  com- 
mon summer  diarrhoea  is  far  more  often  due  to  a  chill  of  the 
surface,  causing  intestinal  catarrh,  than  to  the  fruits  eaten 
in  that  season  which  are  so  often  blamed  for  it.  The  best 
preventative  is  to  wear,  when  exposed  to  great  changes  of  tem- 
perature, a  woollen  or  at  least  a  cotton  garment  over  the  trunk 
of  the  Body;  linen  is  so  good  a  conductor  of  heat  that  it 
permits  any  change  in  the  external  temperature  to  act  almost 
at  once  upon  the  surface  of  the  Body.  After  an  unavoidable 
exposure  to  cold  or  wet  the  thing  to  be  done  is  of  course  to 
restore  the  cutaneous  circulation;  for  this  purpose movemeut 
should  be  persisted  in,  and  a  thick  dry  outer  covering  put  on, 
until  warm  and  dry  underclothing  can  be  obtained. 

For  healthy  persons  a  temporary  exposure  to  cold,  as  a 
plunge  in  a  bath,  is  good,  since  in  them  the  sudden  contrac- 
tion of  the  cutaneous  arteries  soon  passes  off  and  is  succeeded 
by  a  dilatation  causing  a  warm  healthy  glow  on  the  surface. 
If  the  bather  remain  too  long  in  cold  water,  however,  this 
reaction  passes  off  and  is  succeeded  by  a  more  persistent 
chilliness  of  the  surface,  which  may  even  last  all  day.  The 
bath  should  therefore  be  left  before  this  occurs,  but  no  abso- 
lute time  can  be  stated,  as  the  reaction  is  more  marked  and 
lasts  longer  in  strong  persons,  and  in  those  used  to  cold  bath- 
ing, than  in  others. 


VASOMOTOR  NERVES  AND  NERVE-CENTRES.      279 

Vasodilator  Nerves.     We  have  already  noticed,  in  the 
ease  of  the  stomach,  one  method  by  which  a  locally  increased 
blood-supply  may  be  brought  about  in  an  organ  while  it  is 
at  work,  viz.,  by  inhibition  of  local   vaso- constrictor  fibres. 
Frequently,    however,    in    the    Body   this    is    managed    in 
another  way;  by  efferent  vaso-dilator  nerves  which   inhibit 
or  paralyze,  not  the  vaso-constrictor  centre,  but  the  muscles 
of   the  blood-vessels   directly.     The   nerves  of   the   skeletal 
muscles  for  example  contain  two  sets  of  efferent  fibres:  one 
motor  proper  and  the  other  vaso-dilator.     When  the  muscle 
contracts  in  a  reflex  action   or  under  the  influence  of  the 
will  both  sets  of  fibres  are  excited;  so  that  when  the  organ  is 
set  at  work  its  arteries  are  simultaneously  dilated  and  more 
blood  flows  through  it.     But  if  the  animal  have  previously 
administered  to  it  such  a  dose  of  curare  as  to  just  paralyze 
the   true   motor-fibres,   stimulation   of    the   nerve   produces 
dilatation  of  the  arteries  without  a  corresponding  muscular 
contraction.     Quite  a  similar  thing   occurs  in   the  salivary 
glands.     Their  cells,  which  form  the  saliva,  are  aroused  to 
activity  by  special   nerve-fibres;    but  the  gland-nerve   also 
contains  a  quite  distinct  set  of  vaso-dilator  fibres  which  nor- 
mally cause   a  simultaneous  dilatation  of   the  gland-artery, 
though   either   can    be   artificially  stimulated  by  itself   and 
produce  its  effect  alone.     Through  such  arrangements  the 
distribution  of  the  blood  in  the  Body  at  any  moment  is  gov- 
erned :  so  that  working  parts  shall  have  abundance  and  other 
parts  less,  while  at  the  same  time  the  general  arterial  pressure 
remains  the  same  on  the  average;    since  the  expansion  of  a 
few  small  local  branches  but  little  influences  the  total  periph- 
eral resistance  in  the  vascular  system.     Moreover,  commonly 
when  OTie  set  of  organs  is  at  work  with  its  vessels  dilated, 
others  are  at  rest  with  their  arteries  comparatively  contracted, 
and  so  a  general  average  blood-pressure  is  maintained.     Few 
persons,  for  example,  feel  inclined  to  do  brain-work  after  a 
heavy  meal:   Tor  then  a  great  part  of  the  blood  of  the  whole 
Body  is  led  off  into  the  dilated  vessels  of  the  digestive  organs, 
and  the  brain  gets  a  smaller  supply.    On  the  other  hand,  when 
th<-  braiiuis  at  work  its  vessels  are  dilated  and  often  the  whole 
head  flushed:  and  so  excitement  or  hard  thought  after  a  meal 
jg  ,,.!V  apt  to  produce  an  attack  of  indigestion,  by  diverting 

tiie  blood  from  the,  abdominal  organs,  where  it  ought  to  be  at 

thai  time.     5foung  persons,  whose  organs  have  a  superabun- 


280  THE  111  MAX  BODY. 

dance  of  energy  enabling  them  to  work  under  unfavorable 
conditions,  are  less  apt  to  suffer  in  such  ways  than  their  eld- 
ers. One  sees  boys  running  actively  about  after  eating,  when 
older  people  feel  a  desire  to  sit  quiet  and   ruminate— or  even 

to  go  to  sleep. 

When  the  nerve  of  a  limb  is  cut  and  its  peripheral  end  is 
stimulated  the  usual  result  is  arterial  constriction,  because 
the  constrictor  fibres  are  more  numerous  and  more  powerful 
than  the  dilator;  a  day  or  two  after  section,  when  the  nerve 
has  begun  to  degenerate,  stimulation,  however,  causes  dilata- 
tion, apparently  because  tin?  constrictor  fibres  degenerate 
more  quickly:  and  when  the  stimuli  (as  induction  shocks) 
given  to  the  nerve  are  repeated  at  only  a  slow  rate  the  dilator 
effect  frequently  overcomes  the  constrictor. 

The  Vaso-dilator  Centre.  The  vaso-dilator  nerves,  like 
t lie  vaso-constrictor,  seem  to  originate  primarily  in  a  centre 
in  the  medulla  oblongata.  In  regard  to  the  arteries  in  general, 
they  play  a  much  less  conspicuous  part  than  their  analogues, 
the  cardio-inhibitory  fibres,  do  in  regard  to  the  heart. 

The  Vaso-motor  Nerves  of  the  Veins.  Most  veins  have 
a  muscular  coat,  though  it  is  much  less  developed  than  in 
the  arteries,  and  this  coat  is  probably  under  the  control  of 
nerve-fibres.  Satisfactory  evidence  of  their  existence  is  still 
wanting. 

The  Vascular  Phenomena  of  Inflammation.  When 
some  transparent  portion  of  an  animal  (for  example  the 
mesentery  of  a  mouse  or  guinea-pig)  is  carefully  exposed  and 
studied  with  a  microscope,  the  normal  flow  in  the  small  ves- 
sels may  be  studied  for  some  time,  much  as  in  the  web  of  the 
frog.  If  an  irritant  he  applied,  the  immediate  result  is  a 
widening  of  the  small  arteries  and  a  greater  and  more  rapid 
flow  through  them  and  the  capillaries  and  veins.  This  seems 
dependent  mainly  on  a  direct  paralysis  of  the  arteries,  and  if 
the  irritant  be  transient  in  its  influence  the  congested  con- 
dition soon  passes  off.  If  the  irritant  be  more  powerful,  the 
vascular  dilatation  continues  and  other  circulatory  changes 
are  seen.  The  corpuscles,  instead  of  keeping,  as  is  usual  in 
arteries  of  microscope  size,  to  the  central  part  of  .the  tube 
(axial  current),  spread  more  evenly,  and  the  white  cor- 
puscles especially  tend  to  pass  into  the  layer  of  liquid  in  im- 
mediate contact  with  the  inner  coat  of  the  artery,  and  at  the 
same  time  to  exhibit  much  more    marked   amoeboid    move- 


VASO-MOTOR  NERVES  AND  NERVE-CENTRES.      281 

ments  than  they  commonly  do  while  travelling  in  the  blood- 
current.  The  platelets,  also,  which  are  normally  confined  to 
the  axial  currents,  now  pass  towards  the  sides.  If  this  stage 
of  very  early  inflammation  pass  on  to  the  next,  it  is  observed 
that  white  corpuscles  and  platelets  both  stick  to  the  inside  of 
the  vessels.  The  platelets  next  adhere  together  and  break 
down  into  granular  masses,  and  the  white  corpuscles  thrust 
amoeboid  processes  between  the  lining-cells  of  the  capillaries 
and  smallest  veins,  and  begin  to  push  their  way  through.  By 
these  means  a  considerable  impediment  to  the  blood-flow  is 
caused,  and  the  circulation  becomes  slower,  though  all  the 
vessels  of  the  part  may  be  dilated.  If  the  inflammation  con- 
tinue, many  white  corpuscles  pass  quite  out  of  the  vessels 
{migration)  and  enter  the  neighboring  lymph-spaces:  the 
red  corpuscles  get  blocked  and  squeezed  together  into  a  mass 
in  which  their  individual  boundaries  are  indistinguishable, 
and  some  of  them  may  even  be  squeezed  through  the  walls 
of  the  capillaries  (diapedesis).  Next  all  blood-flow  in  the 
area  under  observation  may  be  stopped,  while  more  lymph 
than  normal  collects  in  it.  From  this  state  recovery  may 
take  place;  or  continued  inflammation  may  lead  to  destruc- 
tion of  the  part.  The  primary  local  disturbances  in  the  cir- 
culation seem  due  to  changes  in  the  inner  coats  of  the  vessels 
of  the  irritated  region;  but  an  extensive  continued  inflam- 
mation produces  fever  and  many  other  secondary  general 
results,  partly  through  the  absorption  of  disease  products 
from  the  inflamed  part  and  partly  through  irritation  of 
afferent  nerve-fibres  which  throw  various  nerve-centres  into 
abnormal  action. 


CHAPTER  XIX. 
THE   SECRETORY   TISSUES   AND   ORGANS. 

Definitions.  In  its  broad  etymological  meaning  a  secre- 
tion is  any  substance  separated  or  derived  from  the  blood,  so 
that  in  a  certain  sense  all  the  solid  tissues  of  the  Body,  built 
up  from  materials  supplied  by  the  blood,  are  secretions,  in 
practice  the  name  lias  a  more  limited  application  and  is  given 
to  two  classes  of  substances,  distinguished  as  true  or  extern  til 
secretions  and  internal  secretions. 

Internal  secretions  are  the  results  of  the  vital  activities  of 
various  organs,  their  by-products,  passed  out  directly  into 
the  lymph  and  blood;  and  in  many  cases  are  simple  wastes, 
sent  to  the  blood-stream  for  conveyance  to  other  organs  which 
get  rid  of  them:  such,  for  example,  is  the  carbon  dioxide 
formed  in  every  part  of  the  Body.  In  other  cases  the  by- 
products of  certain  organs,  after  absorption  into  the  blood, 
have  to  be  further  changed  in  a  second  organ  before  elimina- 
tion, and  are  probably  of  use  to  this  second — a  part  of  its 
pabulum:  as  an  instance  we  may  take  leucin  (amido-caproic 
acid),  which  is  formed  in  many  organs  and,  given  by  them  to 
the  blood,  is  carried  to  the  liver,  the  cells  of  which  convert  it 
(or  at  least  a  great  part  of  it)  into  urea,  to  be  subsequently 
eliminated  by  the  kidneys.  A  third  very  important  class  of 
internal  secretions  consists  of  substances  formed  only  in  one 
organ  or  one  pair  of  organs  and  yielded  by  them  to  the  blood 
which  flows  through  them,  the  presence  of  which  substances 
in  the  blood  is  essential  to  the  healthy  nutrition  and  the  con- 
tinuance of  the  life  of  the  Body:  in  such  cases  removal  or 
extensive  disease  of  the  producing  organ  results  in  death. 
Examples  are  to  be  found  in  substances  which  the  thyroid 
body  and  suprarenal  capsules  produce;  they  will  be  consid- 
ered more  fully  in  Chapter  XXIII. 

Excluding  such  things  as  cast  hairs  and  epidermic  scales, 
the  true  or  external  secretions  may  be  defined  as  gases  or 
liquids,  often  of  very  complex   composition,  passed   out  on 

282 


TEE  SECRETO*RY  TISSUES  AND   ORGANS.  283 

some  free  surface  of  the  Body,  either  that  of  the  general 
exterior  or  of  some  internal  cavity,  or  into  recesses  commu- 
nicating with  such  a  surface.  The  true  secretions  fall  into  two 
classes:  one  in  which  the  product  is  of  no  further  use  in  the 
Body  and  is  merely  separated  for  removal,  as  the  urine;  and 
one  in  which  the  product  is  intended  to  be  used,  for  instance 
as  a  solvent  in  the  digestion  of  food.  The  former  group  are 
sometimes  distinguished  as  excretions  and  the  latter  as  secre- 
tions proper,  but  there  is  no  real  difference  between  them,  the 
organs  and  processes  concerned  being  fundamentally  alike  in 
each  case.  A  better  division  is  into  transudata  and  secretions, 
a  transudation  being  a  jwoduct  which  contains  nothing  which 
did  not  previously  exist  in  the  blood,  and  only  in  such  quan- 
tity as  might  be  derivable  from  it  by  merely  physical  processes; 
while  a  secretion  in  addition  to  transudation  elements  contains 
a  specific  element,  due  to  the  special  physiological  activity  of 
the  secretory  organ;  being  either  something  which  does  not 
exist  in  the  blood  at  all  or  something  which,  existing  in  the 
blood  in  small  quantity,  exists  in  the  secretion  in  such  a  high 
proportion  that  it  must  have  been  actively  picked  up  and 
conveyed  there  by  the  secretory  tissues  concerned.  For  in- 
stance, the  gastric  juice  contains  free  hydrochloric  acid  which 
does  not  exist  in  the  blood;  and  the  urine  contains  so  much 
urea  that  we  must  suppose  the  kidney-cells  to  have  a  peculiar 
power  of  removing  that  body  from  the  liquids  flowing  near 
them.  This  subdivision  is  also  justifiable  on  histological 
grounds;  Avherever  there  is  a  secreting  surface  or  recess  it  is 
lined  by  cells,  but  these  cells  where  transudata  are  formed  (as 
on  the  serous  membranes)  are  mere  flat  scales,  with  little  or 
no  protoplasm  remaining  in  them  (Fig.  11b),  while  the  cells 
which  line  a  true  secreting  organ  are  cuboidal,  spherical,  or 
columnar,  and  still  retain,  with  their  high  physiological  activ- 
ity, a  good  deal  of  their  primitive  protoplasm. 

Organs  of  Secretion.  The  simplest  form  in  which  a 
secreting  organ  occurs  {A,  Fig.  104)  is  that  of  a  flat  membrane 
provided  with  a  layer  of  cells,  a,  on  one  side  (that  on  which 
the  secretion  is  poured  out)  and  with  a  network  of  capillary 
blood-vessels,  c,  on  the  other.  The  dividing  membrane,  b,  is 
known  as  the  basement  membrane  and  is  usually  made  ii j *  of 
Hat.  closely  fitting  connective-tissue  corpuscles;  supporting  it 
on  its  deep  side  is  a  layer  of  connective  tissue,  d,  in  which  the 
blood-vessels  and  lymphatics  are  supported.  Such  simple  forms 


284  THE  HUMAN  noDY. 

of  secreting  surfaces  arc  found  on  the  serous  membranes,  but 
are  not  common;  in  most  cases  an  extended  area  is  required 
to  form  the  necessary  amount  of  secretion,  and  if  this  were 
attained  simply  by  spreading  out  plane  surfaces,  these  from 
their  number  and  extent  would  be  hard  to  pack  conveniently 
in  the  Body.  Accordingly  in  most  cases,  the  greater  area  is 
attained  by  folding  the  secreting  surface  in  various  ways  so 
that  a  large  area  can  be  packed  in  a  small  hulk,  just  as  a 
Chinese  lantern  when  shut  up  occupies  much  less  space  than 
when  extended,  although  its  actual  surface  remains  of  the 
same  extent.  In  a  few  cases  the  folding  takes  the  form  of 
protrusions  into  the  cavity  of  the  secreting  organ  as  indicated' 
at  C,  Fig.  104,  and  found  on  some  synovial  membranes;  but 
much  more  commonly  the  surface  extension  is  attained  in 
another  way,  the  basement  membrane,  covered  by  its  epithe- 
lium, being  pitted  in  or  involuted  as  at  B.  Such  a  secreting 
organ  is  known  as  a  gland. 

Forms  of  Glands.  In  some  cases  the  surface  involutions 
are  uniform  in  diameter,  or  nearly  so,  throughout  (B,  Fig. 
104).  Such  glands  are  known  as  I  uhithtr;  examples  are  found 
in  the  lining  coat  of  the  stomach  (Fig.  113);  also  in  the  skin 
(Fig.  135),  where  they  form  the  sweat-glands.  In  other  cases 
the  involution  swells  out  at  its  deeper  end  and  becomes  more  or 
less  sacculated  (E) ;  such  glands  are  racemose  or  acinous.  The 
small  glands  which  form  the  oily  matter  poured  out  on  the 
hairs  are  of  this  type.  In  both  kinds  the  lining  cells  near  the 
deeper  end  are  commonly  different  in  character  from  the  rest; 
and  around  that  part  of  the  gland  the  blood-vessels  form  a 
closer  network.  These  deeper  cells  form  the  true  secreting 
elements  of  the  gland,  and  the  passage,  lined  with  different 
cells,  leading  from  them  to  the  surface,  and  serving  merely  to 
carry  oil'  the  secretion,  is  known  as  the  gland-duct.  When 
the  duct  is  undivided  the  gland  is  simple;  but  when,  as  is 
more  usual,  it  is  branched  and  each  branch  has  a  true  secret- 
ing part  at  its  end,  we  get  a  compound  gland,  tubular  (G)  <>r 
racemose  (F,  //)  as  the  case  may  be.  In  such  castas  the  main 
duet,  into  which  the  rest  open,  is  often  of  considerable  length, 
so  that  the  secretion  is  poured  out  at  some  distance  from  the 
main  mass  of  the  gland. 

A  fully  formed  gland,  If,  thus  comes  to  be  a  complex 
structure,  consisting  primarily  of  a  duct,  c,  ductules,  dd,  and 
secreting  recesses,  ee.     The  ducts  and  ductules  are  lined  with 


THE  SECRETORY  TISSUES  AND   ORGANS. 


285 


epithelium  which  is  merely  protective  and  differs  in  charac- 
ter from  the  secreting  epithelium  which  lines  the  deepest 


|^||g|ii§|||||i~-b      -jJ;"-'''-'^ 


Fig  -  -  "I  glands.    4.  a  simple  secretins:  surface ;  a.  Its  epithelium ; 

emeni  membrane;  c,  capillaries  ;  g,  a  simple  tubular  gland  :  C,  a  secreting 
ice  Increased  by  protrusions;    E,  a  simple  racemose  kIuikI  :    h  and  Q    com- 
pound tubular  glands;  /•',  •■.  compound  racemose  gland     In  all  bui  A    />'   and  a 
the  capillaries  are  <>nntr.-<i  for  the  sake  "i  clearness.    //.  halt  of  a  highly  developed 
racemose  gland  :  c,  Its  main  duct. 

parts.     Surrounding  each  subdivision  and  binding  it  to  its 
neighbors  is  the  gland  .s/i-oma  formed  of  connective  tissue,  a 


286  THE  EUMAN   BODY. 

layer  of  which  also  commonly  envelops  the  whole  gland,  as 
its  capsule.  Usually  on  looking  at  the  surface  of  a  large 
gland  it  is  seen  to  be  separated  by  partitions  of  its  stroma, 
coarser  than  the  rest,  into  lobes,  each  of  which  answers  to  a 
main  division  of  the  primary  duct;  and  the  lobes  arc  often 
similarly  divided  into  smaller  parts  or  lobules.  In  the  con- 
nective tissue  between  the  lobes  and  lobules  blood-vessels 
penetrate,  to  end  in  line  capillary  vessels  around  the  terminal 
recesses.  They  never  penetrate  the  basement  membrane. 
Lymphatics  and  nerves  take  a  similar  course;  there  is  reason 
to  believe  that  the  nerve-fibres  penetrate  the  basement  mem- 
brane ami  become  directly  united  with  the  secreting  cells  of 
some  glands. 

The  Physical  Processes  in  Secretion.  From  the  struc- 
ture of  a  gland  it  is  clear  that  all  matters  derived  from  the 
blood  and  poured  into  its  cavity  must  pass  not  only  through 
the  walls  of  the  capillary  blood-vessels,  but  also,  by  filtra- 
tion or  dialysis,  through  the  basement  membrane  and  the 
lining  epithelium.  By  filtration  is  meant  the  passage  of  a 
fluid  under  pressure  through  the  coarser  mechanical  pores 
of  a  membrane,  as  in  the  ordinary  filtering  processes  of  a 
chemical  laboratory  ;  and  the  higher  the  pressure  on  the 
liquid  to  be  filtered  the  greater  the  amount  which,  other 
things  being  equal,  will  pass  through  in  a  given  time.  Since 
in  the  living  Body  the  liquid  pressure  in  the  blood-capillaries 
is  nearly  always  higher  than  that  outside  them,  filtration  is 
apt  to  take  place  everywhere  to  a  greater  or  less  extent,  and 
will  be  increased  in  amount  in  any  region  by  circum- 
stances raising  blood-pressure  there,  and  diminished  by  those 
lowering  it.  To  a  certain  extent  also  the  nature  of  the 
liquid  filtered  has  an  infiuence.  True  solutions,  as  those  of 
salt  in  water,  passed  through  unchanged  ;  but  solutions  con- 
taining substances  such  as  boiled  starch  or  raw  egg-albumen, 
which  swell  up  greatly  in  water  rather  than  truly  dissolve, 
are  altered  by  filtration  ;  the  filtrate  containing  less  of  the 
imperfectly  dissolved  body  than  the  unfiltered  liquid.  The 
higher  the  pressure  the  greater  the  proportion  of  such  sub- 
stances which  gets  through  ;  and  if  the  pressure  is  slight  the 
water  or  other  solvent  may  alone  pass,  leaving  all  the  rest 
behind  on  the  filter.  Under  moderate  pressure  the  blood 
may  thus  lose  by  filtration  only  such  bodies  as  water  and 
salines  ;  while  an  increase  of  arterial  pressure   may  lead  to 


THE  SECRETORY  TISSUES  AND   ORGANS.  287 

the  passage  of  albumen  and  fibrinogen.  Under  healthy  con- 
ditions, for  example,  the  urine  contains  no  albumen,  but  any- 
thing considerably  increasing  the  capillary  pressure  in  the  kid- 
neys will  cause  it  to  appear.  Dialysis  or  osmosis  has  already 
been  considered  (p.  42);  by  it  substances  pass  through  the  in- 
termolecular  pores  of  a  membrane  independently  of  the  press- 
ure on  either  side,  and  for  its  occurrence  two  liquids  of  dif- 
ferent chemical  constitution  are  required,  one  on  each  side  of 
the  membrane.  At  least  if  diffusion  takes  place,  as  is  proba- 
ble, between  two  exactly  similar  solutions,  the  amount  and 
character  of  the  substances  passing  opposite  ways  in  a  given 
time  are  exactly  equal,  so  that  no  change  is  produced  by  the 
dialysis;  which  practically  amounts  to  the  same  thing  as  if 
none  occurred.  When  a  solution  is  placed  on  one  side  of  a 
membrane  allowing  of  dialysis,  and  pure  water  on  the  other, 
it  is  found  that  for  every  molecule  of  the  dissolved  body  that 
passes  one  way  a  definite  amount  of  water,  called  the  en- 
dosmotic  equivalent  of  that  body,  passes  in  the  opposite 
direction.  Crystalline  bodies  as  a  rule  (haemoglobin  is  an 
exception)  have  a  low  endosmotic  equivalent  or  are  readily 
dialyzable;  while  colloids,  such  as  gum  and  proteids,  have  a 
very  high  one,  so  that  to  get,  by  dialysis,  a  small  amount  of 
albumen  through  a  membrane,  a  practically  infinite  amount 
of  water  must  pass  the  other  way.  Accordingly,  if  we  find 
such  bodies  in  a  secretion  we  cannot  suppose  that  they  have 
been  derived  from  the  blood  by  mere  osmosis. 

The  Chemical  Processes  of  Secretion.  As  above  pointed 
out  certain  secretions,  called  transudata,  seem  to  be  products 
of  filtration  and  dialysis  alone,  containing  only  such  sub- 
stances as  those  which  are  found  in  the  blood-plasma,  more 
or  less  altered  in  relative  quantity  by  the  ease  or  difficulty 
with  which  they  severally  passed  through  the  layers  met 
with  on  their  way  to  the  surface.  But  in  many  cases  the 
composition  of  a  secretion  cannot  be  accounted  for  in  this 
way  ;  it  contains  some  specific  element,  cither  a  substance 
which  does  not  exist  in  the  blood  at  all  and  must  therefore 
have  been  added  by  the  secreting  membrane,  or  somebody 

which,  although  exist  in'/  in  the  blood,  dees  so  in  such  minute 
proportion,  compared  with  that  in  which  it,  is  found  in  the 
secretion,  that  some  special  activity  of  the  secreting  cells  is 
indicated:  some  affinity  in  them  for  these  bodies  by  which 
they  actively  pick  them  up. 


288  Til E  III  MAX  BODY. 

Each  living  cell,  we  have  seen,  is  the  seat  of  constant 
chemical  activity,  taking  up  materials  from  the  medium 
aboul  it,  transforming  and  utilizing  them,  and  sooner  or 
later  restoring  their  elements,  differently  combined,  to  the 
outer  medium.  By  such  means  it  builds  up  and  maintains 
its  living  substance,  and  obtains  energy  to  carry  on  its  daily 
ivork.  While  this  is  true  of  all  cells  in  the  Body,  we  find 
certain  groups  in  which  chemical  metabolism  is  the  promi- 
nent fact — cells  which  are  specialized  for  this  purpose  just 
as  muscular  fibre  is  for  contraction  or  nerve-fibre  for  con- 
duction; and  certain  of  these  prominently  metabolic  tissues 
exist  in  the  true  glands  and  produce  or  collect  the  specific 
elements  of  their  secretions.  Their  chemical  processes  are 
no  doubt  primarily  directed  to  their  own  nutritive  mainte- 
nance; they  live  primarily  for  themselves,  but  their  nutritive 
processes  are  such  that  the  bodies  formed  in  them  and  sent 
into  the  secretion  are  such  as  to  be  useful  to  the  rest  of  the 
cells  of  the  community;  or  the  bodies  which  they  specially 
collect,  and  in  a  certain  sense  feed  on,  are  those  the  removal 
of  which  from  the  blood  is  essential  for  the  general  good. 
Their  individual  nutritive  peculiarities  are  utilized  for  the 
welfare  of  the  whole  Body. 

The  Mode  of  Activity  of  Secretory  Cells.  If  we  con- 
sider the  modes  of  activity  of  living  cells  in  general,  it  be- 
comes clear  that  secretory  cells  may  produce  the  specific 
element  of  a  secretion  in  either  of  two  ways.  They  may, 
as  a  by-result  of  their  living  play  of  forces,  produce  chemical 
changes  in  the  surrounding  medium  ;  or  they  may  build  up 
certain  substances  in  themselves  and  then  set  them  free  as 
specific  elements.  Yeast,  for  example,  in  a  saccharine  solu- 
tion causes  the  rearrangement  into  carbon  dioxide,  alcohol, 
glycerine  and  succinic  acid,  of  many  atoms  of  carbon,  hydro- 
gen and  oxygen  which  previously  existed  as  sugar;  and 
a  very  considerable  quantity  of  sugar  may  be  broken  up  by 
the  activity  of  a  few  living  yeast-cells.  How  the  latter  act 
we  do  not  know  with  certainty,  but  most  likely  by  picking  cer- 
tain atoms  out  of  the  sugar  molecule,  and  leaving  the  rest  to 
fall  down  into  simpler  compounds.  On  the  other  hand,  we  find 
cells  which  form  and  store  up  in  themselves  large  quantities 
of  substances,  which  they  afterwards  liberate;  starch,  for 
instance,  being  formed  and  laid  by  in  many  fruit-cells,  and 


THE  SECRETORY  TISSUES  AND   ORGANS.  289 

afterwards  dissolved  and  sent  in  solution  to  nourish  the 
young  plant. 

Gland-cells  might  a  priori  give  rise  to  the  specific  ele- 
ments of  secretions  in  either  of  these  ways,  and  we  have  to 
seek  in  which  manner  they  work.  Do  they  simply  act  as  fer- 
ments (however  that  is)  upon  the  surrounding  medium;  or 
do  they  form  or  collect  the  bodies  characterizing  their 
secretion,  first  within  their  own  substance,  and  then  liberate 
them,  either  disintegrating  or  not  at  the  same  time?  At 
present  there  is  a  large  and  an  increasing  mass  of  evidence 
in  favor  of  the  second  view.  There  is,  no  doubt,  some 
reason  to  believe  that  every  living  cell  can  act  more  or 
less  as  a  ferment  upon  certain  solutions  should  they  come 
into  contact  with  it.  Not  always,  of  course,  as  an  alcoholic 
ferment,  though  even  as  regards  that  one  fermentative  power 
it  seems  very  generally  possessed  by  vegetable  cells,  and  there 
is  some  evidence  that  alcohol  is  normally  produced  in  small 
amount  (and  presumably  by  the  fermentation  of  sugar)  under 
the  influence  of  certain  of  the  living  tissues  of  the  Human 
Body.  As  regards  distinctively'  secretory  cells,  however,  the 
evidence  is  all  the  other  way,  and  in  many  cases  we  can  see 
the  specific  element  collecting  in  the  gland-cells  before  it  is 
set  free  in  the  secretion.  For  example,  in  the  oil-glands  of 
the  skin  (Chapter  XXVIII)  we  find  the  secreting  cells,  at 
first  granular,  nucleated,  and  protoplasmic,  gradually  under- 
going changes  by  which  their  protoplasm  disappears  and  is 
replaced  by  oil-droplets,  until  finally  the  whole  cell  falls  to 
bits  and  its  detritus  forms  the  secretion;  the  cells  being  re- 
placed by  new  ones  constantly  formed  within  the  gland.  In 
such  cases  the  secretion  is  the  ultimate  product  of  the  cell- 
life,  the  result  of  degenerative  changes  of  old  age  occurring 
in  it. 

In  other  cases,  however,  the  liberation  of  the  specific  ele- 
ment is  not  attended  with  the  destruction  of  the  secreting 
cell;  as  an  example  we  may  take  the  pancreas,  which  is  a 
large  gland  lying  in  the  abdomen  and  forming  a  secretion 
used  in  digestion.  Ajnong  others,  this  secretion  possesses 
the  power,  under  certain  conditions,  of  dissolving  proteids 
and  converting  them  into  dialyzable  peptones  f|».  10).  This 
it  owi  pecific  element  known  as  trypsin,  the  formation 

of  which,  or  rather  of  its  forerunner  trypsinogen,  within  the 
gland-cells  can  he  traced  with  the  microscope. 


290  THE  HUMAN  BODY. 

The  pancreas,  like  the  majority  of  the  glands  connected 
with  the  alimentary  canal,  has  an  intermittent  activity,  de- 
termined by  the  presence  or  absence  of  food  in  various  parts 
of  the  digestive  tract.  If  the  organ  be  taken  from  a  recently 
killed  dog  which  has  fasted  thirty  hours  and,  after  proper 
preparation,  be  stained  with  carmine  and  examined  micro- 
scopically, we  get  specimens  of  what  we  may  call  the  "  rest- 
ing gland  " — a  gland  which  has  not  been  secreting  for  some 
time.  In  these  it  will  be  seen  that  the  cells  lining  the  secret- 
ing recesses  present  two  very  distinct  zones:  an  outer,  next 
the  basement  membrane  which  combines  with  the  coloring 
matter  and  is  not  granular,  and  an  inner  which  is  granular 
and  does  not  pick  up  the  carmine.  The  granules  we  shall 
find  to  be  indications  of  the  presence  of  a  trypsin-yielding 
substance  formed  in  the  cells. 

If  another  dog  be  kept  fasting  until  it  has  a  good  appetite 
and  be  then  allowed  to  eat  as  much  meat  as  it  will,  the  animal 
will  commonly  take  so  much  that  the  stomach  will  only  be  emp- 
tied at  the  end  of  about  twenty  hours.  This  period  may,  so 
far  as  the  pancreas  is  concerned,  be  divided  into  two.  From 
the  time  the  food  enters  the  stomach  and  on  for  about  ten 
hours, the  gland  secretes  abundantly;  after  that  the  secretion 
dwindles,  and  by  the  end  of  the  second  ten  hours  has  nearly 
ceased.  We  have,  then,  a  time  during  which  the  pancreas  is 
working  hard,  followed  by  a  period  in  which  its  activity  is 
very  little,  but  during  which  it  is  abundantly  supplied  with 
food-materials.  The  pancreas  taken  from  an  animal  at  the 
end  of  the  first  period  and  prepared  for  microscopic  exami- 
nation will  be  found  different  from  that  taken  from  a  dog 
killed  at  the  end  of  the  second  digestion  period,  and  also 
from  the  resting  gland.  Towards  the  end  of  the  period  of 
active  work  the  gland-cells  are  diminished  in  size  and  the 
proportions  of  the  granular  and  non-granular  zones  are  quite 
altered.  The  latter  now  occupies  most  of  the  cell,  while 
the  granular  non-staining  inner  zone  is  greatly  diminished. 
During  the  secretion  there  is,  therefore,  a  growth  of  the  non- 
granular and  a  destruction  of  the  granular  zone;  and  the 
latter  process  rather  exceeding  the  former,  the  whole  secret- 
ing cell  is  diminished  in  size.  During  the  second  digestive 
period,  when  secretion  is  languid,  exactly  a  reverse  process 
takes  place.  The  cells  increase  in  size  so  as  to  become  larger 
than  those  of  the  resting  gland:  and  this  growth  is  almost 


THE  SECRETORY  TISSUES  AND   ORGANS.  291 

entirely  due  to  the  granular  zone  which  now  occupies  most 
of  the  cell. 

These  facts  suggest  that  during  secretion  the  granular 
part  of  the  cells  is  used  up:  but  that,  simultaneously,  the 
deejDer  non  -  granular  zone,  being  formed  from  materials 
yielded  by  the  blood,  gradually  renews  the  granular.  Dur- 
ing active  secretion  the  breaking  down  of  the  latter  to 
yield  the  specific  element  occurs  faster  than  its  regenera- 
tion; in  a  later  period,  however,  when  the  secretion  is  ceas- 
ing, the  whole  cell  grows  and,  especially,  the  granular  zone  is 
formed  faster  than  it  is  disintegrated;  hence  the  great  in- 
crease of  that  part  of  the  cell.  If  this  be  so,  then  we  ought 
to  find  some  relationship  between  the  digestive  activity  of  an 
infusion  or  extract  of  the  gland  and  the  size  of  the  granular 
zones  of  the  cells;  and  it  has  been  shown  that  such  exists; 
the  quantity  of  trypsin  which  can  be  obtained  from  a  pan- 
creas being  proportionate  to  the  size  of  that  portion  of  its 
cells. 

The  trypsin,  however,  does  not  exist  in  the  cells  ready 
formed,  but  only  a  body  which  yields  it  under  certain  cir- 
cumstances, and  called  trypsinogen. 

If  a  perfectly  fresh  pancreas  be  divided  into  halves  and 
one  portion  immediately  minced  and  extracted  with  glyce- 
rine, while  the  other  is  laid  aside  for  twenty-four  hours  in  a 
warm  place  and  then  similarly  treated,  it  will  be  found  that 
the  first  glycerine  extract  has  no  power  of  digesting  proteids, 
while  the  second  is  very  active.  In  other  words,  the  fresh 
gland  does  not  contain  trypsin,  but  only  something  which 
yields  it  under  some  conditions;  among  others,  on  being 
kept.  The  inactive  glycerine  extract  of  the  fresh  gland  is, 
however,  rich  in  trypsinogen:  for  if  a  little  acetic  acid  be 
added  to  it,  trypsin  is  formed  and  the  extract  becomes 
powerfully  digestive. 

AVe  may,  then,  sum  up  the  life  of  a  pancreas-cell  in  this 
way.  It  grows  by  materials  derived  from  the  blood  and  first 
laid  down  in  the  non-granular  zone.  This  latter,  in  the  ordi- 
nary course  of  the  cell-life,  gives  rise  to  the  granular  zone; 
and  in  this  is  ;i  Btore  of  trypsinogen  produced  by  the  active 
metabolisms  of  the  cell.  When  the  gland  secretes,  the  tryp- 
sinogen is  converted  into  trypsin  and  set  free  in  the  secre- 
tion; but  in  the  resting  gland  this  transformation  docs  not 
occur.     During   secretory   activity,  therefore,  the   chemical 


292  Til E  HUMAN  BODY. 

processes  hiking  place  in  the  cell  are  different  from  those  at 
other  periods;  and  we  have  next  to  consider  how  this  change 
in  the  mode  of  life  of  the  cells  is  brought  aboat. 

Influence  of  the  Nervous  System  upon  Secretion. 
When  the  gland  is  active  it  is  fuller  of  blood  than  when  ;it 
rest:  its  arteries  are  dilated  and  its  capillaries  gorged  so  that 
it  'jets  a  brighter  pink  color;  this  extra  Mood-supply  might 
he  the  primary  cause  of  the  altered  metabolism.  Again,  the 
activity  of  the  pancreas  is  under  the  influence  of  the  nervous 
system,  as  proved  not  only  by  the  reflex  secretion  called  forth 
when  food  enters  the  stomach,  but  also  by  the  fact  that 
electrical  stimulation  of  the  medulla  oblongata  will  cause  the 
gland  to  secrete.  The  nervous  system  may,  however,  only 
act  through  the  nerves  governing  the  calibre  of  the  gland 
arteries,  and  so  but  indirectly  on  the  secreting  cells;  while 
on  the  other  hand  it  is  possible  that  nerve-fibres  act  directly 
upon  the  gland-cells  and,  controlling  their  nutritive  pro- 
cesses, govern  the  production  of  the  trypsin.  To  decide  be- 
tween the  relative  importance  of  these  possible  agencies  we 
must  pass  to  the  consideration  of  other  glands;  since  the 
question  can  only  be  decided  by  experiment  upon  the  lower 
animals,  and  the  position  of  the  pancreas  and  the  difficulty 
of  getting  at  its  nerves  without  such  severe  operations  as 
upset  the  physiological  condition  of  the  animal  furnish  ob- 
stacles to  its  study  which  have  not  yet  been  overcome. 

In  certain  other  glands,  however,  we  find  conclusive  evi- 
dence of  a  direct  action  of  nerve-fibres  upon  the  secreting 
elements.  When  the  sciatic  nerve  of  a  cat  is  stimulated 
electrically,  the  balls  of  its  feet  sweat.  Under  ordinary  cir- 
cumstances they  become  at  the  same  time  red  and  full  of 
blood;  but  that  this  congestion  is  a  factor  of  subsidiary  im- 
portance as  regards  secretion  is  proved  by  the  facts  that  stim- 
ulation of  the  nerve  is  still  able  to  excite  the  gland-cells  and 
cause  sweating  in  a  limb  which  has  been  amputated  ten  or 
fifteen  minutes  (and  in  which  therefore  no  circulatory  changes 
can  occur)  and  also  by  the  cold  sweats,  with  a  pallid  skin,  of 
phthisis  and  the  death-agony.  It  is,  however,  with  reference 
to  the  submaxillary  and  parotid  salivary  glands  that  our  in- 
formation is  most  precise. 

When  the  mouth  is  empty  and  the  jaws  at  rest  the  sali- 
vary secretion  is  comparatively  little:  but  a  sapid  substance 
placed   on  the  tongue  will  cause  a  copious  flow.     The  phe- 


THE  SECRETORY  TISSUES  AND   ORGANS.  293 

nomenon  is  closely  comparable  to  the  production  of  a  reflex 
muscular  contraction.  A  stimulus  acting  upon  an  irritable 
tissue  excites  through  it  certain  afferent  nerve-fibres;  these 
excite  a  nerve-centre,  which  in  turn  stimulates  efferent  fibres; 
going  to  a  muscle  in  the  one  case,  to  a  gland  in  the  other. 
It  will  be  useful  to  consider  again  for  a  moment  what  occurs 
in  the  case  of  the  muscle,  taking  account  only  of  the  efferent 
fibres  and  the  parts  they  act  upon. 

When  a  muscle  in  the  Body  is  made  to  contract  reflexly, 
through  its  nerve,  two  events  occur  in  it.  One  is  the  short- 
ening of  the  muscular  fibres;  the  other  is  the  dilatation  of 
the  muscular  arteries;  every  muscular  nerve  contains  two 
sets  of  fibres,  one  motor  and  one  vaso-dilator,  and  normally 
both  act  together.  In  this  case,  however,  it  is  clear  that  the 
activities  of  both,  though  correlated,  are  essentially  inde- 
pendent. The  contraction  is  not  due  to  the  greater  blood- 
flow,  for  not  only  can  an  excised  muscle  entirely  deprived  of 
blood  be  made  to  contract  by  stimulating  its  nerves,  but  in 
an  animal  to  which  a  small  dose  of  curari — the  arrow-poison 
of  certain  South  American  Indians — has  been  given,  stimu- 
lation of  the  nerve  will  cause  the  vascular  dilatation  but  no 
muscular  contraction:  the  curari  paralyzing  the  motor  fibres, 
but,  unless  in  large  doses,  leaving  the  vaso-dilators  intact. 
The  muscular  fibres  themselves  are  unacted  upon  by  the  poi- 
son, as  is  proved  by  their  ready  contraction  when  directly 
stimulated  by  an  electric  shock. 

Now  let  us  return  to  the  salivary  glands  and  see  how 
far  the  facts  are  comparable.  The  main  nerve  of  the  sub- 
maxillary gland  is  known  as  the  chorda  tympani.  If  it  be 
divided  in  a  narcotized  dog,  and  a  tube  placed  in  the  gland- 
duct,  no  saliva  will  flow.  But  on  stimulating  the  peripheral 
end  of  the  nerve  (that  end  still  connected  with  the  gland) 
an  abundant  secretion  takes  place.  At  the  same  time  there 
is  a  great  dilatation  of  the  arteries  of  the  organ,  much  more 
blood  than  before  flowing  through  it  in  a  given  time:  the 
chorda  obviously  then  contains  vaso-dilator  fibres.  Now  in 
this  case  it  might  very  well  be  that  the  process  was  different 
from  that  in  a  muscle.  It  is  conceivable  that  tin;  secretion 
may  be  but  a  filtration  due  to  increased  pressure  in  the  gland 
capillaries,  consequent  on  dilatation  of  the  arteries  supplying 
them.  It'  a  greater  filtration  into  tin;  lymph  spaces  of  the 
gland   took   place,   this   liquid   might,   then   merely  ooze  on 


2i>4  THE  III   MAX   HOD 7. 

through  the  secreting  cells  into  the  commencing  ducts  and, 
as  it  passed  through,  dissolve  out  and  carry  on  from  the  cells 
the  specific  organic  elements  of  the  secretion.  Of  these,  in 
the  submaxillary  of  the  dog  at  least,  mucin  is  the  most  im- 
portant and  abundant,  That,  however,  the  process  is  quite 
different,  and  that  there  are  in  the  gland  true  secretory  fibres 
in  addition  to  the  vaso-dilator,  just  as  in  the  muscle  there  are 
true  motor  fibres,  is  proved  by  other  experiments. 

If  the  flow  of  liquid  from  the  excited  gland  were  merely 
the  outcome  of  a  filtration  dependent  on  increased  blood- 
pressure  in  it,  then  it  is  clear  that  the  pressure  of  the  secre- 
tion in  the  duct  could  never  rise  above  the  pressure  in  the 
blood-vessels  of  the  gland.  Now  it  is  found,  not  only  that 
the  gland  can  be  made  to  secrete  in  a  recently  decapitated 
animal,  in  which  of  course  there  is  no  blood-pressure,  but 
that,  when  the  circulation  is  going  on,  the  pressure  of  the 
secretion  in  the  duct  can  rise  far  beyond  that  in  the  gland 
arteries.  Obviously,  then,  the  secretion  is  no  question  of 
mere  filtration,  since  a  liquid  cannot  filter  against  a  higher 
pressure.  Finally,  the  proof  that  the  vascular  dilatation  is 
quite  a  subsidiary  phenomenon  has  been  completed  by  show- 
ing that  we  can  produce  all  the  increased  blood-flow  through 
the  gland  without  getting  any  secretion — that  just  as  in  a 
muscle  nerve  we  can,  by  curari,  paralyze  the  motor  fihres 
and  leave  the  vaso-dilators  intact,  so  we  can  by  atropin,  the 
active  principle  of  deadly  nightshade,  get  similar  phenomena 
in  the  gland.  In  an  atropized  animal  stimulation  of  the 
chorda  produces  vascular  dilatation  but  not  a  drop  of  secretion. 
Bringing  blood  to  the  cells  abundantly  will  not  make  them 
drink:  we  must  seek  something  more  in  the  chorda  than  the 
vaso-dilator  fibres — some  proper  secretory  fihres;  that  the 
atropin  acts  upon  them  and  not  upon  the  gland-cells  is  shown, 
as  in  the  muscle,  by  the  fact  that  the  cells  still  are  capable  of 
activity  when  stimulated  otherwise  than  through  the  chorda 
tympani.  For  example,  by  stimulation  of  the  sympathetic 
fihres  going  to  the  gland. 

So  far.  then,  we  seem  to  have  good  evidence  of  a  direct 
action  of  nerve-fibres  upon  the  -land-cells.  I'.ut  even  that  is 
not  the  whole  matter.  It  is  extremely  probable,  if  not  cer- 
tain, that  there  arc  two  sets  of  secretory  fibres  in  the  gland- 
nerves:  a  set  which  so  acts  upon  the  cells  as  to  make  them 
pass  on  more  abundantly  the  transudation   elements  of  the 


THE  SECRETORY  TISSUES  AND   ORGANS.  295 

secretion  (the  water  and  mineral  salts),  and  another,  quite 
different,  which  governs  the  chemical  transformations  of  the 
cells  so  as  to  make  them  produce  mucin  from  mucigeu  pre- 
viously stored  in  them,  in  a  way  comparable  to  the  production 
of  trypsin  from  trypsiiiogen  in  the  active  pancreas.  These 
latter  fibres  may  be  called  "  trophic,"  since  they  directly  con- 
trol the  cell  metabolism:  while  the  former  may  be  called 
"  transudatory  "  fibres.  Some  of  the  evidence  which  leads  to 
thiscouclusiouis  a  little  complex,  but  it  is  worth  wbile  to  con- 
sider it  briefly.  In  the  first  place,  on  stimulation  of  the 
chorda  of  an  unexhausted  gland  (that  is,  a  gland  not  over- 
fatigued  by  previous  work)  the  following  points  can  be 
noted : — 

With  increasing  strength  of  the  stimulus  the  quantity  of 
the  secretion,  that  is  of  the  water  poured  out  in  a  unit  of 
time,  increases  ;  at  the  same  time  the  mineral  salts  also  in- 
crease, but  more  rapidly,  so  that  their  percentage  in  a  rap- 
idly formed  secretion  is  greater  than  in  a  more  slowly 
formed,  up  to  a  certain  limit.  The  percentage  of  organic 
constituents  of  the  secretion  also  increases  up  to  a  limit;  but 
soon  ceases  to  rise,  or  even  falls  again,  while  the  water  and 
salts  still  increase.  This  of  course  is  readily  intelligible; 
since  the  water  and  salts  can  be  derived  continually  from  the 
blood,  while  the  specific  elements,  coming  from  the  gland- 
cells,  may  be  soon  exhausted ;  and  so  far  the  experiment 
gives  no  evidence  of  the  existence  of  distinct  nerve-fibres 
for  the  salts  and  water,  and  for  the  specific  elements:  all 
vary  together  with  the  strength  of  the  stimulus  applied  to 
the  nerve.  But  under  slightly  different  circumstances  their 
quantities  do  not  run  parallel.  The  proportion  of  specific 
elements  in  the  secretion  is  largely  dependent  on  whether 
the  gland  has  been  previously  excited  or  not.  Prior  stimula- 
tion, not  carried  on  of  course  to  exhaustion,  largely  increases 
the  percentage  of  organic  matters  in  the  secretion  produced 
by  a  subsequent  stimulation;  but  has  no  effect  whatever  on 
the  quantities  of  water  and  salts.  These  are  governed  en- 
tirely by  the  strength  of  the  second  stimulation.  Here, 
then,  we  find  that  under  similar  circumstances  the  transuda- 
tory and  Bpecifc  elements  of  the  secretion  do  not  vary  to- 
gether;  and  are  therefore  probably  dependent  upon  different 
exciting  causes.  And  the  facts  might  lead  as  to  suspect 
that  there  are  in  the  chorda,  besides  the  vaso-dilator,  two 


996  THE  HUMAN  BODY. 

other  sets  of  fibres:  one  governing  the  salts  and  water,  and 
the  other  the  specific  elements  of  the  secretion.  So  far  the 
evidence  is,  perhaps,  Dot  quite  conclusive;  bul  experiments 
upon  the  parotid  gland  of  the  dog  put  the  matter  beyond  a 
doubt. 

The  submaxillary  gland  receives  fibres  from  the  sympa- 
thetic system,  as  well  as  the  chorda  tympani  from  the  cerebro- 
spinal. Excitation  of  the  sympathetic  fibres  causes  the 
gland  to  secrete,  but  the  saliva  poured  out  is  different  from 
that  following  chorda  stimulation,  which  is  in  the  dog  abun- 
dant and  comparatively  poor  in  organic  constituents,  and 
accompanied  by  vascular  dilatation:  while  the  " sympathetic 
saliva,"  as  it  is  called,  is  less  abundant,  very  rich  in  mucin, 
and  accompanied  by  constriction  of  the  gland  arteries. 
According  to  the  above  view  we  might  suppose  that  the 
chorda  contains  many  trausudatory  and  few  trophic  fibres, 
and  the  sympathetic  many  trophic  and  few  trausudatory. 
It  might,  however,  well  be  objected  that  the  greater  rich- 
ness in  organic  bodies  of  the  sympathetic  saliva  was  really 
due  to  the  small  quantity  of  blood  reaching  the  gland,  when 
that  nerve  was  stimulated.  This  might  alter  the  nutritive 
phenomena  of  the  cells  and  cause  them  to  form  mucin  in 
unusual  abundance,  in  which  case  the  trophic  influence  of 
the  nerve  would  be  only  indirect.  Experiments  on  the 
parotid  preclude  this  explanation.  That  gland,  like  the  sub- 
maxillary, gets  nerve-fibres  from  two  sources:  a  cerebral  and 
a  sympathetic.  The  latter  enter  the  gland  along  its  artery, 
while  the  former,  originating  from  the  glossopharyngeal, 
run  in  a  roundabout  course  to  the  gland.  Stimulation  of  the 
cerebral  fibres  causes  an  abundant  secretion,  rich  in  water 
and  salts,  but  with  hardly  any  organic  constituents.  At  the 
same  time  it  produces  dilatation  of  the  gland  arteries.  Stim- 
ulation of  the  sympathetic  causes  contraction  of  the  parotid 
gland  arteries  and  no  secretion  at  all.  Nevertheless  it  causes 
great  changes  in  the  gland-cells.  If  it  be  first  stimulated 
for  a  while  and  then  the  cerebral  gland-nerve,  the  resulting 
secretion  maybe  ten  times  as  rich  inorganic  bodies  as  that 
obtained  without  previous  stimulation  of  the  sympathetic; 
and  a  similar  phenomenon  is  observed  if  the  two  nerves  be 
stimulated  simultaneously.  So  that  the  sympathetic  nerve, 
though  unable  of  itself  to  cause  a  secretion,  brings  about 
great  chemical  changes  in  the  gland-cells.     It  is  a  distinct 


THE  SECRETORY  TISSUES  AND   ORGANS.  297 

trophic  nerve.  This  conclusion  is  confirmed  by  histology. 
Sections  of  the  gland  after  prolonged  stimulation  of  the  sym- 
pathetic show  its  cells  to  be  quite  altered  in  appearance,  and 
in  their  tendency  to  combine  with  carmine,  when  compared 
either  with  those  of  the  resting  gland  or  of  the  gland  which 
has  been  made  to  secrete  by  stimulating  its  glosso-pharyngeal 
branch  alone. 

We  have  still  to  meet  the  objection  that  the  sympathetic 
fibres  may  be  only  indirectly  trophic,  governing  the  meta- 
bolism of  the  cells  through  contraction  of  the  blood-vessels. 
If  this  were  so,  cutting  off  or  diminishing  the  blood-supply 
of  the  gland  in  any  way  ought  to  have  the  same  result  as 
stimulation  of  its  sympathetic  fibres.  Experiment  shows  that 
such  is  not  the  case  and  reduces  us  to  a  direct  trophic  influ- 
ence of  the  nerve.  When  the  arteries  are  closed  and  the  cere- 
bral gland-nerve  stimulated,  it  is  found  that  the  percentage  of 
organic  constituents  in  the  secretion  is  as  low  as  usual;  it  re- 
mains almost  exactly  the  same  whether  the  arteries  are  open 
or  closed  or  have  been  previously  ojjen  or  closed.  We  must 
conclude  that  the  peculiar  influence  of  the  sympathetic  does 
not  depend  upon  its  vaso-constrictor  fibres. 

These  observations  make  it  clear  that  the  phenomena  of 
secretion  are  dependent  on  very  complex  conditions,  at  least 
in  the  salivary  glands  and  presumably  in  others.  Primarily 
dependent  upon  filtration  and  dialysis  from  the  blood-vessels 
and  upon  the  physiological  character  of  the  gland-cells,  both 
of  these  factors  are,  we  find,  controlled  by  the  nervous  system, 
such  secreting  cells  being  no  more  automatic  than  striped 
muscle;  and  the  facts  also  give  us  important  evidence  of  the 
power  of  the  nervous  system  to  influence  cell  nutrition  directly. 
In  other  simpler  cases,  secretion  seems  to  be  a  mere  direct  re- 
sult of  the  growth  and  life  of  the  secreting  cell;  for  example 
the  formation,  storage  and  discharge  of  fatty  matters  by  the 
oil-glands  of  the  skin. 

Summary.  By  secretion  proper  is  meant  the  separation 
of  Buch  substances  from  the  blood  as  are  poured  out  on  free 
surfaces  of  the  Body,  whether  external  or  internal.  In  its 
simplesl  form  it  is  merely  a  physical  process  dependent  on  fil- 
tration and  dialysis;  for  example,  the  elimination  of  carbon 
dioxide  from  the  surfaces  of  the  lungs,  and  very  watery  liquid 
poured  out  on  the  surface  of  the  serous  membranes.  (Such 
secretions  arc  known  as  (nt)ix/f<{/i((i,im<\  their  amount  is  only 


298  THE  HUMAN  BODY. 

indirectly  controlled  by  the  nervous  Bystem,  through  the  in- 
fluence of  the  latter  upon  the  circulation  of  the  blood:  they 
are  however  dependent  <m  the  .structure  of  the  cells  concerned, 
so  that  the  characters  of  the  transudate  and  their  quantity  are 
altered  when  the  cells  are  diseased.  After  death,  too,  the 
process  of  dialysis  through  such  cells  is  different  from  that 
during  life,  for  the  living  cell  controls  to  a  certain  extent  the 
nature  and  amount  of  the  substances  which  it  will  allow  to 
se  through  it.  The  cells  lining  such  surfaces  are  not,  how- 
ever, secretary  tissues  in  any  true  sense  of  the  word.  In 
other  cases  the  lining  cells  are  thicker,  and  more  actively 
concerned  in  the  process:  they  are  then  usually  spread  over 
the  recesses  of  a  much  folded  membrane,  so  that  the  whole 

lied  up  into  a  compact  organ  called  a  gland,  the  secre- 
tion of  which  may  coiitain  only  transudation  elements  (as 
for  example  that  of  the  lachrymal  glands  which  form  the 
- |  or  may  contain  a  specific  element,  formed  in  the 
gland  by  its  cells,  in  addition  to  transudation  elements. 
In  both  cases  the  activity  of  the  organ  may  be  influenced 
by  the  nervous  system,  usually  in  a  reflex  manner  (e.g.  the 
watering  of  the  eyes  when  the  eyeball  is  touched  and  the 
saliva  poured  into  the  mouth  when  food  is  tasted),  but  may 
also  be  otherwise  excited,  as  for  example  the  flow  of  tears 
under  the  influence  of  those  changes  of  the  central  nervous 
system  which  are  associated  with  sad  emotions,  or  the  water- 
ing of  the  mouth  at  the  thought  of  dainty  food.  The  nerves 
going  to  such  glands,  besides  controlling  their  blood-vessels, 
act  upon  the  gland-cells;  one  set  governing  the  amount  of 
transudation  of  water  and  salines  which  shall  take  place 
through  them,  and  another  (in  the  case  of  glands  producing 

*ions  with  one  or  more  specific  elements)  controlling  the 
production  of  these,  by  starting  new  chemical  processes  in 
fhe  cells  by  which  a  substance  built  up  in  them  during  rest 
La  converted  into  the  specific  element,  which  is  soluble  in  and 
carried  off  by  the  transudation  elements.  What  the  specific 
element  of  a  gland  shall  be,  or  whether  its  secretion  contain 
any,  is  dependent  on  the  nature  of  its  special  cells;  how 
much  transudation  and  how  much  specific  element  shall  be 

(ted  at  any  time  is  controlled  by  the  nervous  system; 
just  as  the  contractility  of  a  muscle  depends  on  the  endow- 
ments of  muscular  tissue,  and  whether  it  shall  rest  or  con- 
tract— and  if  the  latter,  how  powerfully — upon  its  nerve. 


CHAPTEB   XX. 

THE  INCOME  AND  EXPENDITURE  OF  THE  BODY. 

The  Material  Losses  of  the  Body.  All  day  long  while 
life  lasts  each  of  us  is  losing  something  from  his  Bodv.  The 
air  breathed  into  the  lungs  becomes  in  them  laden  with 
carbon  dioxide  and  water  vapor,  which  are  carried  off  with 
it  when  it  is  expired.  The  skin  is  as  constantly  giving  off 
moisture,  the  total  quantity  in  twenty-four  hours  being  con- 
siderable, even  when  the  amount  jiassed  out  at  any  one  time 
is  so  small  as  to  be  evaporated  at  once  and  so  does  not  collect 
as  drops  of  visible  perspiration.  The  kidneys  again  are  con- 
stantly at  work  separating  water  and  certain  crystalline  ni- 
trogeneous  bodies  from  the  blood,  along  with  some  mineral 
salts.  The  product  of  kidney  activity,  however,  not  being 
forthwith  carried  to  the  surface  but  to  a  reservoir,  in  which 
it  accumulates  and  which  is  only  emptied  at  intervals,  the  ac- 
tivity of  those  organs  appears  at  first  sight  intermittent.  If  to 
these  losses  we  add  certain  other  waste  substances  passed  into 
the  alimentary  canal  and  got  rid  of  along  with  the  undiges 
residue  of  the  food,  and  the  loss  of  hairs  and  of  dried  cells 
from  the  surface  of  the  skin,  it  is  clear  that  the  total  amount 
of  matter  daily  removed  from  the  Body  is  considerable.  The 
actual  quantity  varies  with  the  individual,  with  the  work 
done,  and  with  the  nature  of  the  food  eaten;  but  the  follow- 
ing table  (}).  300)  gives  approximately  that  of  the  more  im- 
portant daily  material  losses  of  an  average  man. 

The  living  Body  thus  loses  daily  in  round  numbers  4  kilo- 
grams of  matter  (0  lbs.)  and.  since  it  is  unable  to  create  new 
matter,  this  loss  must  be  compensated  for  from  the  exterior 
or  the  tisanes  would  soon  dwindle  away  altogether:  or  at  I 
until  they  were  so  impaired  that  life  came  to  an  end.  After 
death  the  ould  be  of  a  different  kind,  and  their  quan- 

tity much  more  dependent  upon  surrounding  conditions;  but 
except  under  very  unusual  circumstances  the  wasting  away 
wmild  still  continue  in  the  dead  Body.    Moreover,  the  compo> 


3<w> 


THE  HI' MAX  BODY. 


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INCOME  ABB  EXPENDITURE  OF  THE  BODY.      301 

sition  of  the  daily  wastes  of  the  living  Body  is  tolerably  con- 
stant ;  it  does  not  simply  lose  a  quantity  of  matter  weighing 
so  much,  but  a  certain  amount  of  definite  kinds  of  matter, 
carbon,  nitrogen,  oxygen,  and  so  on;  and  these  same  sub- 
stances must  be  restored  to  it  from  outside,  in  order  that  life 
may  be  continued.  To  give  a  stone  to  one  asking  for  bread 
might  enable  him,  if  he  swallowed  it,  to  make  up  the  weight 
of  matter  lost  in  twenty-four  hours:  but  bread  would  be 
needed  to  keep  him  alive.  The  Body  not  only  requires  a 
supply  of  matter  from  outside,  but  a  supply  of  certain  definite 
kinds  of  matter. 

The  Losses  of  the  Body  in  Energy.  The  daily  expendi- 
ture of  matter  by  the  living  Body  is  not  the  only  one:  as 
continuously  it  loses  in  some  form  or  another  energy,  or  the 
power  of  doing  work;  often  as  mechanical  work  expended  in 
moving  external  objects,  but  even  when  at  rest  energv  is  con- 
stantly being  lost  to  the  Body  in  the  form  of  heat,  by  radia- 
tion and  conduction  to  surrounding  objects,  by  the  evaporation 
of  water  from  the  lungs  and  skin,  and  by  removal  in  warm 
excretions.  Unless  the  Body  can  make  energy  it  must  there- 
fore receive  a  certain  supply  of  it  also  from  the  exterior,  or  it 
would  very  soon  cease  to  carry  on  any  of  its  vital  work :  it 
would  be  unable  to  move  and  would  cool  down  to  the  temper- 
ature of  surrounding  objects.  The  discoveries  of  this  centurv 
having  shown  that  energy  is  as  indestructible  and  uncreatable 
(see  Physics)  as  matter,  we  are  led  to  look  for  the  sources  of 
the  supply  of  it  to  the  Body:  and  finding  that  the  living  Bodv 
daily  receives  it  and  dies  when  the  supply  is  cut  off,  we  no 
longer  suppose,  with  the  older  physiologists,  that  it  works  by 
means  of  a  mysterious  vital  force  existing  in  or  created  bv  it : 
but  that  getting  energy  from  the  outside  it  utilizes  it  for  its 
purposes — for  the  performance  of  its  nutritive  and  other  living 
work — and  then  returns  it  to  the  exterior  in  what  the  phvsi- 
cists  know  as  a  degraded  state:  that  is.  in  a  less  utilizable 
condition.  While  energy  like  matter  is  indestructible  it  is. 
unlike  matter,  transmutable:  iron  is  always  iron  and  gold 
always  gold;  neither  can  by  any  means  which  we  possess  be 
converted  into  any  other  form  of  matter:  and  so  the  Body, 
needing  carbon,  hydrogen,  oxygen,  and  nitrogen  to  build  it 
and  to  cover  its  daily  losses,  must  be  supplied  with  those  very 
substances.  As  regards  energy  this  is  not  the  case.  While 
the  total  amount  of  it  in  the  universe  is  constant,  its  form  is 


302  THE  HUMAN  BODY. 

constantly  subject  to  change— and  that  one  in  which  it  enters 
the  Body  need  not  be  that  in  which  it  exists  while  in  it,  nor 
thai  in  which  it  leaves  it.  Daily  losing  heat  and  mechanical 
work  the  Body  does  not  need,  could  not  in  fact  much  utilize 
energy, supplied  to  it  in  these  tonus;  hut  it  does  need  energy 
of  some  form  and  in  amount  equivalent  to  that  which  it  loses. 
The  Conservation  of  Energy.  The  forms  of  energy  vet 
discovered  are  not  nearly  so  numerous  as  the  kinds  of  matter. 
Still  we  all  know  several  of  them;  such  as  light,  heat,  sound, 
electricity,  and  mechanical  work;  and  most  people  nowadays 
know  that  some  of  these  forms  are  interconvertible,  so  that 
directly  or  indirectly  we  can  turn  one  into  another.  In  such 
changes  it  is  found  that  a  definite  amount  of  one  kind  always 
disappears  to  give  rise  to  a  certain  quantity  of  the  other;  or, 
in  other  words,  that  so  much  of  the  first  form  is  equivalent 
to  so  much  of  the  second.  In  a  steam-engine,  heal  is  pro- 
duced in  the  furnace;  when  the  engine  is  at  work  all  of  this 
energy  does  not  leave  it  as  heat;  some  goes  as  mechanical 
work,  and  the  more  work  the  engine  does  the  greater  is  the 
difference  between  the  heat  generated  in  the  furnace  and  that 
leaving  the  machine.  If,  however,  we  used  the  work  for  rub- 
bing two  rough  surfaces  together  we  could  get  the  heat  bark 
again,  and  if  (which  of  course  is  impossible  in  practice)  we 
could  avoid  all  friction  in  the  moving  parts  of  the  machine, 
the  quantity  thus  restored  would  be  exactly  equal  to  the 
excess  of  the  heat  generated  in  the  furnace  over  that  leaving 
the  engine.  Having  turned  some  of  the  heat  into  mechanical 
work  we  could  thus  turn  the  work  back  into  heat  again,  and 
find  it  yield  exactly  the  amount  which  seemed  lost.  Or  we 
might  use  the  engine  to  drive  an  electro-magnetic  machine 
and  so  turn  part  of  the  heat  liberated  in  its  furnace  first  into 
mechanical  work  and  that  into  electricity;  and  if  we  chose  to 
use  the  latter  with  the  proper  apparatus,  we  could  turn  more 
or  less  of  it  into  light,  and  so  have  a  great  part  of  the  energy 
which  first  became  conspicuous  as  heat  in  the  engine  furnace, 
now  manifested  in  the  form  of  light  at  some  distant  point. 
In  fact,  starting  with  a  given  quantity  of  one  kind  of  energy, 
we  may  by  proper  contrivances  turn  all  or  some  of  it  into 
one  or  more  other  forms;  and  if  we  collected  all  the  final 
forms  and  retransformed  them  into  the  first,  we  should  have 
exactly  the  amount  of  it  which  had  disappeared  when  the 
other  kinds  appeared.     This  law,  that  energy  can  change  its 


INCOME  AND  EXPENDITURE  OF  THE  BODY.      303 

form  but  that  its  amount  is  invariable,  that  it  cannot  be  created 
or  destroyed  but  simply  transmuted,  is  known  as  the  law  of 
the  Conservation  of  Energy  (see  Physics),  and,  like  the  inde- 
structibility of  matter,  lies  at  the  basis  of  all  scientific  con- 
ceptions of  the  universe,  whether  concerned  with  animate  or 
inanimate  objects. 

Since  all  forms  of  energy  are  interconvertible  it  is  con- 
venient in  comparing  amounts  of  different  kinds  to  express 
them  in  terms  of  some  one  kind,  by  saying  how  much  of  that 
standard  form  the  given  amount  of  the  kind  spoken  of  would 
give  rise  to  if  completely  converted  into  it.  Since  the  most 
easily  measured  form  of  energy  is  mechanical  work  this  is 
commonly  taken  as  the  standard  form,  and  the  quantities  of 
others  are  expressed  by  saying  how  great  a  distance  against 
the  force  of  gravity  at  the  earth's  surface  a  given  weight  could 
be  raised  by  the  energy  in  question,  if  it  were  all  spent  in 
lifting  the  weight.  The  units  of  mechanical  work  being  the 
kilogrammeter  or  the  foot-pound,  the  mechanical  equivalent 
of  any  given  kind  of  energy  is  the  number  of  kilogrammeters 
or  foot-pounds  of  work  its  unit  quantity  would  perform  if 
converted  into  mechanical  work  and  used  to  raise  a  weight. 
For  example  the  unit  quantity  of  heat  is  that  necessary  to 
raise  one  kilogram  of  water  one  degree  centigrade  in  temper- 
ature; or  sometimes,  in  books  written  in  English,  the  quan- 
tity necessary  to  warm  one  pound  of  water  one  degree  Fahren- 
heit. When  therefore  we  say  that  the  mechanical  equivalent 
of  heat  is  423  kilogrammeters  we  mean  that  the  quantity  of 
heat  which  would  raise  one  kilogram  of  water  in  temperature 
from  4°  C.  to  5°  (J.  would,  if  all  turned  into  mechanical  work, 
be  able  to  raise  one  kilogram  423  meters  against  the  attraction 
of  the  earth;  and  conversely,  that  this  amount  of  mechanical 
work  if  turned  into  heat  would  warm  a  kilogram  of  water 
one  degree  centigrade.  The  mechanical  equivalent  of  heat, 
taking  the  Fahrenheit  thermometric  scale  and  using  feet  and 
ponnds  as  measures,  is  772  foot-pounds. 

Potential  and  Kinetic  Energy.  At  times  energy  seems 
to  be  lost.  Ordinarily  we  only  observe  it  when  it  is  doing 
work  and  prodncing  some  change  in  matter:  but  sometimes 
it  is  at  rest,  stored  away  and  producing  do  changes  that  we 
recognize  and  thus  seems  to  have  been  destroyed.  Energy  at 
work  is  known  as  kinetic  energy;  energy  at  rest,  not  produc- 
ing changes  in  matter,  is  called  potential  energy.     Suppose  a 


304  THE  HUMAN  BODY. 

stone  pulled  up  by  a  string  and  left  suspended  in  the  air. 
We  know  a  certain  amount  of  energy  was  used  to  lift  it;  but 
while  it  hangs  we  have  neither  heat  nor  light  nor  mechanical 
work  to  represent  it.  Still  the  energy  is  not  lost;  we  know 
we  have  only  to  cut  the  string  and  the  weight  will  fall,  and 
striking  something  give  rise  to  heat.  Or  we  may  wind  up  a 
spring  and  keep  it  so  by  a  catch.  In  winding  it  up  a  certain 
amount  of  energy  in  the  form  of  mechanical  work  was  used 
to  alter  the  form  of  the  spring.  Until  the  catch  is  removed 
this  energy  remains  stored  away  as  potential  energy:  but  we 
know  it  is  not  lost.  Once  the  spring  is  let  loose  again  it  may 
drive  a  clock  or  a  watch,  and  in  so  doing  will  perform  again 
just  so  much  work  as  was  spent  in  coiling  it;  and  when  the 
watch  has  run  down  this  energy  will  all  have  been  turned 
into  other  forms — mainly  heat  developed  in  the  friction  of 
the  "parts  of  the  watch  against  one  another:  but  partly  also 
in  producing  movements  of  the  air,  a  portion  of  which  we 
ean  readily  observe  in  the  sound  of  its  ticking.  The  law  of 
the  conservation  of  energy  does  not  say,  then,  that  either  the 
total  potential  or  the  total  kinetic  energy  in  the  universe  is 
constant  in  amount:  but  that  the  sum  of  the  two  is  invariable, 
while  constantly  undergoing  changes  from  kinetic  to  potential 
and  vice  versa :  and  from  one  form  of  kinetic  to  another. 

The  Energy  of  Chemical  Affinity.  Between  every  two 
chemical  atoms  which  are  capable  of  entering  into  combina- 
tion there  exists  a  certain  amount  of  potential  energy:  when 
they  unite  this  energy  is  liberated,  usually  in  the  form  of  heat, 
and  once  they  have  combined  a  certain  amount  of  kinetic 
energy  must  be  spent  to  pull  them  apart  again;  this  being 
exactly  the  amount  which  was  liberated  when  they  united. 
The  more  stable  the  compound  formed  the  more  kinetic 
energy  appears  during  its  formation,  and  the  more  must  be 
spent  to  break  it  up  again.  One  may  imagine  the  separated 
atoms  as  two  halls  pushed  together  by  springs,  the  strength 
of  the  spring  being  proportionate  to  the  degree  of  their 
chemical  affinity.  Once  they  are  let  loose  and  permitted  to 
strike  together  the  potential  energy  previously  represented  by 
the  compressed  springs  disappears,  and  in  its  place  we  have 
the  kinetic  energy,  represented  by  the  heat  developed  when 
the  balls  strike  together.  To  pull  them  apart  again,  against 
the  springs,  to  their  original  positions,  just  so  much  mechani- 
cal work  must  be  spent  as  is  the  equivalent  of  that  amount 


INCOME  AND  EXPENDITURE  OF  THE  BODY.      305 

of  heat  which  appeared  when  they  struck;  and  thus  kinetic 
energy  will  again  become  latent  in  breaking  up  the  compound 
represented  by  tbe  two  in  contact.  The  energy  liberated  in 
chemical  combination  is  the  most  important  source  of  that 
used  in  our  machines:  and  also  of  that  spent  by  the  living 
Body. 

The  Relation  between  the  Matters  Removed,  from  the 
Body  daily  and  the  Energy  Spent  by  it.  A  working  loco- 
motive is,  we  know,  constantly  losing  matter  to  the  exterior 
in  the  form  of  ashes  and  gaseous  products  of  combustion,  the 
latter  being  mainly  carbon  dioxide  and  water  vapor.  The 
engine  also  expends  energy,  not  only  in  the  form  of  heat 
radiated  to  the  air,  but  as  mechanical  work  in  drawing  the 
cars  against  the  resistance  offered  by  friction  or  sometimes, 
up  an  incline,  by  gravity.  Now  the  engine-driver  knows  that 
there  is  a  close  relationship  between'  the  losses  of  matter  and 
the  expenditure  of  energy,  so  that  he  has  to  stoke  his  furnace 
more  frequently  and  allow  a  greater  draft  of  air  through  it  in 
going  up  a  gradient  than  when  running  on  the  level.  The 
more  work  the  engine  does  the  more  coals  and  air  it  needs  to 
make  up  for  its  greater  waste.  If  we  seek  the  cause  of  this 
relationship  between  work  and  waste,  the  first  answer  natu- 
rally is  that  the  engine  is  a  machine  the  special  object  of 
which  is  to  convert  heat  into  mechanical  work,  and  so  the 
more  work  it  has  to  do  the  more  heat  is  required  for  conver- 
sion, and  consequently  the  more  coals  must  be  burnt.  This, 
however,  opens  the  question  of  the  source  of  the  heat — of  all 
that  vast  amount  of  kinetic  energy  which  is  liberated  in  the 
furnace;  and  to  answer  this  we  must  consider  in  what  forms 
matter  and  energy  enter  the  furnace,  since  the  energy  liber- 
ated there  must  be  carried  in  somehow  from  outside.  For 
present  purposes  coals  may  be  considered  as  consisting  of 
carbon  and  hydrogen,  both  of  which  substances  tend  to 
forcibly  combine  with  oxygen  at  high  temperatures,  forming 
in  tin-  one  <;ise  carbon  dioxide  and  in  the  other  water.  The 
oxygen  necessary  to  form  these  compounds  being  supplied 
by  the  air  entering  the  furnace,  all  the  potential  energy  of 
chemical  affinity  which  existed  between  the  uncombined 
elements  becomes  kinetic,  and  is  liberated  as  heat  when  the 
combination  bakes  place.  The  energy  utilized  by  the  engine 
is  therefore  supplied  to  it  in  the  form  of  potential  energy, 
associated  with  the  uncombined  forms  of  matter  which  reach 


306  THE  HUMAN  BODY. 

the  furnace.     Once  the  carbon  ami  hydrogen  have  combined 

with  oxygen  they  are  no  longer  of  any  use  as  liberators  of 
energy;  and  the  compounds  formed  if  retained  in  the  furnace 

would  only  clog  it  and  impede  farther  combustion;  they  are 
therefore  got  rid  of  as  wastes  through  the  smoke-stack.  The 
engine,  in  short,  receives  uncoinhined  elements  associated 
with  potential  energy;  and  loses  combined  elements  (which 
have  lost  the  energy  previously  associated  with  them)  and 
kinetic  energy:  it, so  to  speak,  separates  the  energy  from  the 
matter  with  which  it  was  connected,  utilizes  it,  and  gets  rid 
of  the  exhausted  matter.  The  amount  of  kinetic  energy 
liberated  dining  such  chemical  combinations  is  very  great; 
a  kilogram  of  carbon  uniting  with  oxygen  to  form  carbon 
dioxide  sets  free  S080  units  of  heat,  or  calories.  During  the 
combination  of  oxygen  and  hydrogen  to  form  water  even 
more  energy  is  liberated,  one  kilogram  of  hydrogen  when 
completely  burnt  liberating  more  than  thirty-four  thousand 
of  the  same  units.  The  mechanical  equivalent  of  this  can  be 
calculated  if  it  is  remembered  that  one  heat  unit  =  423 
kilogrammeters. 

Turning  now  to  the  living  Body  we  find  that  its  income 
and  expenditure  agree  very  closely  with  those  of  the  steam- 
engine.  It  receives  from  the  exterior  substances  capable  of 
entering  into  chemical  union;  these  combine  in  it  and  liber- 
ate energy;  and  it  loses  kinetic  energy  and  the  products  of 
combination.  From  the  outside  it  takes  oxygen  through  the 
lungs,  and  oxidizable  substances  (in  the  form  of  foods) 
through  the  alimentary  canal;  these  combine  under  the  con- 
ditions prevailing  in  the  living  cells  just  as  the  carbon  and 
oxygen,  which  will  not  unite  at  ordinary  temperatures,  com- 
bine under  the  conditions  existing  in  the  furnace  of  the 
engine;  the  energy  liberated  is  employed  in  the  work  of  the 
Body,  while  the  useless  products  of  combination  are  got  rid 
of.  To  explain,  then,  the  fact  that  our  Bodies  go  on  working 
we  have  no  need  to  invoke  some  special  mysterious  power 
resident  in  them  and  capable  of  creating  energy,  a  vital  force 
having  no  relation  with  other  natural  forces,  such  as  the 
older  physiologists  used  to  imagine.  The  Body  needs  and 
gets  a  supply  of  energy  from  the  exterior  just  as  the  steam- 
engine  does,  food  and  air  being  to  one  what  coals  and  air  are 
to  the  other;  each  is  a  machine  in  which  energy  is  liberated 
by  chemical  combinations  and  then  used  for  special  work ; 


INCOME  AND   EXPENDITURE  OF  THE  BODY.      307 

the  character  of  which  depends  upon  the  peculiarities  of 
mechanism  which  utilizes  it  in  each  case,  and  not  upon  any 
peculiarity  in  the  energy  utilized  or  in  its  source.  The  Body 
is,  however,  a  far  more  economical  machine  than  any  steam- 
engine;  of  all  the  energy  liberated  in  the  latter  only  a  small 
fraction,  about  one  eighth,  is  usefully  employed,  while  our 
Bodies  can  utilize  for  the  performance  of  muscular  work 
alone  one  fifth  of  the  whole  energy  supplied  to  them;  leaving 
out  of  account  altogether  the  nutritive  and  other  work  carried 
on  in  them,  and  the  heat  lost  from  them. 

The  Conditions  of  Oxidation  in  the  Living  Body.  Al- 
though the  general  principles  applied  in  the  Body  and  the 
steam-engine  for  getting  utilizable  energy  are  the  same,  in 
minor  points  obvious  differences  are  found  between  the  two. 
In  the  first  place  the  coals  of  an  engine  are  oxidized  only  at 
a  very  high  temperature,  one  which  would  be  instantly  fatal 
to  our  Bodies,  which,  although  warm  when  compared  with 
the  bulk  of  inanimate  objects,  are  very  slow  fires  when  com- 
pared with  a  furnace.  Chemistry  and  physics,  however, 
teach  us  that  this  difference  is  quite  unimportant  so  far  as 
concerns  the  amount  of  energy  liberated.  If  magnesium 
wire  be  ignited  in  the  air  it  will  become  white-hot,  flame,  and 
leave  at  the  end  of  a  few  seconds  only  a  certain  amount  of 
incombustible  rust  or  magnesia,  which  consists  of  the  metal 
combined  with  oxygen.  The  heat  and  light  evolved  in  the 
process  represent  of  course  the  energy  which,  in  a  potential 
form,  was  associated  with  the  magnesium  and  oxygen  before 
their  combination.  We  can,  however,  oxidize  the  metal  in  a 
different  way,  attended  with  no  evolution  of  light  and  no 
very  perceptible  rise  of  temperature.  If,  for  instance,  we 
leave  it  in  wet  air  it  will  become  gradually  turned  into  mag- 
nesia without  having  ever  been  hot  to  the  touch  or  luminous 
to  the  eye.  The  process  will,  however,  take  days  or  weeks; 
and  while  in  this  slow  oxidation  just  as  much  energy  is  liber- 
ated ae  in  the  former  case,  it  now  all  takes  the  form  of  heat; 
and  instead  of  being  Liberated  in  a  short  time  is  spread  over 
a  much  longer  one,  as  the  gradual  chemical  combination 
takes  place.  The  slowly  oxidizing  magnesium  is,  therefor*;, 
at  no  moment  noticeably  hot,  since  it  loses  its  heat  to  sur- 
rounding objects  as  fast  as  if  is  generated.  The  oxidations 
occurring  in  our  Bodies  are  of  this  slow  kind.  An  ounce  of 
arrowroot  oxidized   in  a  fire,  and  in  the  Human  Body,  would 


308  THE  II UMAX  BODY. 

liberate  exactly  as  much  energy  in  one  case  as  the  other,  but 
the  oxidation  would  take  place  in  a  few  minutes  and  at  a 
high  temperature  in  the  former,  and  slowly,  at  a  lower  tem- 
perature, in  the  Latter.  In  the  second  place,  the  engine  dif- 
fers from  the  living  Body  in  the  fact  thai  the  oxidations  in  it 
all  take  place  in  a  small  area,  tbe  furnace,  and  so  the  tem- 
perature there  becomes  very  high;  while  in  our  Bodies  the 
oxidations  take  place  all  over,  in  each  of  the  living  cells; 
there  is  no  one  furnace  or  hearth  where  all  the  energy  is  lib- 
erated for  the  whole  and  transferred  thence  in  one  form  or 
another  to  distant  parts:  and  this  is  another  reason  why  no 
one  part  of  the  Body  attains  a  very  high  temperature. 

The  Fuel  of  the  Body.  This  is  clearly  different  from 
that  of  an  ordinary  engine:  no  one  could  live  by  eating  coals. 
This  difference,  again,  is  subsidiary;  a  gas-engine  requires 
different  fuel  from  an  ordinary  locomotive;  and  the  Body  re- 
quires a  somewhat  different  one  from  either.  It  needs,  as 
foods,  substances  which  can,  in  the  first  place,  be  absorbed 
from  the  alimentary  canal  and  carried  to  the  various  tissues; 
and,  in  the  second,  can  be  oxidized  at  a  low  temperature  in 
the  blood  or  tissues,  or  can  be  converted  by  the  living  cells 
into  compounds  which  can  be  so  oxidized.  AVith  some  trivial 
exceptions,  all  substances  which  fulfil  these  conditions  are 
complex  chemical  compounds,  and  to  understand  their  utili- 
zation in  the  Body  we  must  extend  a  little  the  statements 
above  made  as  to  the  liberation  of  energy  in  chemical  com- 
binations. The  general  law  may  be  stated  thus:  Energy  is 
liberated  whenever  chemical  union  takes  ]}lace :  and  whenever 
more  stable  compounds  are  formed  from  less  stable  ones,  in 
which  the  constituent  atoms  were  less  firmly  held  together. 
Of  the  liberation  by  simple  combination  we  have  already  seen 
an  instance  in  the  oxidation  of  carbon  in  a  furnace;  but  the 
anion  need  not  be  an  oxidation.  Every  one  knows  how  hot 
quicklime  becomes  when  it  is  slaked;  the  water  combining 
strongly  with  the  lime,  and  energy  being  liberated  in  the 
form  of  heat  during  the  process.  Of  the  liberation  of  energy 
by  the  breaking  down  of  a  complex  compound,  in  which  the 
atoms  are  only  feebly  united,  into  simpler  and  stabler  ones, 
we  get  an  example  in  alcoholic  fermentation.  During  that 
process  grape-sugar  is  broken  down  into  more  stable  com- 
pounds, mainly  carbon  dioxide  and  alcohol,  while  oxygen  is 
at  the  same  time  taken  up.     To  pull  apart  the  carbon,  hydro- 


INCOME  AND  EXPENDITURE  OF  THE  BODY.      309 

gen,  and  oxygen  of  the  sugar  molecule  requires  a  certain 
expenditure  of  kinetic  energy :  but  in  the  simultaneous  for- 
mation of  the  new  and  stabler  compounds  a  greater  amount  of 
energy  is  set  free,  and  the  difference  appears  as  heat,  so  that 
the  brewer  frequently  has  to  cool  his  vats  with  ice.  It  is  by 
processes  like  this  latter,  rather  than  by  direct  combinations, 
that  most  of  the  kinetic  energy  of  the  Body  is  obtained;  the 
complex  proteids  and  fats  and  starches  and  sugar  taken  as 
food  being  broken  down  (usually  with  concomitant  oxida- 
tion) into  simpler  and  more  stable  compounds. 

Oxidation  by  Successive  Steps.  In  the  furnace  of  an 
engine  the  oxidation  takes  place  completely  at  once.  The 
carbon  and  hydrogen  leaving  it,  if  it  is  well  managed,  are 
each  in  the  state  of  their  most  stable  oxygen  compound. 
But  this  need'not  be  so:  we  might  first  oxidize  the  carbon  so 
as  to  form  carbon  monoxide,  CO,  and  get  a  certain  amount  of 
heat;  and  then  oxidize  the  carbon  monoxide  farther  so  as  to 
form  carbon  dioxide,  CO„,  and  get  more  heat.  If  Ave  add 
together  the  amounts  of  heat  liberated  in  each  stage,  the  sum 
will  be  exactly  the  quantity  which  would  have  been  obtained 
if  the  carbon  had  been  completely  burnt  to  the  state  of  car- 
bon dioxide  at  first.  Every  one  who  has  studied  chemistry 
will  think  of  many  similar  cases.  As  the  process  is  impor- 
tant physiologically,  we  may  take  another  example,  say  the 
oxidation  of  alcohol.  This  may  be  burnt  completely  and  di- 
rectly, giving  rise  to  carbon  dioxide  and  water — 

C,H60    +    06       =       2C02     +     3H20 

1  Alcohol.        (i  Oxygen.        2  Carbon  dioxide.        3  Water. 

But  instead  of  this  we  can  oxidize  the  alcohol  by  stages,  get- 
ting at  each  stage  only  a  comparatively  small  amount  of  heat 
evolved.  By  combining  it  first  with  one  atom  of  oxygen,  we 
get  aldehyde  and  water — 

C,H„0    +    0     =     C2H40   +  H,0 

1  Alcohol.        1  Oxygen.        1  Aldehyde.        1  Water. 

Then  we  add  an  atom  of  oxygen  to  the  aldehyde  and  get 
acetic  acid  (vinegar) — 

0,H40     +     0     =     C„H40, 

1  Aldehyde.        1  Oxygen.        1  Acetic  acid. 


310  THE  HUMAN  BODY 

And  finally  we  may  oxidize  bhe  acetic  acid  so  as  to  get  carbon 

dioxide  and  water — 

C3H40,+  04  =  2COa  +  2H80. 

We  get,  in  both  cases,  from  one  molecule  of  alcohol,  two 
of  carbon  dioxide  and  three  of  water;  and  six  atoms  of  oxy- 
gen are  taken  up.  In  each  stage  of  the  gradual  oxidation  ;i 
certain  amount  of  heat  is  evolved;  and  the  sum  of  these  is 
exactly  the  amount  which  would  have  been  evolved  by  burn- 
ing the  alcohol  completely  at  once. 

The  food  taken  into  the  Body  is  for  the  most  part  oxi- 
dized in  this  gradual  manner;  the  products  of  imperfect 
combustion  in  one  set  of  cells  being  carried  off  and  more 
completely  oxidized  in  another  set,  until  the  final  products, 
no  longer  capable  of  further  oxidation  in  the  Body,  are  car- 
ried to  the  lungs,  or  kidneys,  or  skin,  and  got  rid  of.  A  great 
object  of  physiology  is  to  trace  all  intermediate  compounds 
between  the  food  which  enters  and  the  waste  products  which 
leave;  to  find  out  just  how  far  chemical  degradation  is  carried 
in  each  organ,  and  what  substances  are  thus  formed  in  vari- 
ous parts:  but  at  present  this  part  of  the  science  is  very  im- 
perfect. 

The  Utilization  of  Energy  in  the  Human  Body.  In 
the  steam-engine  energy  is  liberated  as  heat;  some  of  the  heat 
is  used  to  evaporate  water  and  expand  the  resulting  steam; 
and  then  the  steam  to  drive  a  piston.  Hut  in  the  living  Body 
it  is  very  probable  (indeed  almost  certain)  that  a  great  part 
of  the  energy  liberated  by  chemical  transformations  does  not 
first  take  the  form  of  heat;  though  some  of  it  does.  This, 
again,  does  not  affect  the  general  principle:  the  source  of 
energy  is  essentially  the  same  in  both  cases;  it  is  merely  the 
form  which  it  takes  that  is  different.  In  a  galvanic  cell 
energy  is  liberated  during  the  union  of  zinc  and  sulphuric 
acid,  and  we  may  so  arrange  matters  as  to  gel  this  energy  as 
heat;  but  on  the  other  hand  we  may  lead  much  of  it  off,  as 
a  galvanic  current,  and  use  it  to  drive  a  magneto-electric 
machine  before  it  has  taken  the  form  of  heat  at  all.  In  fact, 
that  heat  maybe  used  to  do  mechanical  work  we  must  reduce 
sonic  of  it  to  a  lower  temperature:  an  engine  needs  a  con- 
denser of  some  kind  as  well  as  a  furnace;  and,  other  things 
being  equal,  the  cooler  the  condenser  the  greater  the  propor- 


INCOME  AND  EXPENDITURE  OF  THE  BODY.      311 

tion  of  the  whole  heat  liberated  in  the  furnace  which  can  be 
nsed  to  do  work.  Now  in  a  muscle  there  is  no  condenser;  its 
temperature  is  uniform  throughout.  So  when  it  contracts 
and  lifts  a  weight,  the  energy  employed  must  be  liberated  in 
some  other  form  than  heat — some  form  which  the  muscular 
fibre  can  use  without  a  condenser. 

Summary.  The  living  Body  is  continually  losing  mat- 
ter and  expending  energy.  So  long  as  we  regard  it  as  work- 
ing by  virtue  of  some  vital  force,  the  power  of  generating 
which  it  has  inherited,  the  waste  is  difficult  to  account  for, 
since  it  is  far  more  than  we  can  imagine  as  due  merely  to 
wear  and  tear  of  the  working  parts.  When,  however,  we  con- 
sider tbe  nature  of  the  income  of  the  Body,  and  of  its  ex- 
penditure, from  a  chemico-physical  point  of  view,  we  get  the 
clue  to  the  puzzle.  The  Body  does  not  waste  because  it 
works,  but  works  because  it  wastes.  The  working  power  is 
obtained  by  chemical  changes  occurring  in  it,  associated  with 
the  liberation  of  energy  which  the  living  cells  utilize;  and 
the  products  of  these  chemical  changes,  being  no  longer 
available  as  sources  of  energy,  are  passed  out.  The  chemical 
changes  concerned  are  mainly  the  breaking  down  of  complex 
and  unstable  chemical  compounds  into  simpler  and  more 
stable  ones,  with  concomitant  oxidation.  Accordingly  tbe 
material  losses  of  the  Body  are  highly  or  completely  oxidized, 
tolerably  simple,  chemical  compounds;  and  its  material  in- 
come is  mainly  uncombined  oxygen  and  oxidizable  substances, 
the  former  obtained  through  the  lungs,  the  latter  through 
the  alimentary  canal.  In  energy,  its  income  is  the  potential 
energy  of  uncombined  or  feebly  combined  elements,  which 
are  capable  of  combining  or  of  forming  more  stable  com- 
pounds: and  its  final  expenditure  is  kinetic  energy  almost 
entirely  in  the  form  of  mechanical  work  and  heat.  Given 
oxygen,  all  oxidizable  bodies  will  not  serve  to  keep  the  Body 
alive  and  working,  but  only  those  which  (1)  are  capable  of 
absorption  from  the  alimentary  canal  and  (2)  those  which 
are  oxidizable  at  the  temperature  of  the  Body  under  the  influ- 
ence of  protoplasm.  Just  as  carbon  and  oxygen  will  not 
unite  in  the  furnace  of  an  engine  unless  the  fire  be  lighted  by 
the  application  of  a  match  but,  when  once  started,  the  heat 
evolved  at  one  point  will  serve  to  bring  about  the  conditions 
of  combination  through  the  rest  of  the  mass,  so  the  oxida- 
tions of  the    Body  only  occur   under  special  conditions;  and 


312  THE  HUMAN  BODY. 

these  are  transmitted  from  parent  to  offspring.  Every  new 
Human  Being  starts  as  a  portion  of  protoplasm  separated 
from  a  parent  and  affording  the  conditions  for  those  chemi- 
cal combinations  which  supply  to  living  matter  its  working 
power:  this  serves,  like  the  energy  of  the  burning  part  of  a 
fire  to  start  similar  processes  in  other  portions  of  matter.  At 
present  we  know  nothing  in  physiology  answering  to  the 
match  which  lights  a  furnace;  those  manifestations  of  energy 
which  we  call  life  are  handed  down  from  generation  to  gen- 
eration, as  the  sacred  fire  in  the  temple  of  Vesta  from  one 
watcher  to  another.  Science  may  at  some  time  teach  us  how 
to  bring  the  chemical  constituents  of  protoplasm  into  that 
combination  in  which  they  possess  the  faculty  of  starting 
oxidations  under  those  conditions  which  characterize  life; 
then  we  shall  have  learnt  to  strike  the  vital  match.  For 
the  present  we  must  be  content  to  study  the  properties  of 
that  form  of  matter  which  possesses  living  faculties;  since 
there  is  no  satisfactory  proof  that  it  has  ever  been  produced, 
within  our  experience,  apart  from  the  influence  of  matter 
already  living.  How  the  vital  spark  first  originated,  how 
molecules  of  carbon,  hydrogen,  nitrogen  and  oxygen  first 
united  with  water  and  salts  to  form  protoplasm,  we  have  no 
scientific  data  to  ground  a  positive  opinion  upon,  and  such  as 
we  may  have  must  rest  upon  other  grounds. 


CHAPTER  XX. 
FOODS. 

Foods  as  Tissue-formers.  Hitherto  we  have  considered 
foods  merely  as  source  of  energy,  but  they  are  also  required 
to  build  up  the  substance  of  the  Body.  From  birth  to  man- 
hood we  increase  in  bulk  and  weight,  and  that  not  merely  by 
accumulating  water  and  such  substances,  but  by  forming  more 
bone,  more  muscle,  more  brain,  and  so  on,  from  materials 
which  are  not  necessarily  bone  or  muscle  or  nerve-tissue. 
Alongside  of  the  processes  by  which  complex  substances  are 
broken  down  and  oxidized  and  energy  liberated,  constructive 
processes  take  place  by  which  new  complex  bodies  are  formed 
from  simpler  substances  taken  as  food.  A  great  part  of  the 
energy  liberated  in  the  Body  is  in  fact  utilized  first  for  this 
purpose,  since  to  construct  complex  unstable  molecules,  like 
those  of  protoplasm,  from  the  simpler  compounds  taken  into 
the  Body,  needs  an  expenditure  of  kinetic  energy.  Even 
after  full  growth,  when  the  Body  ceases  to  gain  weight,  the 
same  synthetic  processes  go  on;  the  living  tissues  are  steadily 
broken  down  and  constantly  reconstructed1,  as  we  see  illus- 
trated by  the  condition  of  a  man  who  has  been  starved  for 
some  time,  and  who  loses  not  only  his  power  of  doing  work 
and  of  maintaining  his  bodily  temperature  but  also  a  great 
part  of  his  living  tissues.  If  again  fed  properly  he  soon 
makes  new  fat  and  new  muscle  and  regains  his  original  mass. 
Another  illustration  of  the  continuance  of  constructive 
powers  during  the  whole  of  life  is  afforded  by  the  growth  of 
the  muscles  when  exercised  properly. 

Since  the  tissues,  on  ultimate  analysis,  yield  mainly  car- 
bon, hydrogen,  nitrogen  and  oxygen,  it  might  be  supposed 
a  priori  that  a  supply  of  these  elements  in  the  uncombined 
state  would  serve  as  material  for  the  constructive  forces  of 
the  Body  to  work  with.  Experience,  however,  teaches  us 
that  this  is  not  the  case,  but  that  the  animal  body  requires, 
for  the  most  part,  highly  complex  compounds  for  the  con- 

313 


314  THE  HUMAN  BODY. 

struction  of  new  tissue  elements.  All  the  active  tissues  yield 
on  analysis  large  quantities  of  proteids  which,  as  pointed  out 
in  Chapter  I,  enter  always  into  the  structure  of  protoplasm. 
Now,  so  far  as  we  know  at  present,*  the  animal  body  is  unable 
to  build  up  proteids  from  simpler  compounds  of  nitrogen, 
although  when  given  one  variety  of  them  it  can  convert  thai 
one  into  others,  and  combine  them  with  other  things  to  form 
protoplasm.  Hence  proteids  arc  an  essential  article  of  diet, 
in  order  to  replace  the  proteid  of  the  living  cells  which  is 
daily  broken  down  and  eliminated  in  the  form  of  urea  and 
other  waste  substances.  Even  albuminoids  (p.  LO),  although 
so  nearly  allied  to  proteids,  will  not  serve  to  replace  them 
entirely  in  a  diet;  a  man  fed  abundantly  on  gelatine,  fats, 
and  starches  would  starve  as  certainly,  though  not  so  quickly, 
as  if  he  got  no  nitrogenous  food  at  all :  Ins  tissue  waste  would 
not  be  made  good,  and  he  would  at  last  lie  no  more  able  to 
utilize  the  energy-yielding  materials  supplied  to  him  than  a 
worn-out  steam-engine  could  employ  the  heat  of  a  fire  in  its 
furnace.  So,  too,  the  animal  is  unable  to  take  the  carbon  for 
the  construction  of  its  tissues,  from  such  simple  compounds 
as  carbon  dioxide.*  Its  constructive  power  is  limited  to  the 
utilization  of  the  carbon  contained  in  more  complex  and  less 
stable  compounds,  such  as  proteids,  fats  or  sugars. 

Nearly  all  the  tissue-forming  foods  must  therefore  consist 
of  complex  substances,  and  of  these  a  part  must  be  proteids, 
since  the  Body  can  utilize  nitrogen  for  tissue  formation  only 
when  supplied  with  it  in  that  form.  The  bodies  thus  taken 
in  are  sooner  or  later  broken  down  into  simpler  ones  and 
eliminated;  some  at  once  in  order  to  yield  energy,  others 
only  after  having  first  been  built  up  into  part  of  a  living  cell. 
The  partial  exceptions  afforded  by  such  losses  to  the  Body  as 
milk  for  suckling  the  young,  or  the  albuminous  and  fatty 
bodies  stored  for  the  same  purpose  in  the  egg  of  a  bird,  are 
only  apparent;  the  chemical  degradation  is  only  postponed, 
taking  place  in  the  body  of  the  offspring  instead  of  that  of 
the  parent.  In  all  cases  animals  are  thus,  essentially,  proteid 
consumers  or  wasters,  and  breakers  down  of  complex  bodies; 
the  carbon,  hydrogen,  and  nitrogen  which  they  take  as  foods 
in  the  form  of  complex  unstable  bodies,  ultimately  leaving 

*  There  is  some  reason  to  believe  that  sorne  few  of  the  lower  animals 
which  contain  chlorophyl  can  manufacture  proteids  and  utilize  carbon 
dioxide. 


FOODS.  315 

them  in  the  simpler  compounds,  carbon  dioxide,  water,  and 
urea;  which  are  incapable  of  either  yielding  energy  or  build- 
ing tissue  for  any  other  animal  and  so  of  serving  it  as  food. 
The  question  immediately  suggests  itself — How,  since  animals 
are  constantly  breaking  up  these  complex  bodies  and  cannot 
again  build  them,  is  the  supply  kept  up  ?  For  example,  the 
supply  of  proteids,  substances  which  cannot  be  made  arti- 
ficially by  any  process  which  we  know,  and  yet  are  necessary 
foods  for  all  animals,  and  daily  destroyed  by  them. 

The  Food  of  Plants.  As  regards  our  own  Bodies  the 
question  at  the  end  of  the  last  paragraph  might  perhaps  be 
answered  by  saying  that  we  get  our  proteids  from  the  flesh 
of  the  other  animals  which  we  eat.  But,  then,  we  have  to 
account  for  the  possession  of  them  by  those  animals;  since 
they  cannot  make  them  from  urea  and  carbon  dioxide  and 
water  any  more  than  we  can.  The  animals  eaten  get  them, 
in  fact,  from  plants  which  are  the  great  proteid  formers  of 
the  world,  so  that  the  most  carnivorous  animal  really  depends 
for  its  most  essential  foods  upon  the  vegetable  kingdom ;  the 
fox  that  devours  a  hare  in  the  long-run  lives  on  the  proteids 
of  the  herbs  that  the  hare  had  previously  eaten.  All  animals 
are  thus,  in  a  certain  sense,  parasites;  they  only  do  half  of 
their  own  nutritive  work,  just  the  final  stages,  leaving  all  the 
rest  to  the  vegetable  kingdom  and  using  the  products  of  its 
labor  ;  and  plants  are  able  to  meet  this  demand  because  they 
can  live  on  the  simple  compounds  of  carbon,  hydrogen,  and 
nitrogen  eliminated  by  animals,  building  up  out  of  them  new 
complex  substances  which  animals  can  use  as  food.  A  green 
plant,  supplied  with  ammonium  salts,  carbon  dioxide,  water, 
and  some  minerals,  will  grow  and  build  up  large  quantities 
of  proteids,  fats,  starches,  and  similar  things;  it  will  pull  the 
stable  compounds  eliminated  by  animals  to  pieces,  and  build 
them  up  into  complex  unstable  bodies,  capable  of  yielding 
energy  when  again  broken  down.  However,  to  do  such  work, 
to  break  up  stable  combinations  and  make  from  them  less 
stable,  needs  a  supply  of  kinetic  energy  which  disappears  in 
the  process,  being  stored  away  as  potential  energy  in  the  new 
compound ;  and  we  may  ask  whence  it  is  that  the  plant  gets 
the  supply  of  energy  which  it  thus  utilizes  for  chemical  con- 
st ruction,  since  its  simple  and  highly  oxidized  foods  can  yield 
it  none.  It  has  been  proved  that  for  this  purpose  the  green 
plant  uses  the  energy  of  sunlight:  those  of  its  cells  which  con- 


316  THE  HUMAN  BODY. 

tain  the  substance  called  chloroph //l  (leaf  green)  have  the  power 
of  utilizing  energy  in  the  form  of  light  for  the  performance 
of  chemical  work,  just  as  a  strain-engine  can  utilize  heat  for  the 
performance  of  mechanical  work.  Exposed  to  light,  and  re- 
ceiving carbon  dioxide  from  the  air,  and  water  and  ammonia 
(which  is  produced  by  the  decomposition  of  urea)  and  other 
simple  nitrogen  compounds  from  the  soil,  the  plant  builds 
them  up  again,  with  the  elimination  of  oxygen,  into  complex 
bodies  like  those  which  animals  broke  down  with  fixation  of 
oxygen.  Some  of  the  bodies  thus  formed  it  uses  for  its  own 
growth  and  the  formation  of  new  protoplasm,  just  as  an  animal 
does;  but  in  sunlight  it  forms  more  than  it  uses,  and  the 
excess  stored  up  in  its  tissues  is  used  by  animals.  In  the  long- 
run,  then,  all  the  energy  spent  by  our  Bodies  comes  through 
millions  of  miles  of  space  from  the  sun;  but  to  seek  the  source 
of  its  supply  there  would  take  us  far  out  of  the  domain  of 
Physiology. 

Non-oxidizable  Foods.  Besides  our  oxidizable  foods,  a 
large  number  of  necessaiy  food-materials  are  not  oxidizable, 
or  at  least  are  not  oxidized  in  the  Body.  Typical  instances 
are  afforded  by  water  and  common  salt.  The  use  of  these  is 
in  great  part  physical:  the  water,  for  instance,  dissolves  ma- 
terials in  the  alimentary  canal,  and  carries  the  solutions 
through  the  walls  of  the  digestive  tube  into  the  blood  and 
lymph  vessels,  so  that  they  can  be  carried  from  part  to  part; 
and  it  permits  interchanges  to  go  on  by  diffusion.  The 
salines  also  influence  the  solubility  and  chemical  interchanges 
of  other  things  present  with  them.  Serum  albumen,  the 
chief  proteid  of  the  blood,  for  example,  is  insoluble  in  pure 
water,  but  dissolves  readily  if  a  small  quantity  of  neutral  salts 
is  present.  Besides  such  uses  the  non-oxidizable  foods  have 
probably  others,  in  what  we  may  call  machinery  formation. 
In  the  salts  which  give  their  hardness  to  the  bones  and  teeth, 
we  have  an  example  of  such  an  employment  of  them:  and  to 
a  less  extent  the  same  may  be  true  of  other  tissues.  The 
Body,  in  fact,  is  not  a  mere  store  of  potential  energy,  but 
something  more — it  is  a  machine  for  the  disposal  of  it  in  cer- 
tain ways;  and,  wherever  practicable,  it  is  clearly  advanta- 
geous to  have  the  purely  energy-expending  parts  made  of 
non-oxidizable  matters,  and  so  protected  from  change  and 
the  necessity  of  frequent  renewal.  The  Body  is  a  self-build- 
ing and  self-repairing  machine,   and   the    material   for  this 


FOODS.  317 

building  and  repair  must  be  supplied  in  the  food,  as  well  as 
the  fuels,  or  oxidizable  foods,  which  yield  the  energy  the 
machine  expends;  and  while  experience  shows  us  that  even 
for  machinery  construction  oxidizable  matters  are  largely 
needed,  it  is  nevertheless  a  gain  to  replace  them  by  non-oxi- 
dizable  substances  when  possible;  just  as  if  practicable  it 
would  be  advantageous  to  construct  an  engine  out  of  mate- 
rials which  would  not  rust,  although  other  conditions  deter- 
mine the  use  of  iron  for  the  greater  part  of  it. 

Definition  of  Foods.  Foods  may  be  defined  as  substances 
which,  when  taken  into  the  alimentary  canal,  are  absorbed 
from  it,  and  then  serve  either  to  supply  material  for  the 
growth  of  the  Body,  or  for  the  replacement  of  matter  which 
has  been  removed  from  it,  either  after  oxidation  or  without 
having  been  oxidized.  Foods  to  replace  matters  which  have 
been  oxidized  must  be  themselves  oxidizable;  they  axe  force- 
generators,  but  may  be  and  generally  are  also  tissue-formers; 
and  are  nearly  always  complex  organic  substances  derived 
from  other  animals  or  from  plants.  Foods  to  replace  matters 
not  oxidized  in  the  Body  are  force-regulators,  and  are  for  the 
most  part  tolerably  simple  inorganic  compounds.  Among 
the  force-regulators  we  must,  however,  include  certain  organic 
foods  which,  although  oxidized  in  the  Body  and  serving  as 
liberators  of  energy,  yet  produce  effects  totally  dispropor- 
tionate to  the  energy  they  set  free,  and  for  which  effects  they 
are  taken.  In  other  words,  their  influence  as  stimuli  in  excit- 
ing certain  tissues  to  liberate  energy,  or  as  inhibitory  agents 
checking  the  activity  of  parts,  is  more  marked  than  their 
direct  action  as  force -generators.  As  examples,  we  may  take 
condiments:  mustard  and  pepper  are  not  of  much  use  as 
sources  of  energy,  although  they  no  doubt  yield  some;  we 
take  them  for  their  stimulating  effect  on  the  mouth  and 
other  parts  of  the  alimentary  canal,  by  which  they  promote 
an  increased  flow  of  the  digestive  secretions  or  an  increased 
appetite  for  food.  Thein  and  caffein,  the  active  principles  of 
tea  and  coffee,  are  taken  for  their  stimulating  effect  on  the 
nervous  system,  rather  than  for  the  amount  of  energy  yielded 
by  their  own  oxidation. 

Conditions  which  a  Food  must  Fulfil.  (].)  A  food 
must  contain  the  elements  which  it  is  to  replace  in  the  Body: 
but  that  alone  is  not  sufficient.  The  elements  leaving  the 
Body  being  usually  derived  from  the  breaking  down  of  com- 


3 1 8  THE  HUMAN  BOD  ) . 

plex  substances  in  it,  the  food  must  contain  them  either  in 
the  form  of  such  complex  substances,  or  in  forms  which  the 
Body  can  build  up  into  them,  Free  nitrogen  and  hydrogen 
arc  no  use  as  foods,  since  they  are  neither  oxidizable  under 
the  conditions  prevailing  in  the  Body  (and  consequently  fan- 
not  yield  it  energy),  nor  are  they  capable  of  construction  by 
it  into  its  tissues.  (2)  Pood  after  it  has  been  swallowed  is 
still  in  a  strict  sense  outside  the  Body;  the  alimentary  canal 
is  merely  a  tube  running  through  it,  and  so  long  as  food  lies 
there  it  does  not  form  any  part  of  the  Body  proper.  Hence 
foods  must  be  capable  of  absorption  from  the  alimentary 
canal;  either  directly,  or  after  they  have  been  changed  by  the 
processes  of  digestion.  Carbon,  for  example,  is  useless  as 
food,  not  merely  because  the  Body  could  not  build  it  up  into 
its  own  tissues,  but  because  it  cannot  be  absorbed  from  the 
alimentary  canal.  (3)  Neither  the  substance  itself  nor  any 
of  the  products  of  its  transformation  in  the  Body  must  be 
injurious  to  the  structure  or  activity  of  any  organ.  If  so  it 
is  a,  poison,  not  a  food. 

Alimentary  Principles.  The  articles  which  in  common 
language  we  call  foods  are,  in  most  cases,  mixtures  of  several 
foodstuffs,  with  substances  which  are  not  foods  at  all.  Bread, 
for  example,  contains  water,  salts,  gluten  (a  proteid),  some 
fats,  much  starch,  and  a  little  sugar;  all  true  foodstuffs:  but 
mixed  with  these  is  a  quantity  of  cellulose  (the  chief  chemical 
constituent  of  the  walls  which  surround  vegetable  cells),  and 
this  is  not  a  food  since  it  is  incapable  of  absorption  from  the 
alimentary  canal.  Chemical  examination  of  all  the  common 
articles  of  diet  shows  that  the  actual  number  of  important 
foodstuffs  is  but  small:  they  are  repeated  in  various  propor- 
tions in  the  different  things  we  eat,  mixed  with  small  quan- 
tities of  different  flavoring  substances,  and  so  give  us  a  pleas- 
ing variety  in  our  meals;  but  the  essential  substances  are 
much  the  same  in  the  fare  of  the  workman  and  in  the 
"  delicacies  of  the  season."  These  primary  foodstuffs,  which 
are  found  repeated  in  so  many  different  foods,  are  known  as 
" alimentary  principles";  and  the  physiological  value  of  any 
article  of  diet  depends  on  them  far  more  than  on  the  traces 
of  flavoring  matters  which  cause  certain  things  to  be  espe- 
cially sought  after  and  so  raise  their  market  value.  The 
alimentary   principles   may   be    conveniently   classified   into 


FOODS.  319 

proteids,  albuminoids,  hydrocarbons,  carbohydrates,  and  inor- 
ganic bodies. 

Proteid  or  Albuminous  Alimentary  Principles.  Of  the 
nitrogenous  foodstuffs  the  most  important  are  proteids:  they 
form  an  essential  part  of  all  diets,  and  are  obtained  both  from 
animals  and  plants.  The  most  common  and  abundant  are 
myosin  and  syntonin,  which  exist  in  the  lean  of  all  meats;  egg 
albumen;  casein,  found  in  milk  and  cheese;  gluten  and  vege- 
table casein  from  various  plants. 

Gelatinoid  or  Albuminoid  Alimentary  Principles.  These 
also  contain  nitrogen,  but  cannot  replace  the  proteids  entirely 
as  foods;  though  a  man  can  get  on  with  less  proteids  when  he 
has  some  albuminoids  in  addition.  The  most  important  is 
gelatin,  which  is  yielded  by  the  white  fibrous  tissue  of  animals 
when  cooked.  On  the  whole  the  gelatinoids  are  not  foods  of 
high  value,  and  the  calf's-foot  jelly  and  such  compounds, 
often  given  to  invalids,  have  not  nearly  the  nutritive  value 
they  are  commonly  supposed  to  possess. 

Hydrocarbons  (Fats  and  Oils).  The  most  important  are 
stearin,  palmatin,  and  olein,  which  exist  in  various  propor- 
tions in  animal  fats  and  vegetable  oils;  the  more  fluid  contain- 
ing more  olein.  Butter  contains  also  a  little  of  a  fat  named 
butyrin.  Fats  are  compounds  of  glycerine  and  fatty  acids, 
and  any  such  substance  which  is  fusible  at  the  temperature 
of  the  Body  will  serve  as  a  food.  The  stearin  of  beef  and 
mutton  fats  is  not  by  itself  fusible  at  the  body  temperature, 
but  is  mixed  in  those  foods  with  so  much  olein  as  to  be  melted 
in  the  alimentary  canal.  Beeswax,  on  the  other  hand,  is  a 
fatty' body  which  will  not  melt  in  the  intestines  and  so  passes 
on  unabsorbed;  although  from  its  composition  it  would  be 
useful  as  a  food  could  it  be  digested.  A  distinction  is  some- 
times made  between  fats  proper  (the  adipose  tissue  of  ani- 
mals consisting  of  fatty  compounds  inclosed  in  albuminous 
cell-walls)  and  oils,  or  fatty  bodies  which  are  not  so  organized. 

Carbohydrates.  These  are  mainly  of  vegetable  origin. 
The  most  important  are  starch,  found  in  nearly  all  vegetable 
foods  ;  dextrin  ;  gums  ;  grape-sugar,  called  also  dextrose  or 
glucose  (into  which  starch  is  converted  during  digestion);  and 
cane-sugar.  Sugar  of  milk  and  glycogen  are  alimentary  prin- 
ciples of  this  group,  derived  from  animals.  All  of  them,  like 
the  fats,  consist  of  carbon,  hydrogen  and  oxygen;  but  the  per- 


320  THE  HUMAN  BODY. 

centage  of  oxygen  in  them  is  much  higher,  there  being 
one  atom  of  oxygen  for  every  two  of  hydrogen  in  their 
molecule. 

Inorganic  Foods.  Water;  common  salt;  and  the  chlo- 
rides, phosphates,  and  sulphates  of  potassium,  magnesium 
and  calcium.  More  or  less  of  these  bodies,  or  the  materials 
for  their  formation,  exists  in  all  ordinary  articles  of  diet,  so 
that  we  do  not  swallow  them  in  a  separate  form.  Phosphates, 
for  example,  exist  in  nearly  all  animal  and  vegetable  foods; 
while  other  foods,  as  casein,  contain  phosphorus  in  combina- 
tions which  in  the  Body  yield  it  up  to  be  oxidized  to  form 
phosphoric  acid.  The  same  is  true  of  sulphates,  which  are 
partially  swallowed  as  such  in  various  articles  of  diet,  and  are 
partly  formed  in  the  Body  by  the  oxidation  of  the  sulphur  of 
various  proteids.  Calcium  salts  are  abundant  in  bread,  and 
are  also  found  in  many  drinking-waters.  "Water  and  table- 
salt  form  exceptions  to  the  rule  that  inorganic  bodies  are 
eaten  imperceptibly  along  with  other  things,  since  the  Body 
loses  more  of  each  daily  than  is  usually  supplied  in  that  way. 
It  has,  however,  been  maintained  that  salt,  as  such,  is  an 
unnecessary  luxury;  and  there  seems  some  evidence  that 
certain  savage  tribes  live  without  more  than  they  get  in  the 
meat  and  vegetables  they  eat.  Such  tribes  are,  however, 
said  to  suffer  especially  from  intestinal  parasites;  and  there 
is  no  doubt  that  to  civilized  man  the  absence  of  salt  is  a  great 
privation. 

Calcium  seems  to  be  an  essential  constituent  of  all  living 
cells  and  in  some  way  closely  connected  with  the  manifestation 
of  their  activity.  As  previously  mentioned  the  heart  of  a 
frog  after  thorough  irrigation  with  dilute  solution  of  sodium 
chloride  ceases  to  beat,  but  resumes  its  pulsations  when  a 
minute  trace  of  calcium  chloride  is  added  to  the  solution; 
and  while  ordinary  serum  restores  the  beat  of  such  a  washed- 
out  heart,  serum  from  which  all  its  calcium  has  been  removed 
does  not.  Moreover  if  defibrinated  blood  to  which  a  little 
more  sodium  oxalate  than  is  sufficient  to  precipitate  all  its 
calcium  has  been  added,  be  circulated  through  the  vessels  of  a 
muscle,  the  latter  loses  its  contractility,  apparently  because 
the  slight  excess  of  oxalate  precipitates  the  calcium  of  the 
muscle-fibres;  for  the  contractility  may  be  restored  by  sup- 
plying some  dissolved  calcium  chloride.  Nerves  treated  simi- 
larly lose   their  irritability;    and  the  eggs  of   some   aquatic 


FOODS.  321 

animals  will  not  develop  normally  in  water  from  which  all 
calcium  salts  have  been  removed. 

Mixed  Foods.  These,  as  already  pointed  out,  include 
nearly  all  common  articles  of  diet;  they  contain  more  than 
one  alimentary  principle.  Among  them  we  find  great  differ- 
ences; some  being  rich  in  pvoteids,  others  in  starch,  others  in 
fats,  and  so  on.  The  formation  of  a  scientific  dietary  depends 
on  a  knowledge  of  these  characteristics.  The  foods  eaten  by 
man  are,  however,  so  varied  that  we  cannot  do  more  than 
consider  the  most  important. 

Flesh.  This,  whether  derived  from  bird,  beast,  or  fish, 
consists  essentially  of  the  same  things — muscular  fibres, 
connective  tissue  and  tendons,  fats,  blood-vessels,  and  nerves. 
It  contains  several  proteids,  especially  myosin;  gelatin-yield- 
ing matters  in  the  white  fibrous  tissue;  stearin,  palmatin, 
and  olein  as  representatives  of  the  fats;  and  a  small  amount  of 
carbohydrates  in  the  form  of  glycogen  and  grape-sugar,  or 
some  chemically  allied  substances.  Flesh  also  contains  much 
water  and  a  considerable  number  of  salines,  the  most  important 
and  abundant  being  potassium  phosphate.  Osmazome  is  a 
crystalline  nitrogenous  body  which  gives  much  of  its  taste  to 
flesh:  and  small  quantities  of  various  similar  substances 
exist  in  different  kinds  of  meat.  There  is  also  more  or  less 
yellow  elastic  tissue  in  flesh;  it  is  indigestible  and  useless  as 
food. 

When  meat  is  cooked  its  white  fibrous  tissue  is  turned 
into  gelatin,  and  the  whole  mass  becomes  thus  softer  and 
more  easily  disintegrated  by  the  teeth.  When  boiled  some 
of  the  proteid  matters  of  the  meat  pass  out  into  the  broth, 
and  there  in  part  coagulate  and  form  the  scum  :  this  loss  may 
be  prevented  in  great  part  by  putting  the  raw  meat  at  once 
into  boiling  water  which  coagulates  the  surface  albumen  be- 
fore it  dissolves  out,  and  this  keeps  in  the  rest,  while  the 
subsequent  cooking  is  continued  slowly.  In  any  case  the 
myosin,  being  insoluble  in  water,  remains  behind  in  the  boiled 
meat.  In  baking  or  roasting,  all  the  solid  parts  of  the  flesh  are 
preserved  and  certain  agreeably  flavored  bodies  are  produced, 
as  to  the  nature  of  which  little  is  known. 

Eggs.  These  contain  a  large  amount  of  egg  albumen 
and,  in  the  yolk,  another  proteid,  known  as  vitellin.  Also 
fats,  and  a  substance  known  as   lecithin,  which    is  important 

containing  a  considerable  quantity  of  phosphorus.    Leci- 


322  THE  J I  [MAN  BODY. 

thin,  or  rather  a  substance  yielding  it,  is  an  important  con- 
stituent of  the  nervous  tissues. 

Milk  contains  a  proteid,  caseinogen;  several  fats  in  the 
butter;  a  carbohydrate,  milk-sugar;  much  water;  and  salts, 
especially  potassium  and  calcium  phosphates.  Butter  consists 
mainly  of  the  same  fats  as  those  iu  beef  and  mutton;  but  has 
in  it  about  one  per  cent  of  a  special  fat,  butyrin.  In  the  milk 
it  is  disseminated  in  the  form  of  minute  globules  which,  for 
the  most  part,  float  up  to  the  top  when  the  milk  is  let  stand 
and  then  form  the  cream.  In  this  each  fat-droplet  is  sur- 
rounded by  a  pellicle  of  albuminous  matter;  by  churning, 
these  pellicles  are  broken  up  and  the  fat-droplets  then  run  to- 
gether to  form  the  butter.  Caseinogen  is  insoluble  in  water; 
in  milk  it  is  dissolved  by  the  alkaline  salts  present.  When 
milk  is  kept,  its  sugar  ferments  and  gives  rise  to  lactic  acid, 
which  neutralizes  the  alkali  and  precipitates  the  caseinogen 
as  curds.  In  cheese-making  the  caseinogen  is  acted  upon  by 
a  ferment  (rennin)  present  in  the  extract  of  stomach  used, 
and  converted  into  tyrein  which  is  precipitated:  this  clotting 
does  not  take  place  unless  a  calcium  salt  be  present.  Tyrein, 
which  forms  the  main  bulk  of  a  true  cheese,  is  different  from 
the  curd  precipitated  from  milk  by  acids;  cheese  made  from 
the  latter  does  not  "  ripen."'  Caseinogen  is  frequently  called 
casein,  which  name  should  be  given  to  the  tyrein  formed  from 
caseinogen  by  ferment  action. 

Vegetable  Foods.  Of  these  wheat  affords  the  best.  In 
1000  parts  it  contains  135  of  proteids,  568  of  starch,  46  of 
dextrin  (a  carbohydrate),  49  of  grape-sugar,  19  of  fats,  and 
32  of  cellulose,  the  remainder  being  water  and  salts.  The 
proteid  of  wheat  is  mainly  gluten,  which  when  moistened 
with  water  forms  a  tenacious  mass,  and  this  it  is  to  which 
wheaten  bread  owes  its  superiority.  When  the  dough  is 
made  yeast  is  added  to  it,  and  produces  a  fermentation  by 
which,  among  other  things,  carbon  dioxide  gas  is  produced. 
This  gas,  imprisoned  in  the  tenacious  dough,  and  expanded 
during  baking,  forms  cavities  in  it  and  causes  it  to  "rise" 
and  make  "light  bread,"  which  is  not  only  more  pleasant  to 
eat  but  more  digestible  than  heavy.  Other  cereals  may  con- 
tain a  larger  percentage  of  starch,  but  none  have  so  much 
gluten  as  wheat;  when  bread  is  made  from  them  the  carbon 
dioxide  gas  escapes  so  readily  from  the  less  tenacious  dough 
that  it  does  not  expand  the  mass  properly.     Corn  contains  in 


FOODS.  323 

1000  parts,  79  of  proteids,  037  of  starch,  and  from  50  to  87 
of  fats;  much  more  than  any  other  kind  of  grain.  Rice  is 
poor  in  proteids  (50  parts  in  1000)  but  very  rich  in  starch 
(823  parts  in  1000).  Peas  and  beans  are  rich  in  proteids 
(from  220  to  200  parts  in  1000),  and  contain  about  half  their 
weight  of  starch.  Potatoes  are  a  poor  food.  They  contain  a 
great  deal  of  water  and  cellulose,  and  only  about  13  parts  of 
proteids  and  15-1  of  starch  in  1000.  Other  fresh  vegetables, 
as  carrots,  turnips,  and  cabbages,  are  valuable  mainly  for  the 
salts  they  contain;  their  weight  is  mainly  due  to  water,  and 
they  contain  but  little  starch,  proteids,  or  fats.  Fruits,  like 
most  fresh  vegetables,  are  mainly  valuable  for  their  saline 
constituents,  the  other  foodstuffs  in  them  being  onlv  present 
in  small  proportion.  Some  fruit  or  vegetable  is,  however,  a 
necessary  article  of  diet ;  as  shown  by  the  scurvy  which  used 
to  prevail  among  sailors  before  fresh  or  canned  vegetables 
and  lime-juice  were  supplied  to  them. 

The  Cooking  of  Vegetables.  This  is  of  more  importance 
even  than  the  cooking  of  flesh,  since  in  most  the  main  ali- 
mentary principle  is  starch,  and  raw  starch  is  difficult  of 
digestion.  In  plants  starch  is  nearly  always  stored  up  in  the 
form  of  solid  granules,  which  consist  of  alternating  layers  of 
starch  cellulose  and  starch  rjranalose.  The  digestive  fluids 
turn  the  starch  into  sugars  which  are  soluble  and  can  be 
absorbed  from  the  alimentary  canal,  while  starch  itself  can- 
not. These  fluids  act  slowly  and  imperfectly  on  raw  starch, 
and  then  only  on  the  granulose;  but  when  boiled,  the  starch 
granules  swell  up,  and  become  more  readily  converted  into 
sugars,  and  the  starch  cellulose  is  so  altered  that  it  too  un- 
dergoes that  change.  When  starch  is  roasted  it  is  in  part 
turned  into  a  substance  known  as  soluble  starch  which  is  read- 
ily dissolved  in  the  alimentary  canal.  There  is,  therefore,  a 
scientific  foundation  for  the  common  belief  that  the  crust  of 
a  loaf  is  more  digestible  than  the  crumb,  and  toast  than  ordi- 
nary bread. 

Alcohol.  There  are  perhaps  no  common  articles  of  diet 
concerning  which  more  contradictory  statements  have  been 
made  than  alcoholic  drinks.  This  depends  upon  their  pe- 
culiar position :  according  to  circumstances  alcohol  may  be  a 
poison  or  be  useful;  when  useful  it  maybe  regarded  either 
as  a  force-regulator  or  a  force-generator.  It  is  sometimes  a 
valuable  medicine,  but  it  does  no  good  to  the  healthy  body. 


324  THE  HUMAN  BODY. 

If  not  more  than  two  ounces  (which  would  be  contained  in 
about  four  ounces  of  whiskey  or  two  quarts  of  lager-beer)  are 
taken  in  the  twenty-four  hours,  they  arc  completely  oxidized 
in  the  Body  and  excreted  as  wain-  and  carbon  dioxide.  In 
this  oxidation  energy  is  of  course  liberated  and  can  be  util- 
ized. Commonly,  however,  alcohol  is  not  taken  for  this  pur- 
pose but  as  ;i  force-regulator,  for  its  influence  on  the  nervous 
system  or  digestive  organs,  and  it  is  in  this  capacity  that  it 
becomes  dangerous.  For  not  oidy  may  it  be  taken  in  quan- 
tities so  great  that  it  is  not  at  all  oxidized  in  the  Body  but  is 
passed  through  it  as  alcohol,  or  even  that  it  acts  as  a  narcotic 
poison  instead  of  a  stimulant,  but  when  taken  in  what  is 
called  moderation  there  can  be  no  doubt  that  the  constant 
"  whipping  up"  of  the  flagging  organs,  if  continued,  must  be 
dangerous  to  their  integrity.  Hence  the  daily  use  of  alcohol 
merely  in  such  quantities  as  to  produce  slight  exhilaration  or 
to  facilitate  work  is  by  no  means  safe;  though  in  disease 
when  the  system  wants  rousing  to  make  some  special  effort, 
the  physician  cannot  dispense  with  it  or  some  other  similarly 
acting  substance.  In  fact,  as  a  force-generator  alcohol  may 
be  advantageously  replaced  by  other  foods  in  nearly  all  cases; 
and  there  is  no  evidence  that  it  helps  in  the  construction  of 
the  working  tissues,  though  its  excessive  use  often  leads  to  an 
abnormal  accumulation  of  fat.  Its  proper  use  is  as  a  "  wdiip," 
and  one  has  no  more  right  to  use  it  to  the  healthy  Body  than 
the  lash  to  overdrive  a  willing  horse.  The  physician  is  the 
proper  person  to  determine  whether  it  is  wanted  under  any 
given  circumstances. 

If  alcohol  is  used  as  a  daily  article  of  diet  it  should  be 
borne  in  mind  that  when  concentrated  it  may  chemically  alter 
the  proteids  of  the  cells  of  the  stomach  with  which  it  conies 
in  contact,  in  the  same  sort  of  way,  though  of  course  to  a 
much  less  degree,  as  it  shrivels  and  dries  up  an  animal  pre- 
served in  it.  Dilute  alcoholic  drinks,  such  as  claret  and  beer, 
are  therefore  far  less  baneful  than  whiskey  or  brandy,  and 
these  are,  so  far  as  direct  action  on  the  stomach  is  con- 
cerned, worse  the  less  they  are  diluted.  For  the  same  reason 
alcoholic  drinks  are  far  more  injurious  on  an  empty  stomach 
than  after  a  meal.  When  the  stomach  is  full  the  liquor 
is  diluted,  is  more  slowly  absorbed,  and,  moreover,  is  largely 
used  up  in  coagulating  the  proteids  of  the  food  instead 
of  those  of  the  gastric  lining  membrane.     The  old  "  three 


foods.  325 

bottle  "  men  who  drank  their  port-wine  after  a  heavy  dinner, 
got  off  far  more  safely  than  the  modern  tippler  who  is  taking 
"  nips "  all  day  long,  although  the  latter  may  imbibe  a 
smaller  quantity  of  alcohol  in  the  twenty-four  hours.  By  far 
the  best  way,  however,  is  to  avoid  alcohol  altogether  in  health. 
If  the  facts  lead  us  to  conclude  that  under  some  conditions 
it  may  be  to  a  certain  extent  a  food,  it  is  a  dangerous  one: 
even  in  what  we  may  call  "physiological "  quantities,  or  such 
amounts  as  can  be  totally  oxidized  in  the  Body. 

The  Advantage  of  a  Mixed  Diet.  The  necessary  quan- 
tity of  daily  food  depends  upon  that  of  the  material  daily  lost 
from  the  Body,  and  this  varies  both  in  kind  and  amount  with 
the  energy  expended  and  the  organs  most  used.  In  children 
a  certain  excess  beyond  this  is  required  to  furnish  materials 
for  growth.  Although  it  is  impossible  to  lay  down  with  per- 
fect accuracy  how  much  daily  food  any  individual  requires, 
still  the  average  quantity  may  be  derived  from  the  table  of 
daily  losses  given  on  page  300,  which  shows  that  a  healthy 
man  needs  daily  in  assimilable  forms  about  274  grams  (4220 
grains)  of  carbon  and  19  grams  (292  grains)  of  nitrogen. 
The  daily  loss  of  hydrogen,  which  is  very  great  (352  grams 
or  5428  grains),  is  for  the  most  part  made  good  by  water  which 
has  been  drunk  and,  so  to  speak,  merely  filtered  through  the 
Body,  after  having  assisted  in  the  solution  and  transference 
through  it  of  other  substances.  About  300  grams  (4620 
grains)  of  water  containing  33.3  grams  (513  grains)  of  hy- 
drogen are,  however,  formed  in  the  Body  by  oxidation,  and 
the  hydrogen  for  this  purpose  must  be  supplied  in  the  form 
of  some  oxidizable  foodstuff,  whether  proteid,  fat,  or  carbo- 
hydrate. The  oxygen  eliminated  is  mainly  received  from  the 
air  through  the  lungs,  but  some  is  taken  in  combination  in 
the  food. 

Since  proteid  foods  contain  carbon,  nitrogen  and  hydro- 
gen, life  may  be  kept  up  on  them  alone,  with  the  necessary 
salts,  water  and  oxygen;  but  such  a  form  of  feeding  would 
be  anything  but  economical.  Ordinary  proteids  contain  in 
100  parts  (p.  9)  about  52  of  carbon  and  15  of  nitrogen,  so  a 
man  fed  on  them  alone  would  get  about  3.1  parts  of  carbon 
\ov  every  1  of  nitrogen.  His  daily  losses  are  not  in  this  ratio, 
but  about  that  of  274  grama  (4220  grains)  of  carbon  to  20 
grama  (308  grains)  of  nitrogen,  or  as  13.7  to  1;  and  so  to  get 
enough   carbon  from   proteids   far  more   than   the  necessary 


326  THE  HUMAN  BODY. 

amount   of   nitrogen    must  be  taken.     Of   dry  proteids  527 

grams  (SI  16  grains)  would  yield  the  necessary  carbon,  but 
would  contain  79  grams  (1217  grains)  of  nitrogen;  or  four 
times  more  than  is  required  to  cover  the  necessary  daily 
losses  of  that  element.  Fed  on  a  purely  proteid  diet  a  man 
would,  therefore,  have  to  digest  a  vast  quantity  to  get  enough 
carbon,  and  in  eating  and  absorbing  it,  as  well  as  in  getting 
rid  of  the  extra  nitrogen  which  is  useless  to  him,  a  great  deal 
of  unnecessary  labor  would  be  thrown  upon  the  various  or- 
gans of  his  Body.  Similarly,  if  a  man  were  to  live  on  bread 
alone  he  would  burden  his  organs  with  much  useless  work. 
For  bread  contains  but  little  nitrogen  in  j)roportion  to  its 
carbon,  and  so,  to  get  enough  of  the  former,  far  more  carbo- 
naceous substances  than  could  be  utilized  would  have  to  be 
eaten,  digested  and  eliminated  daily. 

Accordingly,  we  find  that  mankind  in  general  employ  a 
mixed  diet  when  they  can  get  it,  using  richly  proteid  sub- 
stances to  supply  the  nitrogen  needed,  but  deriving  the  car- 
bon mainly  from  non-nitrogenous  foods  of  the  fatty  or  carbo- 
hydrate groups,  and  so  avoiding  excess  of  either.  For  instance, 
lean  beef  contains  about  \  of  its  weight  of  dry  proteid,  which 
contains  15  per  cent  of  nitrogen.  Consequently  the  133 
grams  (2048  grains)  of  proteid  which  would  be  found  in  532 
grams  (1  lb.  3  oz.)  of  lean  meat  would  supply  all  the  nitrogen 
needed  to  compensate  for  a  day's  losses.  But  the  proteid 
contains  52  per  cent  of  carbon,  so  the  amount  of  it  in  the 
above  weight  of  fatless  meat  would  be  09  grams  (1062  grains) 
of  carbon,  leaving  205  grams  (3157  grains)  to  be  got  either 
from  fats  or  carbohydrates.  The  necessary  amount  would  be 
contained  in  about  256  grams  (3942  grains)  of  ordinary  fats 
or  460  grams  (7084  grains)  of  starch;  hence  either  of  these, 
with  the  above  quantity  of  lean  meat,  would  form  a  far  better 
diet,  both  for  the  purse  and  the  system,  than  the  meat  alone. 

As  already  pointed  out,  nearly  all  common  foods  contain 
several  foodstuffs.  Good  butcher's  meat,  for  example,  con- 
tains nearly  half  its  dry  weight  of  fat;  and  bread,  besides 
proteids,  contains  starch,  fats  and  sugar.  In  none  of  them, 
however,  are  the  foodstuffs  mixed  in  the  physiologically  best 
proportions,  and  the  practice  of  employing  several  of  them  at 
each  meal,  or  different  ones  at  different  meals,  during  the  day, 
is  thus  not  only  agreeable  to  the  palate  but  in  a  high  degree 
advantageous  to  the  Body.     The  strict  vegetarians  who  do 


FOODS.  327 

not  employ  even  such  substances  as  eggs,  cheese  and  milk, 
but  confine  themselves  to  a  purely  vegetable  diet  (such  as  is 
always  poor  iu  proteids),  daily  take  far  more  carbon  than  they 
require,  and  are  to  be  congratulated  on  their  excellent  diges- 
tions which  are  able  to  stand  the  strain.  Those  who  use  eggs, 
cheese,  etc.,  can  of  course  get  on  very  well,  since  such  sub- 
stances are  extremely  rich  in  proteids,  and  supply  the  nitro- 
gen needed  without  the  necessity  of  swallowing  the  vast  bulk 
of  food  which  must  be  eaten  in  order  to  get  it  from  plants 
directly. 


CHAPTER   XXII. 

THE   ALIMENTARY   CANAL   AND   ITS   APPENDAGES. 

G-eneral  Arrangement.  The  alimentary  canal  is  essen- 
tially a  tube  running  through  the  Body  (Fig.  2)  and  lined  by 
a  vascular  membrane,  most  of  which  is  specially  adapted  for 
absorption;  it  communicates  with  the  exterior  at  three  points 
(the  nose,  the  mouth,  and  the  anal  aperture),  at  which  the 
lining  mucous  membrane  is  continuous  with  the  general  outer 
integument.  Supporting  the  absorbent  membrane  are  layers 
which  strengthen  the  tube,  and  are  in  part  muscular  and,  by 
their  contractions,  serve  to  pass  materials  along  it  from  one 
end  to  the  other.  In  the  walls  of  the  canal  are  numerous 
blood  and  lymphatic  vessels  which  carry  off  the  matters  ab- 
sorbed from  its  cavity;  and  there  also  exist  in  connection  with 
it  numerous  glands,  whose  function  it  is  to  pour  into  it  various 
secretions  which  exert  a  solvent  influence  on  such  foodstuffs  as 
would  otherwise  escape  absorption.  Some  of  these  glands  are 
minute  and  imbedded  in  the  walls  of  the  alimentary  tube  it- 
self, but  others  (such  as  the  salivary  glands)  are  larger  and  lie 
away  from  the  main  channel,  into  which  their  products  are 
carried  by  ducts  of  various  lengths. 

The  alimentary  tube  is  not  uniform  but  presents  several 
dilatations  on  its  course;  nor  is  it  straight,  since,  being  much 
longer  than  the  Body,  a  large  part  of  it  is  packed  away  by 
being  coiled  up  in  the  abdominal  cavity. 

Subdivisions  of  the  Alimentary  Canal.  The  mouth- 
opening  leads  into  a  chamber  containing  the  teeth  and 
tongue,  the  mouth-chamber  or  buccal  cavity.  This  is  suc- 
ceeded by  the  pharynx  or  throat-cavity,  which  narrows  at 
the  top  of  the  neck  into  the  gullet  or  oesophagus;  this  runs 
down  through  the  thorax  and,  passing  through  the  dia- 
phragm, dilates  in  the  upper  part  of  the  abdominal  cavity 
into  the  stomach.  Beyond  the  stomach  the  channel  again 
narrows  to  form  a  long  and  greatly  coiled  tube,  the  small 
intestine,  wdiich  terminates  by  opening  into  the  large  intes- 

328 


THE  ALIMENTARY  CANAL   AND  ITS  APPENDAGES.     329 


tine,  much  shorter  although  wider  than  the  small,  and  ter- 
minating   by   an    opening    on 
the  exterior. 

The  Mouth  -  cavity  (Fig. 
105)  is  bounded  in  front  and 
on  the  sides  by  the  lips  and 
cheeks,  below  by  the  tongue, 
h,  and  above  by  the  palate; 
which  latter  consists  of  an  an- 
terior part,  /,  supported  by 
bone  and  called  the  hard  pal- 
ate, and  a  posterior,  /,  con- 
taining no  bone,  and  called 
the  soft  palate.  The  two  can 
readily  be  distinguished  by  ap- 
plying the  tip  of  the  tongue 
to  the  roof  of  the  mouth  and 
drawing  it  backwards.  The 
hard  palate  forms  the  parti- 
tion between  the  mouth  and 
nose.  The  soft  palate  arches 
down  over  the  back  of  the 
mouth,  hanging  like  a  cur- 
tain between  it  and  the  pharynx, 
as  can  be  seen  by  holding  the 
mouth  open  in  front  of  a 
looking-glass.  From  the  mid- 
dle of  its  free  border  a  conical 
process,  the  uvula,  hangs 
down. 

The  Teeth.  Immediately 
within  the  cheeks  and  lips  are  two  semicircles,  formed  by  the 
borders  of  the  upper  and  lower  jaw-bones,  which  are  covered 
by  the  gums,  except  at  intervals  along  their  edges  where 
they  contain  sockets  in  which  the  teeth  are  implanted. 
During  life  two  sets  of  teeth  are  developed;  the  first  or  milk 
set  appears  soon  after  birth  and  is  shed  during  childhood, 
when  the  second  or  permanent  set  appears. 

The  teeth  differ  in  minor  points  from  one  another,  but 
in  each  three  parts  are  distinguishable;  one,  seen  in  the  mouth 
and  called  the  crown  of  the  tooth;  a  second,  imbedded  in  the 
jaw-bone  and   called  the  root  or  fang;  and  between  the  two, 


Fig.  105. — The  mouth,  nose  and  pha- 
rynx, with  the  commencement  of  the 
gullet  and  larynx,  as  exposed  by  a 
section,  a  little  to  the  left  of  the  me- 
dian plane  of  the  head.  a.  vertebral 
column  ;  b,  gullet  :  c,  windpipe  ;  d, 
larynx  :  e,  epiglottis  :  /.  soft  palate  ; 
fir,  opening  of  Eustachian  tube  ;  k, 
tongue  ;  I,  hard  palate  ;  to,  the  sphe- 
noid bone  on  the  base  of  the  skull;  n, 
the  fore  part  of  the  cranial  cavity ; 
<>.  p,  q,  the  tubinate  bones  of  the  out- 
er side  of  the  left  nostril-chamber. 


330 


THE  111  MAX  BODY. 


embraced  by  the  edge  of  the  gum,  is  a  narrowed  portion,  the 
neck  or  cervix.  From  differences  in  their  forms  and  uses 
the  teeth  are  divided  into  incisors,  canines,  bicuspids  and 
molars,  arranged  in  a  definite  order  in  each  jaw.     Beginning 

at  the  middle  line  we  meel  in  cadi  half  of  each  jaw  with, 
successively,  two  incisors,  one  canine,  and  two  molars  in  the 
milk  set;  making  twenty  altogether  in  the  two  jaw.-'.  The 
teeth  of  the  permanent  set  are  thirty-two  in  number,  eight  in 
each  half  of  each  jaw,  viz. — beginning  at  the  middle  line — 
two  incisors,  one  canine,  two  bicuspids,  and  three  molars. 
The  bicuspids,  or  premolars,  of  the  permanent  set  replace  the 
milk  molars,  while  the  permanent  molars  are  new  teeth  added 
on  as  the  jaw  grows,  and  not  substituting  any  of  the  milk- 
teeth.  The  hindmost  permanent  molars  are  often  called  the 
wisdom-teeth. 

Characters  of  Individual  Teeth.  The  incisors  (Fig.  L06) 
are  adapted  for  cutting  the  food.  Their  crowns  are  chisel- 
shaped  and  have  sharp  horizontal  cutting  edges,  which  be- 
come worn  away  by  use  so  that  they  are  bevelled  off  behind 
in  the  upper  row,  and  in  the  opposite  direction  in  the  lower. 
Each  has  a  single  long  fang.  The  canines  (Fig.  107)  are 
somewhat  larger  than  the  incisors.  Their  crowns  are  thick 
and  somewhat  conical,  having  a  central  point  or  cusp  on  the 
cutting  edge.  In  dogs,  cats  and  other  carnivora  the  canines 
are  very  large  and  adapted  for  seizing  and  holding  prey. 
The  bicuspids  or  premolars  (Fig.  108)  are  rather  shorter  than 


Fig.  106. 


Fig.  10 


Fig.  1 


Fig.  109. 


Fig.  106.— An  incisor  tooth. 
Fig.  107. — A  canine  or  eye  tooth. 

Fig.  108.— A  bicuspid  tooth  seen  from  its  outer  side;  the  inner  cusp  is,  accord- 
ingly, not  visible. 

Fig.  109.— A  molar  tooth. 


the  canines  and  their  crowns  are  somewhat  cuboidal.  Each 
has  two  cusps,  an  outer  towards  the  cheek,  and  an  inner  on 
the  side  turned  towards  the  interior  of  the  mouth.     The  fang 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES.     331 

is  compressed  laterally,  and  has  usually  a  groove  partially 
subdividing  it  into  two.  At  its  tip  the  separation  is  often 
complete.  The  molar  teeth  or  grinders  (Fig.  109)  have  large 
crowns  with  broad  surfaces,  on  which  are  four  or  five  project- 
ing tubercles,  which  roughen  them  and  make  them  better 
adapted  to  crush  the  food.  Each  has  usually  several  fangs. 
The  milk-teeth  only  differ  in  subsidiary  points  from  those  of 
the  same  names  in  the  permanent  set. 

The  Structure  of  a  Tooth.  If  a  tooth  be  broken  open,  a 
cavity  extending  through  both  crown  and  fang  will  be  found 
in  it.  This  is  filled  during  life  with  a  soft  vascular  pulp,  and 
hence  is  known  as  the  "  pulp-cavity"  (r,  Fig.  110).  The  hard 
parts  of  the  tooth  disposed  around  the  pulp-cavity  consist  of 
three  different  tissues.  Of  these  one  immediately  surrounds 
the  cavity  and  makes  up  most  of  the  bulk  of  the  tooth;  it  is 
dentine  (2,  Fig.  110);  covering  the  dentine  on  the  crown  is 
the  enamel  (1,  Fig.  110)  and,  on  the  fang,  the  cement 
(3,  Fig.  110). 

The  pulp-cavity  opens  below  by  a  narrow  aperture  at  the 
tip  of  the  fang,  or  at  the  tip  of  each  if  the  tooth  have  more 
than  one.  The  pulp  consists  mainly  of  connective  tissue, but 
its  surface  next  the  dentine  is  covered  by  a  layer  of  columnar 
cells.  Through  the  opening  on  the  fang  blood-vessels  and 
nerves  enter  the  pulp. 

The  dentine  (ivory)  yields  on  analysis  the  same  materials 
as  bone  but  is  somewhat  harder,  earthy  matters  constituting 
72  per  cent  of  it  as  against  CG  per  cent  in  bone.  Under  the 
microscope  it  is  recognized  by  the  fine  dentinal  tubules 
which,  radiating  from  the  pulp-cavity,  perforate  it  through- 
out, finally  ending  in  minute  branches  which  open  into 
irregular  cavities,  the  interglobular  spaces,  which  lie  just 
beneath  the  enamel  or  cement.  At  their  widest  ends,  close 
to  the  pulp-cavity,  the  dentinal  tubules  are  only  about  0.005 
millimeter  (^V.y  of  an  inch)  in  diameter.  The  cement  is 
much  like  bone  in  structure  and  composition,  possessing 
lacunae  and  oanaliculi,  but  rarely  any  Haversian  canals.  It  is 
thickest  at  the  tip  of  the  fang  and  thins  away  towards  the 
cervix.  Enamel  is  the  hardest  tissue  in  the  Body,  yielding 
on  analysis  only  from  two  per  cent  to  three  per  cent  of 
organic  matter,  the  rest  being  mainly  calcium  phosphate  and 
carbonate.  Its  histological  (dements  are  minute  hexagonal 
prisms,  closely  packed,  and  set  on  vertically  to  the  surface  of 


332 


THE  HUMAN  BODY. 


the  subjacent  dentine.     Jt  is  thickest  over  the  free  end  of  the 
crown,  until  worn   away  by  use.     Covering   the  enamel  in 


Fig.  110.— Section  through  a  premolar  tooth  of  the  cat  still  imbedded  in  its 
socket.  1,  enamel;  2,  dentine;  3,  cement ;  4,  the  gum;  5,  the  bone  of  the  lower 
jaw  ;  c,  the  pulp-cavity. 

unworn  teeth  is  a  thin  structureless  horny  layer,  the  enamel 
cuticle. 

The  Tongue  (Fig.  Ill)  is  a  muscular  organ  covered  by 
mucous  membrane,  extremely  mobile,  and  endowed  not 
only  with  a  delicate  tactile  sensibility  but  with  the  terminal 
organs  of  the  special  sense  of  taste;  it  is  attached  by  its  root 
to  the  hyoid  bone.     On  its  upper  surface  are  numerous  small 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES.     333 

eminences  or  papillm,  such  as  are  found  more  highly  devel- 
oped on  the  tongue  of  a  cat,  where  they  may  be  readily  felt. 
On  the  human  tongue  there  are  three  forms  of  papilla?,  the 
circumvallate,  the  fungiform,  and  the  filiform.  The  circum- 
vallate  papilla?,  1  and  2,  the  largest  and  least  numerous,  are 
from  seven  to  twelve  in  number  and  lie  near  the  root  of  the 
tongue  arranged  in  the  form  of  a  V  with  its  open  angle  turned 


Fio.  111  .—The  upper  surface  of  the  tongue  with  part  of  the  pillars  of  the  fauces 
and  the  tonsils.  i,  8,  circumvallate  papillae  ;  8,  fungiform  papillae  ;  4,  filiform 
papilla-  :  »i.  mucous  k'lanils  ;  7,  tonsils  ;  8,  lip  of  epiglottis. 

forwards.  Each  is  an  elevation  of  the  mucous  membrane, 
ired  by  epithelium,  and  surrounded  by  a  trench.  On  the 
-  of  these  papilla,  imbedded  in  the  epithelium,  are  many 

small    oval    bodies    richly    supplied    with     nerves,     and     sup- 


334  THE  II UMAX   BODY. 

posed  to  be  concerned  in  the  sense  of  taste,  and  hence  culled 
the  taste-buds  (Chap.  XXXV).  The  fungiform  papilla,  3, 
arc  rounded  elevations  attached  In  somewhal  narrowed  stalks, 
and  found  all  over  the  middle  and  fore  part  of  the  upper 
surface  of  the  tongue.  They  are  easily  recognized  on  the 
livingtongue  by  their  bright  red  color.  The  filiform  papilla, 
most  numerous  and  smallest,  are  scattered  all  over  the  dorsum 
of  the  tongue  except  near  its  base.  Each  is  a  conical  emi- 
nence covered  by  a  thick  horny  layer  of  epithelium.  It  is 
these  papillae  which  are  so  highly  developed  on  the  tongues 
of  Cam i vora,  and  serve  them  to  scrape  hones  clean  of  even 
such  tough  structures  as  ligaments. 

In  health  the  surface  of  the  tongue  is  moist,  covered  by 
little  "fur,"  and  in  childhood  of  a  red  color.  In  adult  life 
the  natural  color  of  the  tongue  is  less  red,  except  around  the 
edges  and  tip;  a  bright-red  glistening  tongue  being  then, 
usually  a  symptom  of  disease.  When  the  digestive  organs 
are  deranged  the  tongue  is  commonly  covered  with  a  thick 
yellowish  coat,  composed  of  a  little  mucus,  some  cells  of 
epithelium  shed  from  the  surface,  and  numerous  microscopic 
organisms  known  as  bacteria;  and  there  is  frequently  a  "  had 
taste  in  the  mouth."  The  whole  alimentary  mucous  mem- 
brane is  in  close  physiological  relationship;  and  anything 
disordering  the  stomach  is  likely  to  produce  a  "furred 
tongue." 

The  Salivary  Glands.  The  saliva,  which  is  poured  into 
the  mouth  and  which,  mixed  with  the  secretion  of  minute 
glands  imbedded  in  its  lining  membrane,  moistens  it,  is 
secreted  by  three  pairs  of  glands,  the  parotid,  the  submaxil- 
lary and  the  sublingual.  The  parotid  glands  lie  in  front  of 
the  ear  behind  the  ramus  of  the  lower  jaw;  each  sends  its 
secretion  into  the  mouth  by  a  tube  known  as  Stenon's  duct, 
which  crosses  the  cheek  and  opens  opposite  the  second  upper 
molar  tooth.  In  the  disease  known  as  mumps  *  the  parotid 
glands  are  inflamed  and  enlarged.  The  submaxillary  glands 
lie  between  the  halves  of  the  lower  jaw-bone,  near  its  angles, 
and  their  ducts  open  beneath  the  tongue  near  the  middle  line. 
The  sublingual  glands  lie  beneath  the  floor  of  the  mouth, 
covered  by  its  mucous  membrane,  between  the  back  part  of 
the  tongue  and  the  lower  jaw-bone.     Each  has   many  ducts 


*  Parotitis,  in  technical  language. 


THE  ALIMENTARY   CANAL  AND  LTS  APPENDAGES.     335 

(8  to  20),  some  of  which  join  the  submaxillary  duct,  while 
the  rest  open  separately  in  the  floor  of  the  mouth. 

The  Fauces  is  the  name  given  to  the  aperture  which  can 
be  seen  at  the  back  of  the  mouth  below  the  soft  palate  (Fig. 
105),  and  leading  into  the  pharynx.  It  is  bounded  above  by 
the  soft  palate  and  uvula,  below  by  the  root  of  the  tongue, 
and  on  the  sides  by  muscular  elevations  covered  by  mucous 
membrane,  which  reach  from  the  soft  palate  to  the  tongue. 
These  elevations  are  the  ■pillars  of  the  fauces.  Each  bifur- 
cates below,  and  in  the  hollow  between  its  divisions  lies  a 
tonsil  (7,  Fig.  Ill),  a  soft  rounded  body  about  the  size  of  an 
almond,  and  containing  numerous  minute  glands  which  form 
mucus. 

The  tonsils  not  unfrequently  become  enlarged  during  a 
cold  or  sore  throat,  and  then  pressing  on  the  Eustachian  tube 
(Chap.  XXXIV),  which  leads  from  the  pharynx  to  the  mid- 
dle ear,  keep  it  closed  and  produce  partial  deafness. 

The  Pharynx  or  Throat-cavity  (Fig.  105).  This  por- 
tion of  the  alimentary  canal  may  be  described  as  a  conical 
bag  with  its  broad  end  turned  upwards  towards  the  base  of 
the  skull,  and  its  narrow  end  downwards  and  passing  into  the 
gullet.  Its  front  is  imperfect,  presenting  openings  which 
lead  into  the  nose,  the  mouth,  and  (through  the  larynx 
and  windpipe)  the  lungs.  Except  during  swallowing  or 
speech  the  soft  palate  hangs  down  between  the  mouth  and 
pharynx;  during  deglutition  it  is  raised  into  a  horizontal 
position  and  separates  an  upper  or  respiratory  portion  of  the 
pharynx  from  the  rest.  Through  this  upper  part,  therefore, 
air  alone  passes,  entering  it  from  the  posterior  ends  of  the 
two  nostril-chambers;  while  through  the  lower  portion  both 
food  and  air  pass,  one  on  its  way  to  the  gullet,  b,  Fig.  105, 
the  other  through  the  larynx,  d,  to  the  windpipe,  c;  when 
a  morsel  of  food  "  goes  the  wrong  way  "  it  takes  the  latter 
course.  Opening  into  the  upper  portion  of  the  pharynx  on 
each  side  is  an  Eustachian  tube,  g:  so  that  the  apertures 
leading  out  of  it  are  seven  in  number;  the  two  posterior 
Mares,  the  two  Eustachian  tubes,  the  fauces,  the  opening  of 
the  larynx, and  that  of  the  gullet.  At  the  root  of  the  tongue, 
over  the  opening  of  the  larynx,  is  a  plate  of  cartilage,  the 
epiglottis,  >\  which  can  lie  Been  if  the  mouth  is  widely  opened 
and  tin;  back  of  the  tongue  pressed  down  by  some  each  thing 
as  the  handle  of  a  spoon.     During  swallowing  the  epiglottis 


336 


TUK   II r MAN  BODY. 


is  pressed  down  like  a  lid  over  the  air-tube  and  helps  to  keep 
food  or  saliva  from  entering  it.  In  structure  the  pharynx 
consists  essentially  of  a  bag  of  connective  tissue  lined  by 
mncons  membrane,  and  having  muscles  in  its  walls  which 
drive  the  food  on. 

The  (Esophagus  or  Gullet  is  a  tube  commencing  at  the 
lower  termination  of  the  pharynx  ami  which,  passing  on 
through  the  neck  and  chest,  ends  below  the  diaphragm  by 
joining  the  stomach,  \\\  the  neck  it  lies  close  behind  the 
windpipe.  It  consists  of  three  coats — a  mucous  membrane 
within;  next,  a  submucous  coat  of  areolar  connective  tissue; 
and,  outside,  a  muscular  coat  made  up  of  two  layers,  an  inner 
with  transversely  and  an  outer  with  longitudinally  arranged 
fibres.  In  and  beneath  its  mucous  membrane  are  numerous 
small  mucous  glands  whose  ducts  open  into  the  tube. 

The  Stomach  (Fig.  11:3)  is  a  somewhat  conical  bag  placed 
transversely  in  the  upper  part  of  the  abdominal  cavity.     Its 

larger  end  is  turned  to  the 
left  and  lies  close  beneath 
the  diaphragm;  opening 
into  its  upper  border, 
through  the  cardiac  orifice 
at  a,  is  the  gullet  d.  The 
narrower  right  end  is  con- 
tinuous at  c  with  the  small 
intestine;  the  aperture  be- 
tween the  two  is  the  pyloric 
orifice.  The  pyloric  end  of 
the  stomach  lies  lower  in  the 
abdomen  than  the  cardiac, 
and  is  separated  from  the 
diaphragm  by  the  liver  (see  Fig.  1).  The  concave  border  be- 
tween the  two  orifices  is  known  as  the  .small  curvature,  and 
the  convex  as  the  great  curvature,  of  the  stomach.  From 
the  latter  hangs  down  a  fold  of  peritoneum  (nc,  Fig.  1) 
known  as  the  great  omentum.  It  is  spread  over  the  rest  of 
the  abdominal  contents  like  an  apron.  After  middle  life 
much  fat  frequently  accumulates  in  the  omentum,  so  that  it 
is  largely  responsible  for  the  "fair  round  belly  with  good 
capon  lin'd."  The  protrusion  b  to  the  left  side  of  the  cardiac 
orifice.  Fig.  112,  is  the  fundus  or  great  cul  de  sac.  The  size 
of  the  stomach  varies  greatly  with  the  amount  of  food  in  it; 


Fig.  112. — The  stomach,  d.  lower  end  of 
the  gullet  ;  <i.  position  of  the  e;irdiac  aper- 
ture ;  /).  tlie  fundus  ;  c,  the  pylorus  ;  e.  the 
commencement  of  the  small  intestine; 
along  a.  I),  v.  the  trieat  curvature  ;  hetween 
the  pylorus  and  d.  the  lesser  curvature. 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES.     337 

just  after  a  moderate  meal  it  is  about  ten  inches  long,  by  five 
wide  at  its  broadest  part. 

Structure  of  the  Stomach.  This  organ  has  four  coats, 
known  successively  from  without  in  as  the  serous,  the  mus- 
cular, the  submucous,  and  the  mucous.  The  serous  coat  is 
formed  by  a  reflection  of  the  peritoneum,  a  double  fold  of 
which  slings  the  stomach;  after  separating  to  envelop  it  the 
two  layers  again  unite  and,  hanging  down  beyond  it,  form  the 
great  omentum.  The  muscular  coat  (Fig.  59)  consists  of 
unstriped  muscular  tissue  arranged  in  three  layers:  an  outer, 
longitudinal,  most  developed  about  the  curvatures;  a  circu- 
lar, evenly  spread  over  the  whole  organ,  except  around  the 
pyloric  orifice  where  it  forms  a  thick  ring;  and  an  inner, 
oblique  and  very  incomplete,  radiating  from  the  cardiac 
orifice.  The  submucous  coat  is  made  up  of  lax  areolar  tissue 
and  binds  loosely  the  mucous  coat  to  the  muscular.  The 
mucous  coat  is  a  moist  pink  membrane  which  is  inelastic,  and 
large  enough  to  line  the  stomach  evenly  when  it  is  fully  dis- 
tended. Accordingly,  when  the  organ  is  empty  and  shrunken, 
this  coat  is  thrown  into  folds,  which  disappear  when  the  organ 
is  distended.  During  digestion  the  arteries  supplying  the 
stomach  become  dilated  and,  its  capillaries  being  gorged,  its 
mucous  membrane  is  then  much  redder  than  during  hunger. 

The  blood-vessels  of  the  stomach  run  to  it  between  the 
folds  of  peritoneum  which  sling  it.  After  giving  off  a  few 
branches  to  the  outer  layers,  most  of  the  arteries  break  up 
into  small  branches  in  the  submucous  coat,  from  which  twigs 
proceed  to  supply  the  close  capillary  network  of  the  mucous 
membrane. 

The  nerves  of  the  stomach  are  chiefly  derived  from  the 
pneumogastrics.  In  the  lower  part  of  the  thorax  these  nerves 
consist  mainly  of  nonmedullated  fibres,  and  lie  on  the  sides 
of  the  gul let,  across  which  they  interchange  fibres  by  means 
of  several  branches.  On  entering  the  abdomen  the  left  pneu- 
mogastric  passes  to  the  ventral  side  of  the  stomach,  in  which 
it  ends:  the  right  supplies  the  dorsal  side  of  the  stomach,  but 
a  considerable  portion  of  it  passes  on  to  enter  the  solar  plextts, 
which  lies  behind  the  stomach  and  contains  several  large 
ganglia.  The  sympathetic  also  supplies  gastric  nerves  which 
mainly  go  to  the  blood-vessels.  In  tin1  muscular  coat  of  the 
stomach  are  many  nerve-cells. 

Histology  of  trie  Gastric  Mucous  Membrane.     Examina- 


338 


THE  III  MAS  BODY. 


lion  of  the  inner  surface  of  the  stomach  with  a  hand  lens 
shows  it  to  be  covered  with  minute  shallow  pits.  Into  these 
open  the  mouths  of  minute  tubes,  the  gastric  glands,  which 
are  closely  packed  side  by  side  in  the  mucous  membrane; 
something  like  the  cells  of  a  honeycomb,  except  that  each  is 
open  at  one  end.  Between  them  lie  a  small  amount  of  con- 
nective tissue,  a  close  network  of  lymph-channels,  and  capil- 
lary hlood-vessels.  The  connective  tissue  is  of  a  peculiar 
variety  closely  packed  with  lymph-cells  ami  will  he  more  mi- 
nutely described  later  (Chap.  XX  111).  The  whole  surface  of 
the  mucous  membrane  is  lined  by  a  single  layer  of  columnar 
mucus-making  epithelium  cells  (Fig.  113).  These  dip  down 
and  line  the  necks  of  the  tubular 
glands.  The  deeper  portions  of  the 
glands  are  lined  by  a  layer  of 
shorter  and  somewhat  cuboid al  cells, 
the  central  or  chief  cells.  In  speci- 
mens taken  from  a  healthy  animal 
killed  during  digestion  these  cells  are 
large  and  do  not  stain  deeply  with 
carmine.  .  Similar  specimens  taken 
from  an  animal  an  hour  or  two 
after  a  good  meal  has  been  swallowed 
show  the  chief  cells  shrunken  and 
staining  more  deeply.  They,  thus, 
store  up  during  rest  a  material  which 

called  oxyntic calls';  o,  retiform     t]  Q{  ^  Qf  wl,on  t])e  gastric  juice 

connective  tissue.  .    .->  &  J 

is  being  secreted.  This  material  is, 
in  part,  pepsinogen,  which  during  activity  of  the  gland  is 
changed,  giving  rise  among  other  things  to  pepsin,  the  chief 
enzvme  of  gastric  juice.  The  deeply  staining  protoplasmic 
portion  of  the  cell  which  is  left  behind,  forms  and  stores  more 
pepsinogen  during  the  next  period  during  which  the  stomach 
is  not  digesting.  In  the  pyloric  end  of  the  stomach  only  the 
chief  cells  line  the  glands,  but  elsewhere  there  is  found  out- 
side them,  in  nmst  of  the  glands,  an  incomplete  layer  of  larger 
oral  cells  (d,  Fig.  113).  These  are  sometimes  called  the 
oxyntic  cells,  from  the  belief  that  they  are  especially  con- 
cerned in  secretin-'  the  acid  of  the  gastric  juice.  The  glands 
frequently  branch  at  their  deeper  ends. 

The  Pylorus.     If  the  stomach  he  opened  it  is  seen  that 
the  mucous  membrane  projects  in  a  fold  around  the  pyloric 


Fig.  113  —  A  thin  section 
through  the  gastric  mucous 
membrane,  perpendicular  to  its 
surface,  magnified  about  25  di- 
ameters, a,  a  simple  gastric 
gland  ;  b,  a  compound  gastric 
gland  ;  c.  a  gland  containing 
only  chief  cells  ;  d,  oval  or  so- 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES.     339 

orifice  and  narrows  it.  This  is  due  to  a  thick  ring  of  the 
circular  muscular  layer  there  developed,  and  forming  around 
the  orifice  a  sphincter  muscle,  which,  by  its  contraction,  keeps 
the  passage  to  the  small  intestine  closed  except  when  portions 
of  food  are  to  be  passed  on  from  the  stomach  to  succeeding 
divisions  of  the  alimentary  canal. 

Since  the  cardiac  end  of  the  stomach  lies  immediately  be- 
neath the  diaphragm,  which  has  the  heart  on  its  upper  side, 
its  over-distension,  due  to  indigestion  or  flatulence,  may  im- 
pede the  action  of  the  thoracic  organs,  and  cause  feelings  of 
oppression  in  the  chest,  or  palpitation  of  the  heart. 

The  Small  Intestine  (Fig.  120),  commencing  at  the  py- 
lorus, ends,  after  many  windings,  in  the  large.  It  is  about  six 
meters  (twenty  feet)  long,  and  about  five  centimeters  (two 
inches)  wide  at  its  gastric  end,  narrowing  to  about  two  thirds 
of  that  width  at  its  lower  portion.  Externally  there  are  no 
lines  of  subdivision  on  the  small  intestine,  but  anatomists 
arbitrarily  describe  it  as  consisting  of  three  parts;  the  first 
twelve  inches  being  the  duodenum,  D,  the  succeeding  two 
fifths  of  the  remainder  the  jejunum,  J,  and  the  rest  the 
ileum,  I. 

Like  the  stomach,  the  small  intestine  possesses  four  coats; 
a  serous,  a  muscular,  a  submucous,  and  a  mucous.  The 
serous  coat  is  formed  by  a  duplicative  of  the  peritoneum,  but 
presents  nothing  answering  to  the  great  omentum;  this 
double  fold,  slinging  the  intestine  as  the  small  omentum 
slings  the  stomach,  is  named  the  mesentery.  The  muscular 
rout  is  composed  of  plain  muscular  tissue  arranged  in  two 
strata,  an  outer  longitudinal,  and  an  inner  transverse  or  cir- 
cular.  The  submucous  coat  is  like  that  of  the  stomach;  con- 
sisting of  loose  areolar  tissue,  binding  together  the  mucous 
and  muscular  coats,  and  forming  a  bed  in  which  the  blood 
and  lymphatic  vessels  (which  reach  the  intestine  in  the  fold 
of  the  mesentery)  break  up  into  minute  branches  before  en- 
tering the  mucous  membrane. 

The  Mucous  Coat  ofthe  Small  Intestine.  This  is  pink, 
.-oft  and  extremely  vascular.  It  does  not  present  temporary 
or  effaceable  folds  like  those  of  the  stomach,  but,  is,  through- 
out a  greal  portion  of  its  length,  raised  up  into  permanent 
transverse  folds  in  the  form  of  crescentic  ridges,  each  of 
which  runs  transversely  \'<>r  a  greater  or  less  way  round  the 
tuhe  (Fig.  114).    These  folds   are  the  valvules  conniventes. 


340 


THK  HUMAN   IlfWY. 


They  are  first  found  about  two  inches  from  the  pylorus,  and 
are  most  thickly  set  and  largest  in  the  upper  half  of  the 
jejunum,  in  the  lower  half  of  winch  they  become  gradually 
less  conspicuous;  and  they  finally  disappear  altogether  about 
the  middle  of  the  ileum.  The  folds  serve  greatly  to  increase 
the  surface  of  the  mucous  membrane  both  for  absorption  and 
secretion,  and  they  also  delay  the  food  Bomewhal  in  its  pas- 
sage, since  it  must  collect  in  the  hollows  between  them,  and 
so  be  longer  exposed  to  the  action  of  the  digestive  liquids. 
Examined  closely  with  the  eye  or,  better,  with  aid  of  a  lens, 
the  niueous  membrane  of  the  small  intestine  is  seen  to  be  not 
smooth  but  shaggy,  being  covered  everywhere  (both  over  the 
valvulae  COnniventes  and  between  them)  with  closely  packed 
minute  processes,  standing  up  somewhat  like  the  "pile  "on 
velvet,  and  known  as  the  villi.  Each  villus  is  from  0.5  to  0.7 
millimeter  (-g^  to  .}:>  inch)  in  length;  some  are  conical  and 
rounded,  but  the  majority  are  compressed  at  the  base  in  one 
diameter  (Fig.  11")).  In  structure  a  villus  is  somewhat  com- 
plex. Covering  it  is  a  single  layer  of  columnar  epithelial  cells, 
the  exposed  ends  of  the  majority  having  a  peculiar  bright 
striated  border  and  being  probably  of  great  importance  in  ab- 
sorption. Mixed  with  these  cells  are  others  in  which  most  of 
the  cell  has  become  filled  with  a  clear  mass  which  does  not 
stain  readily  with  reagents;  the  deep  narrow  end  of  the  cell 
stains  easily  and  contains  the  nucleus.  From  time  to  time  the 
clear  substance  (mucigen)  is  converted  into  mucus  and  dis- 
charged into  the  intestine,  leaving  behind  only  the  nucleus  and 
the  protoplasm  around  it.  These  reconstruct  the  cell  and  form 
more  mucigen.  These  mucus-forming  cells  are  named  goblet- 
celh,  from  their  shape.    Beneath  the  epithelium  the  villus  may 


Fig.  1i4. — A  portion  of  thf  small  intestine  opened  t<>  show  the  valvules connivente& 


be  regarded  as  made  up  of  a  framework  of  connective  tissue, 
mainly  of  the  adenoid  variety  (Chap.  XXIII),  supporting  the 


TEE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES.     341 

more  essential  constituents.  Near  the  surface  is  an  incomplete 
layer  of  plain  muscular  tissue,  continuous  below  with  a  mus- 
cular stratum  forming  the  deepest  layer  of  the  mucous  mem- 
brane and  named  the  muscularis  mucosw.  In  the  centre  is  an 
offshoot  of  the  lymphatic  system;  sometimes  in  the  form  of  a 
single  vessel  with  a  closed  dilated  end,  and  sometimes  as  a  net- 
work  formed  by  two  main  vessels  with  cross-branches.  During 
digestion  these  lymphatics  are  filled  with  a  milky-white  liquid 
absorbed  from  the  intestines,  and  they  are  accordingly  called 
the  lacteals.  They  communicate  with  larger  branches  in  the 
submucous  coat,  which  end  in  trunks  that  pass  out  through 
the  mesentery  to  join  the  main  lymphatic  system.  Finally, 
in  each  villus,  outside  the  lacteals  and  beneath  the  muscular 
layer  of  the  villus,  is  a  close  network  of  blood-vessels. 

Opening  on  the  surface  of  the  small  intestine,  between 
the  bases  of  the  villi,  are  small  glands,  the  crypts  of  Lieber- 
Jcuhn.  Each  is  a  simple  unbranched  tube  lined  by  a  layer  of 
columnar  cells  some  of  which  have  a  striated  free  border, 
though  less  marked  than  that  on  the  corresponding  cells   of 


Fir;.  115.— Villi  of  (lie  small  Intestine;  magnified  about  80  diameters.  In  the 
rijrht-hand  figure  the  lacteals,  a.,  b,  c,  are  tilled  willi  whin-  injection;  <l,  blood-ves- 
■els.  In  the  left  hand  figure  the  lacteals  alone  are  represented,  filled  with  a  dark 
injection.  The  epithelium  covering  the  villi,  and  their  muscular  fibres,  are  omitted. 

the  villi,  and  others  arc  goblet-cells.  The  crypts  of  Lieber- 
kiihu  arc  closely  packed,  side  by  side,  like  the  glands  of  the 
stomach.  In  (lie  duodenum  are  found  olhcr  minute  glands, 
the  glands  of  Brunner.     They   lie   in    the  submucous  coat 


342  THE  HUMAN  BODY. 

and  Bend  their  ducts  through  the  mucous  membrane  to  open 

on  its  inner  side. 

The  Large  Intestine  (Fig.  120),  forming  the  final  por- 
tion of  the  alimentary  canal,  is  aboul  L.o  meters  (5  feet) 
Inn--,  and  varies  in  diameter  from  about  ij  tn  4  ceutimetera 
(•j.l  to  ij  inches).  Anatomists  describe  it  as  consisting  of 
the  ccBCum  with  the  vermiform  appendix,  the  colon,  and  the 
rectum.  The  small  intestine  does  not  open  into  the  com- 
mencement of  the  large  hut  into  its  side,  some  distance  from 
its  closed  upper  end,  and  the  caecum,  CC,  is  that  par!  of  the 
large  intestine  which  extends  beyond  the  communication. 
From  it  projects  the  vermiform  appendix,  a  narrow  tube 
not  thicker  than  a  cedar  pencil,  and  about  10  centimeters 
(4  inches)  long.  The  colon  commences  on  the  righl  side  of 
the  abdominal  cavity  where  the  small  intestine  communicates 
with  the  large,  runs  up  for  some  way  on  that  side  {ascending 
colon,  AC),  then  crosses  the  middle  line  {transverse  colon, 
TC)  below  the  stomach,  and  turns  down  {descending  colon, 
DC)  on  the  left  side  and  there  makes  an  S-shaped  bend 
known  as  the  sigmoid  flexure,  SF;  from  this  the  rectum,  R, 
the  terminal  straight  portion  of  the  intestine,  proceeds  to 
the  anal  opening,  by  which  the  alimentary  canal  communi- 
cates with  the  exterior.  In  structure  the  large  intestine 
presents  the  same  coats  as  the  small.  The  external  stratum 
of  the  muscular  coat  is  not,  however,  developed  uniformly 
around  it,  except  on  the  rectum,  but  occurs  in  three  bands 
separated  by  intervals  in  which  it  is  wanting.  These  bands 
being  shorter  than  the  rest  of  the  tube  cause  it  to  be  puck- 
ered, or  sacculated,  between  them.  The  mucous  coat  pos- 
sesses no  villi  or  valvulae  conniventes,  but  is  usually  thrown 
into  effaceable  folds,  like  those  of  the  stomach  but  smaller. 
It  contains  numerous  closely  set  glands  much  like  the  crypts 
of  Lieberkiihn  of  the  small  intestine. 

The  Ileo-colic  Valve.  Where  the  small  intestine  joins 
the  large  there  is  a  valve,  formed  by  two  flaps  of  the  mucous 
membrane  sloping  down  into  the  colon,  and  so  disposed  as  to 
allow  matters  to  pass  readily  from  the  ileum  into  the  large 
intestine  hut  not  the  other  way. 

The  Nerves  of  the  Intestines.  These,  like  those  of  the 
heart  with  which  we  shall  later  have  to  compare  them 
physiologically,  are  intrinsic  and  extrinsic.  The  former  are 
connected    with    small    ganglia   found    abundantly    on    the 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES.     343 

plexus  of  Auerbach  which  lies  between  the  two  muscular 
coats,  and  the  plexus  of  Meissner  found  in  the  submucous 
coat.  The  extrinsic  fibres  proceed  immediately  from  the 
gaugliated  solar  plexus  already  referred  to  and  from  a  similar 
mesenteric  plexus  which  lies  lower  in  the  abdomen;  except  a 
few  branches  to  the  longitudinal  muscular  coat  of  the  rectum 
which  pass  directly  from  some  of  the  sacral  spinal  nerves. 
Some  of  the  fibres  distributed  from  the  solar  plexus  are 
those  running  from  the  brain  in  the  right  pneumogastric, 
and  probably  also  from  the  left,  having  crossed  over  to  the 
left  in  branches  joining  the  two.  Others  reach  the  solar 
plexus  by  means  of  the  splanchnics  and  other  nerves  pro- 
ceeding from  the  thoracic  parts  of  the  two  sympathetic  chains. 
These  are  partly  vaso-constrictor  fibres  (Chap.  XVIII.),  but 
in  part  go  to  the  muscular  coats  of  the  intestine.  They  may 
be  traced  back  through  the  communicating  branches  from 
sympathetic  ganglia  to  the  corresponding  spinal  nerves  and 
thence  by  the  ventral  nerve-roots  into  the  spinal  cord. 
The  fibres  passing  to  the  intestines  from  the  mesenteric 
plexus  reach  that  plexus  from  the  posterior  thoracic  and 
anterior  lumbar  sympathetic  ganglia,  and  can  also  be  tracked 
by  experiment  to  the  spinal  cord. 

The  Liver.  Besides  the  secretions  formed  by  the  glands 
imbedded  in  its  walls,  the  small  intestine  receives  those  of 
two  large  glands,  the  live?-  and  the  pancreas,  which  lie  in  the 
abdominal  cavity.  The  ducts  of  both  open  by  a  common 
aperture  into  the  duodenum  about  10  centimeters  (4  inches) 
from  the  pylorus. 

The  liver  is  the  largest  gland  in  the  Body,  weighing  from 
1400  to  1T00  grams  (50  to  G4  ounces).  It  is  situated  in  the 
upper  part  of  the  abdominal  cavity  (le,  W,  Fig.  1),  rather 
more  on  the  right  than  on  the  left  side  and  immediately 
below  tin;  diaphragm,  into  the  concavity  of  which  its  upper 
surface  fits,  and  reaches  across  the  middle  line  above  the 
pyloric  end  of  the  stomach.  It  is  of  dark  reddish-brown 
color,  and  of  a  soft  friable  texture.  A  deep  fissure  incom- 
pletely divides  the  orgau  into  right  and  left  lobes,  of  which 
the  riLrht  is  much  the  larger;  on  its  under  surface  (Fig.  116) 
shallower  grooves  mark  off  several  minor  lobes.  Its  upper 
surface  is  smooth  and  convex.  The  vessels  carrying  blood 
to  the  liver  are  the  portal  vein,  Vp,  and  the  hepatic  artery; 
both  enter  it  at  a  fissure  (///''  portal  fissure)  on  its  under  side, 


344 


TEE  HUMAN   HODY. 


and  there  also  a  duct  passes  out  from  each  half  of  the  organ. 
The  ducts  unite  to  form  the  hepatic  duct,  Dh,  which  meets 
at  an  acute  angle,  the  cystic  duct,  Dc,  proceeding  from  the 
gall-bladder,  \'l\  a  pear-shaped  sat-  in  which  the  bile,  or  gall, 
formed  by  the  liver,  accumulates  when  food  is  not  being 
digested    in    the    intestine.     The    common    bile-dud,   JJc/i, 


Fig.  116.— The  under  surface  of  the  liver,  d,  right,  and  s.  left  lobe;  Vh.  hepatic 
vein;  Vp,  portal  vein;  TV.  vena  cava  inferior;  Dch,  common  bile-duct;  Dc,  cystic 
duct;  Dh,  hepatic  duct;  Vf,  gall-bladder, 

formed  by  the  union  of  the  hepatic  and  cystic  ducts,  opens 
into  the  duodenum.  The  blood  which  enters  the  liver  by 
the  portal  vein  and  hepatic  artery  passes  out  by  the  hepatic 
veins,  17/,  which  leave  the  posterior  border  of  the  organ  close 
to  the  vertebral  column,  and  there  open  into  the  inferior  vena 
cava  just  before  it  passes  up  through  the  diaphragm. 

The  Structure  of  the  Liver.  On  closely  examining  the 
surface  of  the  liver,  it  will  be  seen  to  be  marked  out  into 
small  angular  areas  from  one  to  two  millimeters  (-^  to  ,'., 
inch)  in  diameter.  These  are  the  outer  sides  of  the  super- 
ficial layer  of  a  vast  number  of  minute  polygonal  masses,  or 
lobules,  of  which  the  liver  is  built  up;  similar  areas  are  seen 
on  the  surface  of  any  section  made  through  the  organ. 
Each  lobule  (Fig.  117)  consists  of  a  number  of  hepatic 
cell*  supported  by  a  close  network  of  capillaries;  and  is 
separated    from    neighboring   lobules    by   connective  tissue, 


THE  ALIMENTARY  CANAL  AND  ITS   APPENDAGES.     345 

larger  blood-vessels,  and  branches  of  the  hepatic  duct.  The 
hepatic  cells  are  the  proper  tissue  elements  of  the  liver,  all 
the  rest  being  subsidiary  arrangements  for  their  nutrition 
and  protection.  Each  is  polygonal,  nucleated  and  very 
granular,  and  has  a  diameter  of  about  .025  millimeter  (ToVo 
of  an  inch).  In  each  lobule  they  are  arranged  in  rows  or 
strings,  which  form  a  network,  in  the  meshes  of  which  the 
blood-capillaries  run.  Covering  the  surface  of  the  liver 
is   a    layer  of   the   peritoneum,   beneath  which    is   a   dense 


Fig.  117— A  lobule  of  the  liver,  magnified,  showing  the  hepatic  cells  radiately 
arranged  around  the  central  intralobular  vein,  and  the  lobular  capillaries  inter- 
laced with  thein. 

connective-tissue  layer,  forming  the  capsule  of  Glisson.  At 
the  portal  fissure  offsets  from  this  capsule  run  in,  and  line 
canals,  the  portal  canals,  which  are  tunnelled  through  the 
organ.  These,  becoming  smaller  and  smaller  as  they  branch, 
finally  become  indistinguishable  close  to  the  ultimate 
lobules.  From  their  walls  and  from  the  external  capsule, 
connective-tissue  partitions  radiate  in  all  directions  through 
the  liver  and  support  its  other  parts.  In  each  portal  canal 
lie  three  vessels— a  branch  of  the  portal  vein,  a  branch  of 
the  hepatic  artery,  and  a  branch  of  the  hepatic  duct;  the 
division  of  the  portal  vein  being  much  the  largest  of  the 
three.  These  vessels  break  up  as  the  portal  canals  do,  and 
all  end  'm  minute  branches  around  the  lobules.  The  blood 
carried  in  by'  tin;  portal  vein  (which  has  already  circulated 
through  the  capillaries  of  the  stomach,  spleen,  intestines  and 


346 


I'lIK  //(MAN   BODY. 


pancreas)  is  tints  conveyed  to  a  fine  vascular  interlobular 
plexus  around  the  liver-lobules,  from  which  it  Hows  on 
through  the  capillaries  (lobular  plexus)  of  the  lobules  them- 
selves. These  (Fig.  117)  unite  in  the  centre  of  the  lobule  to 
form  a  small  intralobular  rein,  which  carries  the  blood  out, 
and  pours  it  into  one  of  the  branches  of  origin  of  the  hepatic 
vein,  called  the  sublobulay  vein.  Each  of  the  latter  has 
many  lobules  emptying  blood  into  it,  and  if  dissected  out 
with  them  (Fig.  118)  would  look  something  like  a  branch  of 
a  tree  with  apples  attached  to  it  by  short  stalks,  represented 


Fig.  118.— A  small  portion  of  the  liver,  injected,  and  magnified  about  twenty 
diameters.  The  blood  vessels  are  represented  white;  the  large  vessel  is  a  sub- 
lobular  vein,  receiving  the  intralobular  veins,  which  in  turn  are  derived  from  the 
capillaries  of  the  lobules. 


by  the  intralobular  veins.  The  blood  is  finally  carried,  as 
already  pointed  out,  by  the  hepatic  veins  into  the  inferior  vena 
cava.  The  hepatic  artery,  a  direct  offshoot  of  the  cceliac 
axis,  ends  mainly  in  Glisson's  capsule  and  the  walls  of  the 
blood-vessels  and  bile-duets,  but  some  of  its  blood  reaches 
the  lobular  plexuses;  it  all  finally  leaves  the  liver  by  the 
hepatic  veins. 

The  bile-ducts  can  be  readily  traced  to  the  periphery 
of  the  lobules,  and  there  communicate  with  a  network  of 
extremely  minute  commencing  bile  ducts,  ramifying  in  the 
lobule  between  the  hepatic  cells  composing  it. 

The  Pancreas  or  Sweetbread.  This  is  an  elongated 
soft  organ  of  a  pinkish  yellow  color,  lying  along  the  great 
curvature  of  the  stomach.  Its  right  end  is  the  larger, 
and  is  embraced  by  the  duodenum  (Fig.  119),   which  there 


THE  ALIMENTARY  CANAL  AND  ITS  APPENDAGES.     347 

makes  a  curve  to  the  left.  A  duct  traverses  the  gland  and 
joins  the  common  bile-duct  close  to  its  intestinal  opening 
The  pancreas  produces  a  watery-looking  secretion  which  is  of 
great  importance  in  digestion;  the  gland  also  (Chap.  XXIII) 
exerts  an  important  influence  on  the  general  nutritional 
processes  of  the  Body.    It  is  of  the  compound  racemose  type. 


'  ";  tlfl  The  stotuacn,  pancreas,  liver,  and  duodenum,  with  part  of  the  rest 
of  the  small  intestine  an.i  the  mesentery;  the  stomach  and  liver  have  been 
turned  up  so  as  to  expose  the  pancreas.  V,  stomach  i  />.  />'.  D"  duodenum  ;  L 
spleen  ;  P.  pancreas:  B.  i-itriit  kidney  :  T.  jejunum  ;  Vf,  gall-bladder:  h,  hepatic 
duct:  c.  cystic  duel  :  cA.  common  bile-duct:  i.  aorta;  •,'.  an  artery  Heft  coronary) 
of  the  stomach;  8.  hepatic  artery:  i  splenic  artery;  5.  superior  mesenteric  artery; 
fi.  superior  mesenteric  vein;  7,  splenic  rein;  l>.  portal  vein. 


The  Blood-vessels  of  Alimentary  Canal,  Liver,  Spleen 
and  Pancreas      The   portal  vein   (  Vp,   Fig.  L19)  has  already 


348 


77/ K  HUMAN    liODY. 


been  referred  to  as  differing  from  all  other  veins  in  that  it 
not  only  receives  blood  from  a  system  of  capillaries  but  ends 
in  a  second  set  of  capillaries,  which  lie  in  the  liver.  The 
quantity  of  blood  brought  to  supply  the  hepatic  capillaries 
by  the  hepatic  artery  is  in  fact  much  less  than  that  brought 
by  the  portal  vein.     The  stomach,  the  intestines,  the  pancreas 

and  the  spleen  are  supplied 
with  arterial  blood  from 
three  great  branches  of  the 
aorta.  The  most  anterior 
of  these,  the  cceliac  axis, 
springs  from  the  aorta  close 
beneath  the  diaphragm  and 
divides  into  the  hepatic 
artery,  splenic  artery,  and 
arteries  for  the  stomach; 
some  of  these  divisions  may 
be  seen  in  Fig.  119.  The 
pancreas  is  supplied  partly 
from  the  hepatic,  partly 
from  the  splenic  artery. 
The  two  other  branches 
{superior  and  inferior  mes- 
enteric artery)  are  given  off 
from  the  aorta  lower  clown 
in  the  abdominal  cavity; 
the  former  (5,  Fig.  119) 
supplies  the  small  intestine 
and  half  of  the  large,  the 
Fig.  120.— Diagram  of  abdominal  part  of  latter  the  remainder  of  the 

alimentary  canal.      C,  the  cardiac,  and  P,  m,  .  ,       , 

the    pyloric    end    of    the    stomach:    />.    the  large.        1  lie  hJoOCl  passing 

duodenum:    J.  I,   the    convolutions    of    the  ,              ,         ,,  >i  „„  n*.4-n™na 

small    intestine:    CC.   the  csecum    with  the  through     all  these  ai  teiies 

venous  in  the 
capillaries  of  the  organs 
they  supply,  and  is  gathered  into  corresponding  veins  (Fig. 
119)  which  unite  near  the  liver  to  form  the  portal  vein. 
The  further  course  of  the  blood  carried  to  the  liver  (partly 
arterial  from  the  hepatic  artery,  partly  venous  from  the  portal 
system)  has  been  described  already  (p.  34.1). 


vermiform  appendix:  AC.  ascending,  TC,  i 
transverse,  arid  DC,  descending  colon;  R,  I 
the  rectum. 


CHAPTEE  XXIII. 

THE  LYMPHATIC  SYSTEM  AND  THE  DUCTLESS  GLANDS. 

The  Lymphatics  or  Absorbents  are  very  widely  distrib- 
uted in  the  Body.  Most  organs,  as  has  been  pointed 
out  (p.  63),  possess  a  sort  of  internal  skeleton  made  up 
of  connective  tissue,  which  consists  mainly  of  bundles  of 
fibres,  united  together  and  covered-in  by  a  "cement"  sub- 
stance. In  this  substance  are  found  numerous  cavities,  usu- 
ally branched,  and  communicating  with  one  another  by  their 
branches.  They  frequently  contain  connective-tissue  cor- 
puscles, which,  however,  do  not  completely  fill  them;  and 
they  thus,  with  their  branches,  form  a  set  of  intercommuni- 
cating channels  known  as  the  "  lymph-canaliculi"  because 
they  are  filled  with  lymph.  As  the  connective  tissues  accom- 
panv  blood-vessels  wherever  the  latter  run,  the  canaliculi, 
which  are  frequently  relatively  large  around  the  blood-capil- 
laries, take  up  the  liquid  which  transudes  through  their  walls, 
and  this  transudation  liquid  is  the  origin  of  the  lymph. 
Even  where  blood-vessels  and  connective  tissue  do  not  pene- 
trate, as  in  bone  between  the  Haversian  canals,  lyraph-canal- 
iculi  penetrate,  being  connected  with  the  cavities  in  which 
the  bone-corpuscles  lie;  and  in  the  deeper  layers  of  the 
epidermis  the  cells  are  covered  with  prickle-like  projections 
ami  unite  by  the  tips  of  these  so  as  to  leave  minute  channels 
which  apparently  are  lymph-canaliculi.  These  very  minute 
channels,  with  no  definite  lining  cells,  but  mere  crevices  be- 
tween tissue  elements,  or  tubes  hollowed  out  in  the  matrix  of 
connective  tissue,  bone  and  (possibly)  cartilage,  constitute 
the  origin  of  the  lymphatic  system.  The  transudation  liquid 
which  enters  them  from  the  blood-vessels  is  rapidly  altered 
by  interchange  with  the  neighboring  tissues,  losing  certain 
materials  and  gathering  others;  and  as  the  substances  taken 
and  the  waste  ami  other  products  returned  vary  very  much 
in  different  organs,  tin;  lymph  leaving  them  must,  differ  also. 
Nevertheless  it,  retains  certain  common  features,  histological 

•6VJ 


350  THE  HUMAN  BODY. 

and  chemical  (pp.  49,  62),  which  justify  us  in  speaking  of  it 
in  general  as  the  lymph.  'The  lymphatic  vessels  collect  this 
lymph  or  at  least  Buch  part  of  it  as  does  not  pass  hack  locally 
by  diffusion  into  the  blood,  and  pour  it  into  the  veins. 

The  Structure  of  Lymph-vessels.  The  smallest  lymph- 
vessels  proper  arc  the  lymph- capillaries;  tubes  rather  wider 
than  the  blood-capillaries,  but  like  them  having  a  wall  con- 
sisting of  a  single  layer  of  flattened  epithelium  cells.  The 
cells  have,  however,  a  wavy  margin  and  are  not  as  a  rule  much 
longer  in  one  diameter  than  another,  in  both  of  which  respects 
they  differ  from  the  cells  of  the  corresponding  blood-vessels. 
In  some  regions,  as  in  many  glands,  the  lymph-capillaries  are 
much  dilated  and  form  irregular  lymph  lacuna,  everywhere 
bounded  by  their  peculiar  wavy  cells,  lying  in  the  interstices 
of  organs;  and  sometimes  they  form  tubes  around  small  blood- 
vessels, us  in  the  brain  (perivascular  lymph-channel).  In 
some  places  they  commence  by  blind  ends  as  in  the  lacteal 
vessels  of  the  villi  of  the  small  intestine  (Fig.  115)  which  are 
lymph-capillaries;  but  usually  they  branch  and  join  to  form 
networks.  Lymph  from  the  canaliculi  enters  them  (whether 
by  passing  through  their  boundary  cells  or  through  clefts  left 
between  these  is  not  certain)  and  is  passed  on  to  larger  vessels 
which  much  resemble  veins  of  corresponding  size,  having  the 
same  three  coats,  and  being  abundantly  provided  with  valves. 

The  Thoracic  Duct.  The  lymph-vessels  proceeding  from 
the  capillaries  in  various  organs  become  larger  and  fewer  by 
joining  together,  and  all  end  finally  in  two  main  trunks  which 
open  into  the  venous  system  on  the  sides  of  the  neck,  at  the 
point  of  junction  of  the  jugular  and  subclavian  veins.  The 
trunk  on  the  right  side  is  much  smaller  than  the  other  and 
is  known  as  the  "  right  lymphatic  duct."  It  collects  lymph 
from  the  right  side  of  the  thorax,  from  the  right  side  of  the  head 
and  neck,  and  the  right  arm.  The  lymph  from  all  the  rest 
of  the  Body  is  collected  into  the  thoracic  duct.  It  com- 
mences at  the  upper  part  of  the  abdominal  cavity  in  a  dilated 
reservoir  (the  receptaculum  chyli),  into  which  the  lacteals 
from  the  intestines,  and  the  lymphatics  of  the  rest  of  the 
lower  part  of  the  Body,  open.  From  thence  the  thoracic 
duct,  receiving  tributaries  on  its  course,  runs  up  the  thorax 
alongside  of  the  aorta  and,  passing  on  into  the  neck,  ends  on  the 
left  side  at  the  point  already  indicated;  receiving  on  its  way 
the  main  stems  from  the  left  arm  and   the  left  side  of  the 


LYMPHATIC  SYSTEM  AND  DUCVLESS   GLANDS.     351 

head  aud  neck.  The  thoracic  duct,  thus,  brings  back  to  the 
blood  much  more  lymph  than  the  right  lymphatic  duct. 

The  Serous  Cavities.  These  are  great  dependencies  of 
the  lymphatic  system  and  may  be  regarded  as  large  lacunae. 
Each  of  them  (peritoneal,  pleural,  arachnoidal  and  pericar- 
diac) is  lined  by  a  definite  epithelioid  layer  of  close-fitting 
polygonal  cells.  At  certain  points,  however,  openings  or 
stomata  occur,  surrounded  by  a  ring  of  smaller  cells,  and 
leading  into  tubes  which  open  into  subjacent  lymphatic 
vessels.  The  liquid  moistening  these  cavities  is,  then,  really 
lymph:  in  some  dropsical  diseases  it  collects  in  great  excess 
in  them. 

Lymphoid  or  Adenoid  Tissue  is  the  name  given  to  cer- 
tain aggregations  of  slightly  differentiated  cells  (leucocytes) 
supported  by  a  peculiar  form  of  tissue  and  found  in  con- 
nection with  the  lymphatic  system  in  many  parts  of  the  body. 
The  cells  much  resemble  white  blood -corpuscles,  though  their 
nuclei  usually  have  a  more  distinct  network,  and  they  are 
capable  of  executing  amoeboid  movements.  Many  of  them 
ultimately  are  carried  by  the  lymph  into  the  blood  to  be- 
come pale  corpuscles,  and  from  the  blood  some  again  pass 
back  into  the  lymph  by  migrating  through  the  walls  of 
the  blood-capillaries.  By  amoeboid  movement  these  lymph- 
corpuscles  can  take  up  foreign  particles  into  themselves 
and  creep  with  the  absorbed  material  along  lymph-canaliculi 
and  lymph-capillaries.  Lymphoid  tissue  is  extensively  devel- 
oped in  the  mucous  membrane  of  a  great  part  of  the  ali- 
mentary canal. 

The  deepest  layer  of  the  mucous  membrane  of  stomach 
and  intestines,  lying  next  to  the  submucous  coat  is  the  mus- 
cularis  mucosCB,  a  thin  layer  of  unstriped  muscular  tissue  quite 
distinct  from  the  proper  muscular  coats  of  those  viscera.  Above 
it  and  forming  the  main  bulk  of  the  mucous  membrane  lying 
between  the  glands  (o,  Fig.  112)  and,  in  the  small  intestine, 
the  main  mass  of  the  villi,  is  a  delicate  connective  tissue  con- 
sisting of  very  line  fibres  which  originated  by  the  branch- 
ing of  cells;  in  many  places  the  nuclei  of  these  cells  have  quite 
disappeared,  and  the  original  central  part  of  the  cell  is  only 
recognizable  as  the  place  from  which  the  branches  spread :  such 
tissue ia  reticular  connective  tissue,  [ts  meshes  contain  many 
leucocytes, and  the  mixture  of  reticular  tissue  with  these  cells 
titutee  adenoid  or  lymphoid  tissue.     At  numerous  soots, 


352  THE  HUMAN  BODY. 

especially  in  the  small  intestine,  the  cells  arc  peculiarly  abun- 
dant, forming  local  aggregations  of  about  the  size  of  the  head 
of  a  small  pin:  these  are  named  closed  or  solitary  follicles, 
A  minute  artery  enters  each  ami  gives  rise  to  a  capillary  net- 
work in  it.  from  which  the  Mood  is  carried  off  by  a  small  vein. 
The  follicle  lies  in,  or  rather  projects  into,  a  lymph-lacunas 
which  closely  invests  it,  and  is  in  direct  communication  with 
other  lymphatic  vessels  of  the  neighborhood.  The  central 
leucocytes  of  the  follicle  are  smaller  than  the  outer,  and  their 
nuclei  are  often  found  in  various  stages  of  karyokinesis. 
Each  follicle  must  therefore  be  regarded  as  a  seat  of  forma- 
tion of  new  leucocytes,  new-made  ones  being  pushed  to  the 
outside,  growing,  and  finally  being  cast  out  into  the  sur- 
rounding lymph-lacuna,  to  be  carried  away  in  the  lymph- 
current. 

Near  the  lower  part  of  the  ileum  large  numbers  of  solitary 
follicles  are  closely  collected  side  by  side  at  intervals  along 
the  part  of  the  bowel  opposite  to  that  at  which  the  mesentery 
joins  it:  these  aggregations  are  known  as  Peyer's  patchesj  and 
are  easily  recognizable  by  the  unaided  eye,  as  villi  are  absent 
from  the  part  of  the  mucous  membrane  opposite  them,  and 
they  also  cause  a  bulging,  visible  on  the  outside  of  the  intes- 
tine.    They  disappear  after  middle  life. 

The  Lymphatic  Glands  are  essentially  Peyer's  patches 
more  complicated  in  structure  by  the  fact  that  the  constitu- 
ent follicles  are  more  closely  united  and  are  gathered  into 
roundish  masses  instead  of  being  spread  out  in  a  single  layer. 
They  are  found  in  various  regions  on  the  course  of  lymphatic 
vessels;  especially  in  the  mesentery,  groin  and  neck.  In  the 
latter  posit  ion  they  often  inflame  and  give  rise  to  abscesses, 
especially  in  tuberculous  persons;  and  still  more  of  ten  enlarge, 
harden  and  become  more  or  less  tender,  so  as  to  attract  at- 
tention to  them.  In  common  parlance  it  is  then  frequently 
said  that  the  person's  "  kernels  have  come  down,"  or  that  he 
has  '•  waxing  kernels."  Each  lymphatic  gland  is  enveloped 
in  a  connective-tissue  capsule, partitions  of  which  incomplete- 
ly divide  it  into  chambers  in  which  the  lymphoid  tissue  lies. 
The  partitions  are  more  complete  in  the  outer  parts  of  the 
gland  (cortical  portion),  which  accordingly  looks  different 
from  the  central  portion  (medulla)  in  sections.  In  the  lym- 
phoid tissue  are  contained  many  leucocytes  in  process  of 
division.     "  Afferent "  lymphatic   vessels   open  into   the   pe- 


LYMPHATIC  SYSTEM  AND  DUCTLESS  GLANDS.     353 

riphery  of  the  gland,  and  efferent  vessels  arise  in  its  centre. 
Hence  the  lymph  in  its  flow  traverses  the  cellular  gland  sub- 
stance, and  in  its  course  picks  up  extra  corpuscles  which  it 
carries  on  to  the  blood.  In  the  lymphoid  tissue  there  is  a 
close  network  of  blood-capillaries.  It  is  clear  that  these 
organs  are  not  true  glands,  in  the  proper  sense  of  the  word: 
they  are  sometimes  called  lymphatic  ganglia,  but  that  sug- 
gests a  connection  with  nerve-centres;  a  good  name  for  them 
is  lymphatic  »<><l<>s.  In  Fig.  120  is  given  a  diagrammatic  rep- 
resentation of  a  lymphatic  node. 


Fir;.  121  —  Diagram  of  cross-sectio"  of  a  lymphatic  gland  :  al.  afferent  lymphatic 
vessels:  el,  efferent  lymphatic  vessel;  Ir.  one  of  the  connective-tissue  bands  sub- 
dividing: the  pland :  C.  cortical  portion:  M.  medullary  portion.  The  leucocytes  are 
represented  only  in  a  part  of  'he  rigrht  half  of  the  figure,  where  they  are  seen,  Ih, 
to  lie  closely  packed  in  the  centre  of  a  gland-chamber,  while  towards  the  walls  of 
the  chamber,  la,  where  they  are  naturally  less  closely  packed,  they  have  been 
washed  away,  as  often  happens  in  preparing  a  specimen,  leaving  the  reticular  sup- 
porting tissue  conspicuous. 

The  Movement  of  the  Lymph.  This  is  no  doubt  some- 
what irregular  in  the  commencing  vessels,  but,  on  the  whole, 
sets  on  to  the  larger  trunks  and  through  them  to  the  veins. 
In  many  animals  (as  the  frog)  at  points  where  the  lymphatics 
communicate  witli  the  veins,  there  are  found  regularly  con- 
tractile "lymph-hearts"  which  bent,  with  a  rhythm  independ- 
ent of  that  of  the  blood-heart,  and  pump  the  lymph  into  a 
vein.  In  the  human  Body, however,  there  are  no  such  hearts, 
and  the  flow  of  the  lymph  is  dependent  on  less  definite 
arrangements.  It  seems  to  he  maintained  mainly  by  three 
things.  (1)  Tin'  pressure  on  the  blood-plasma  in  the  capil- 
Iariee  Is  greater  than  that  in  the  great  veins  of  the  neck; 
hence  any  plasma  filtered  through  the  capillary-walls  will  be 


354  THE  HUMAN  BODY. 

under  a  pressure  which  will  tend  to  make  it  flow  to  the  ve- 
nous termination  of  the  thoracic  or  the  righl  lymphatic  duct. 
(2)  On  account  of  the  numerous  valves  in  the  lymphatic 
vessels  (which  all  only  allow  the  lymph  to  How  pasl  them  to 
larger  trunks)  any  movement  compressing  a  Lymph-vessel  will 
cause  an  onward  flow  of  its  contents.  The  influence  thus 
exerted  is  very  important.  If  a  tube  be  put  in  a  large  lym- 
phatic, say  at  the  top  of  the  leg  of  an  animal,  it  will  he  seen 
that,   the   lymph   only  Hows  out  very   slowly  while  the  animal 

is  quiet;   hut  as  si as  it  moves   the  leg  the  flow  is  greatly 

accelerated.  (:i)  During  each  inspiration  the  pressure  on  the 
thoracic  duct  is  less  than  that  in  the  lymphatics  in  part.-  of 
the  Body  outside  the  thorax  (see  Chap.  XXV).  Accord- 
ingly, at  that  time,  lymph  is  pressed,  or.  in  common  phrase, 
is  "sucked,"  into  the  thoracic  duct,  During  the  succeeding 
expiration  the  pressure  on  the  thoracic  duct  becomes  greater 
again,  and  some  of  its  contents  are  pressed  out;  hut  on 
account  of  the  valves  of  the  vessels  which  unite  to  form  the 
duct,  they  can  only  go  towards  the  veins  of  the  neck. 

During  digestion,  moreover,  contractions  of  the  villi  press 
on  the  lymph  or  chyle  within  them  and  force  it  on;  and  in 
certain  parts  of  the  Body  gravity,  of  course,  aids  the  flow, 
though  it  will  impede  it  in  others. 

The  Ductless  Glands— Spleen,  Thyroid,  Thymus,  Pit- 
uitary Body,  Suprarenals. — There  are  in  the  Body  several 
organs  of  such  considerable  size  and  so  constantly  present 
in  vertebrate  animals  that  a  priori  they  would  seem  to  be  of 
functional  importance.  Until  quite  recently,  however,  the 
functions  of  nearly  all  of  them  were  quite  problematical,  al- 
though it  has  long  been  known  that  pathological  changes  in 
some  of  them  were  associated  with  grave  conditions  of  general 
disease.  Even  yet  their  physiology  is  very  incompletely  known. 

When  we  speak  of  a  true  gland  we  mean  an  organ  that 
forms  some  definite  secretion  which  it  pours  out  in  a  separate 
form,  but  the  organs  we  are  about  to  consider  have  no  secret- 
ing recesses  and  no  duets:  nevertheless  some  of  them  un- 
doubtedly make,  and  pass  into  the  lymph  and  blood,  substances 
of  great  importance  to  the  healthy  working  of  the  Body. 
Some  true  glands  indeed  do  this,  quite  apart  from  the  manu- 
facture of  what  is  usually  spoken  of  as  their  secretion.  Why 
so  large  an  organ  as  the  liver  should  he  set  apart  for  the  for- 
mation of  so  comparatively  unimportant   a  digestive  fluid   as 


LYMPHATIC  SYSTEM  AND  DUCTLESS  GLANDS.     355 

the  bile  was  long  a  puzzle.  We  now  know  that  the  chief  use 
of  the  liver  is  connected  with  the  storage  and  formation  of  car- 
bohydrate materials  (see  Chap.  XXIX),  and  that,  quite  apart 
from  the  use  of  bile  in  digestion  or  the  elimination  of  part  of 
the  bile  as  waste,  the  liver  exerts  an  essential  influence  on  the 
whole  normal  nutritional  processes  of  the  Body.  Again,  in 
the  pancreas  we  have  an  organ  which  forms  a  very  important 
digestive  secretion,  and  it  might  well  be  that  this  was  its  sole 
use  in  the  economy.  But  when  the  pancreas  is  carefully  re- 
moved from  an  animal  great  nutritional  disturbances  follow, 
as  shown,  among  other  things,  by  diabetes,  i.e.,  the  presence 
of  sugar  in  the  urine.  Since  the  pancreatic  secretion  poured 
into  the  intestine  by  the  gland  duct  has  much  to  do  with  the 
digestion  of  starch  and  its  conversion  into  sugar,  it  might  be 
supposed  that  mere  digestive  disturbances  due  to  its  absence 
led  to  the  diabetic  and  general  changes.  But  this  is  not  so. 
If  a  piece  of  living  pancreas  be  transplanted  from  one  animal 
to  beneath  the  skin  of  another,  and  left  until  it  has  grown 
there,  the  pancreas  of  the  second  animal  may  be  removed 
without  causing  diabetes.  Moreover  it  is  possible  by  inject- 
ing melted  paraffin  into  the  pancreatic  duct  of  an  animal  not 
only  to  prevent  the  gland  secretion  from  reaching  the  intes- 
thie,  but  to  cause  atrophy  of  the  true  gland-cells.  Yet 
animals  so  treated  do  not  become  diabetic.  It  is  then  clear 
that  there  is  some  material  necessary  to  health  and  quite 
distinct  from  pancreatic  juice  formed  by  pancreatic  tissue  and 
taken  up  from  it  by  the  circulating  liquids.  Scattered  through 
the  pancreas,  and  quite  distinct  from  its  proper  gland  tissue, 
are  peculiar  patches  of  cells  very  richly  supplied  with  blood- 
vessels. Probably  these  cells  are  concerned  in  the  antidiabetic 
function  of  the  gland;  but  whether  through  special  cells 
or  not,  the  organ  has  an  important  inter  mil  secretion  to 
blood  and  lymph,  in  addition  to  its  external  secretion  to  its 
•  I net.  This  fact  may  have  a  very  wide  bearing:  it  may  be 
that  all  organs,  or  many  organs,  in  addition  fco  their  more  ob- 
vious functions,  do,  as  the  result  of  the  chemical  processes 
taking  place  in  them,  manufacture  substances  a  supply  of 
which, to  lymph  or  blood,  is  required  for  the  life  or  health  of 
distant  parts  of  the  Body.  Tint  waste  of  one  organ  before  its 
final  conversion  info  carbon  dioxide,  water,  or  urea,  for  elimi- 
nation from  tin;  system,  may  be  a  necessary  food  of  another. 
It  is,  for  example,  quite  possible  that  the  kreatin   formed  in 


356  TEE  HUMAN  BOD  Y. 

muscles  and  passed  from  them  to  the  circulating  fluid  is 
essential  to  the  general  health  of  the  Body.  There  are,  how- 
ever, so  many  muscles  that  the  removal  of  some  of  them,  as 
when  a  limb  is  amputated,  does  not  cut  off  the  kreatin  supply, 
and  so  disease  does  not  result.  When,  on  the  other  hand,  an 
organ  is  unique,  as  the  thyroid,  or  exists  only  in  a  single  pair, 
as  the  suprarenals,  then  removal  or  extensive  disease,  by  de- 
priving the  system  of  the  peculiar  internal  secretion  of  the 
organ  concerned  or,  possibly,  from  the  accumulation  within 
the  blood  of  substances  which  it  is  the  function  of  the  missing 
part  to  absorb  and  destroy,  may,  often  in  fact  does,  lead  to 
widespread  nutritional  changes,  resulting  in  death. 

The  Spleen.  This  is  an  organ  situated  at  the  left  end  of 
the  stomach  (L,  Fig.  110)  and  is  about  L70  grams  (6  oz. )  in 
weight.  Its  size  is,  however,  very  variable;  it  enlarges  dur- 
ing digestion  and  shrinks  after  it  until  the  next  meal.  In 
many  fevers,  especially  in  those  of  malarial  nature,  it  also 
becomes  enlarged,  frequently  to  a  very  great  extent,  and  this 
enlargement  may  become  permanent,  constituting  the  so- 
called  "' ague-cake."'  In  color  the  spleen  is  dark  red,  but  if 
cut  across  numerous  white  spots  of  about  1  mm.  (.,K  inch) 
diameter  are  seen  scattered  over  the  surface  of  the  section:  it 
is  very  richly  supplied  with  blood  which  is  carried  away  by 
the  splenic  vein  (7,  Fig.  119)  and  poured  into  the  portal  vein. 
The  spleen  possesses  on  its  exterior  a  connective-tissue  capsule 
very  rich  in  elastic  fibres  and  giving,  off  numerous  bands 
[trabecules)  which  branch  and  interlace  throughout  the  organ 
forming  a  spongy  mass,  in  the  spaces  of  which  is  contained  a 
soft  red  pulp  of  peculiar  structure.  The  arteries  of  the  organ 
by  frequent  branching  are  reduced  to  almost  capillary  size, 
and  these  terminal  twigs  enter  into  the  pulp,  and  there,  los- 
ing all  coats  but  the  lining  epithelium,  assume  the  structure 
of  capillaries.  The  cells  forming  the  walls  of  these  ca- 
pillaries next  separate  from  one  another  so  as  to  leave 
clefts  between  them,  and  at  the  same  time  become  irregu- 
larly branched  and,  joining  by  their  branches,  form  a  sup- 
porting framework  or  reticulum  through  the  pulp,  into 
which  latter  the  blood  is  poured  freely  through  the  spaces 
between  the  cells.  The  main  mass  of  the  splenic  pulp  con- 
sists of  red  blood-corpuscles,  some  normal  in  appearance, 
some  appearing  partly  broken  down;  mixed  with  these  are 
some  white  corpuscles,  and  some  larger  colorless  amoeboid 


LYMPHATIC  SYSTEM  AND  DUCTLESS  GLANDS.     357 

cells  in  which  are  often  found  one  or  more  red  corpuscles 
which  have  apparently  been  swallowed  by  them.  There  are 
also  many  pigmented  granules,  some  free  and  some  within 
amoeboid  cells;  they  are  apparently  the  debris  of  red  corpus- 
cles which  have  been  broken  down.  In  early  life  the  splenic 
pulp  also  contains  granular  colorless  cells  within  which  red 
corpuscles  are  seen  in  the  process  of  development.  The  whole 
histological  structure  of  the  adult  pulp  suggests  that  in  it 
many  red  blood-corpuscles  are  finally  destroyed,  setting  free 
haemoglobin  and  other  coloring  matters  derived  from  it.  This 
breaking  down  of  haemoglobin  must  also  give  rise  to  proteids 
and  substances  derived  from  the  chemical  degradation  of 
proteids,  and  the  spleen  is  extremely  rich  in  nitrogenous 
crystalline  substances.  The  increase  in  size  of  the  spleen 
during  digestion,  when  the  veins  of  the  alimentary  canal  are 
pouring  great  quantities  of  blood  laden  with  absorbed  mat- 
ters into  the  portal  system,  suggests  that  the  spleen  supplies 
things  to  the  liver  at  that  time  which  are  of  importance  to  it. 
There  is  reason  to  believe  that  the  main  coloring  matter 
of  the  bile  {bilirubin)  is  derived  from  the  haemoglobin  of  red 
corpuscles  which  have  completed  their  life-period  and  been 
destroyed,  and  it  may  be  that  the  spleen  takes  the  first  steps  in 
the  preparation  of  bilirubin  for  its  elimination  from  the  Body 
as  a  waste  product.  There  still  is,  however,  much  doubt  as 
to  the  real  function  of  the  spleen;  it  almost  certainly  plays 
an  important  part  in  the  proteid  metabolisms  of  the  Body. 
Though  so  large  an  organ  it  is  not  essential;  animals  from 
whom  it  has  been  completely  removed  can  live  a  long  time 
in  good  health.  The  red  marrow  of  spongy  bone  greatly  re- 
sembles the  splenic  pulp  in  histological  characters  and  may 
have  similar  functions  and  be  able  to  entirely  take  the  place  of 
the  spleen  when  the  organ  has  been  excised.  The  white  spots 
Been  on  the  cut  surface  of  a  spleen  are  sections  of  masses  of 
adenoid  tissue  attached  to  the  smaller  splenic  arteries  and 
named  Malpighian  corpuscles;  they  resemble  the  elosed  fol- 
licles of  the  intestine  in  structure. 

The  Thyroid  Body  or  Gland.  This  organ  lies  in  the 
neek  on  the  .-ides  6t  the  windpipe  and  consists  usually  of  a 
right  and  a  left  lobe  united  by  a  narrow  isthmus  across  the 
front  of  the  air-tube.  It  LB  about  thirty  grams  (two  ounces) 
iii  weight;  in  the  disease  known  as  goitre  it.  is  greatly  en- 
larged and  its   structure  altered.     The  thyroid  is  dark  red  in 


358  THE  in  ma  V  r.ODY. 

color  and  very  vascular,  richly  .supplied  with  nerves,  and  is 
subdivided  by  connective  tissue  into  cavities  or  alveoli,  the 
largesl  of  which  arc  jusl  visible  to  the  unaided  eye.  Each 
alveolus  is  lined  by  a  single  layer  of  cuboidal  cells,  and  filled 
by  a  glairy  fluid  which  appears  to  contain  mucin. 

The  very  abundant  blood-supply  of  the  thyroid  suggests 
that  it  is  the  seat  of  important  metabolic  or  chemical  changes, 
and  observation  and  experiment  confirm  this.  Extensive 
disease  of  the  thyroid  leads  to  great  changes  in  the  general 
nutrition  of  the  Hotly,  ending  in  the  condition  named 
myzodcema;  muciginous  liquid  collects  in  the  connective  tis- 
sues, nervous  and  muscular  activity  are  much  impaired, 
tremors  and  convulsions  occur.,  and  finally  a  semi  idiotic  con- 
dition (cretinism)  comes  on  and  is  followed  by  death  if  all  the 
gland  be  diseased.  Quite  similar  symptoms  follow  the  com- 
plete removal  of  the  thyroid  body  from  animals,  or  from  man 
for  tumors;  but  if  even  a  small  part  of  healthy  gland-tissue 
be  left  behind  the  symptoms  do  not  occur.  Moreover,  if  a 
portion  of  living  thyroid  from  one  animal  be  grafted  beneath 
the  skin  of  another,  the  thyroid  of  the  latter  can  be  com- 
pletely removed  without  influencing  the  general  health.  It 
would  seem  then  that  the  gland  is  the  place  of  formation  of 
some  substance  essential  to  the  healthy  working  of  the  Body, 
but  that  under  ordinary  conditions  of  life  the  whole  organ 
is  not  required  to  produce  the  necessary  minimum  of  this 
substance.  This  view  is  strengthened  by  the  fact  that  in 
patients  with  thyroid  disease  and  in  animals  deprived  of  the 
organ  the  symptoms  of  myxodcema  may  be  relieved  or  removed 
by  adding  raw  thyroid  tissue  to  the  food,  or  by  subcutaneous 
injection  of  the  expressed  juice  of  a  fresh  gland.  When  in- 
jected into  a  healthy  animal  extract  of  thyroid  causes  arterial 
dilatation,  and  a  lowering  of  blood  pressure. 

The  Thymus.  This  is  a  temporary  organ  of  unknown 
function.  It  has  its  greatest  size  in  proportion  to  the  whole 
weight  of  the  Body  a  short  time  before  birth.  After  birth 
it  grows  in  absolute  weight  for  some  time,  but  then  begins 
to  dwindle  away  and  has  usually  completely  disappeared  by 
the  twelfth  or  fourteenth  year.  It  lies  in  front  of  the  wind- 
pipe in  the  lower  part  of  the  neck  and  the  upper  part  of  the 
thorax,  and  is  the  "neck  "  sweetbread  of  the  butcher  as  dis- 
tinguished   from    the    true    sweetbread    or   pancreas.       The 


LYMPHATIC  SYSTEM  AXD  DUCTLESS  GLANDS.     359 

thymus  essentially  consists  of  adenoid  tissue,  and  is  well  sup- 
plied with  blood-vessels  and  lymphatics. 

The  Pituitary  Body  (Fig.  75)  is  in  part  an  offshoot  of  the 
brain,  and  probably  that  portion  of  it  is,  like  the  pineal  bod  v. 
a  remnant  of  a  once  functionally  important  ancestral  organ. 
The  anterior  lobe  of  the  pituitary  body,  however,  is  derived 
in  development  from  the  pharynx,  of  which  it  is  an  embryonic 
outgrowth.  This  part  of  it  somewhat  resembles  the  thyroid 
in  structure.  Complete  removal  of  the  pituitary  body  in  the 
case  of  cats  and  dogs  causes  a  lowering  of  temperature,  mus- 
cular twitchings  and  spasms,  difficulty  in  breathing,  general 
lassitude,  and  death  within  a  fortnight.  These  symptoms 
improve  when  extract  of  the  gland  is  injected.  The  organ 
has  therefore  been  supposed  to  form  an  internal  secretion  use- 
ful in  maintaining  the  nutrition  of  the  muscular  and  nervous 
systems.  Disease  of  the  pituitary  body  in  man  has  been  found 
to  be  associated  with  the  curious  condition  named  acromegaly, 
in  which  there  is  hypertrophy  of  the  bones  of  the  limbs  and 
face,  and  of  parts  of  the  skin  and  mucous  membranes.  In- 
jection of  the  extract  of  the  gland  causes,  in  a  normal  animal, 
a  more  powerful  but  not  quicker  heart-beat,  and  constriction 
of  the  arteries. 

The  Suprarenal  Capsules  or  Adrenals  are  a  pair  of 
small  organs,  weighing  together  about  12  grams  (foz.)  placed 
one  on  the  top  of  each  kidney.  They  have,  however,  no  inti- 
mate connection  with  the  kidneys,  and  in  many  animals  are 
placed  at  some  distance  from  them.  Each  consists  of  a  denser 
less  colored  external  cortex,  and  a  central  deep  yellow-brown 
softer  medulla.  The  cortex  is  subdivided  into  chambers  by 
connective  tissue,  and  the  chambers  are  filled  by  closely 
packed,  polygonal  nucleated  cells.  Similar  cells  are  found 
in  tin;  medulla,  which  is,  moreover,  closely  connected  with 
the  sympathetic  system  and  is  richly  supplied  with  nerves. 

It  was  noticed  some  fifty  years  ago  by  a  physician  named 
Addison  thai  certain  obscure  diseased  conditions  characterized 
by  greal  debility  and  by  the  appearance  of  bronzed  patches 
on  the  skin,  and  leading  to  death,  were  found  on  post-mortem 
examination  to  be  accompanied  by  disease  of  the  adrenals. 
The  disease  lias  hence  been  named  Addison's  disease.  When 
the  suprarenal  capsules  are  completely  removed  from  animals 
a  similar  fata!  diseased  condition  results,  death  taking  place 
in  warm-blooded  animals  within  two  or  three  days,  and  be- 


360  THE  HUMAN  BODY. 

ing  preceded  by  muscular  weakness,  dilatatton  of  the  arteries, 
mental  feebleness  and  general  prostration.  The  exact  rule 
played  in  the  organism  by  these  small  but  essential  organs  is 
still  unknown,  but  they  form  substances  which  have  a  pro- 
found effect  on  the  nerves  of  the  heart  and  blood-vessels. 
A  very  minute  portion  of  the  watery  or  alcoholic  extract  of  a 
suprarenal  capsule  when  injected  into  avein  of  an  animal  causes 
a  very  slow  heart-beat,  or  even  complete  inhibition  of  the 
auricles.  If  the  cardio-inhibitory  nerves  have  first  been  cut, 
on  the  other  hand,  the  injection  causes  a  great  increase  in 
the  rate  of  heart-beat  and  a  great  increase  of  its  force,  espe- 
cially that  of  the  auricles.  The  small  arteries  become  greatly 
contracted,  and  this  combined  wit  li  the  powerful  heart-beats 
leads  to  a  very  great  increase  of  arterial  pressure.  The  arterial 
constriction  is  not  due  to  stimulation  of  the  vaso-constrictor 
centre,  but  to  a  direct  action  on  the  muscular  coats  of  the 
arteries:  it  is  very  transient.  The  skeletal  muscles  are  also 
affected,  the  period  of  a  simple  muscular  contraction  being 
greatly  prolonged,  and  this  effect  lasts  much  longer  than  the 
changes  produced  in  the  organs  of  circulation.  The  active 
material  exists  only  in  the  medulla  of  the  adrenal,  is  efficient 
in  extremely  minute  doses,  is  dialyzable,  and  its  efficacy  is  not 
impaired  by  short  boiling. 

It  would  appear  then  that  the  suprarenale  are  constantly 
forming  and  passing  into  the  blood  minute  quantities  of  a 
substance  which  is  of  great  importance  for  the  maintenance 
of  the  "tone"  of  the  muscles,  especially  of  the  cardiac  and 
arterial  muscles.  Whether  in  addition  they  also  remove 
noxious  substances  from  the  blood,  tli£  accumulation  of  which 
after  their  removal  is  one  cause  of  the  death  which  results,  is 
still  undecided.  The  blood  of  such  animals  acts  as  a  poison 
to  other  animals,  and  this  has  been  supposed  to  be  due  to  the 
presence  in  it  of  a  specific  poison  which  the  adrenals  normally 
pick  up  and  destroy:  but  it  is  clear  that  the  blood  of  an  ani- 
mal dying  from  extensive  malnutrition  produced  in  any  way 
would  be  quite  abnormal,  and  might  well  be  poisonous  to  other 
animals.  The  same  remark  may  be  made  as  to  the  poisonous 
character  of  the  blood  of  animals  dying  as  a  result  of  removal 
of  the  thyroid:  there  is  no  satisfactory  evidence  that  it  is  due 
to  the  accumulation  of  any  one  special  toxic  substance  which 
it  is  a  function  of  the  thyroid  to  remove:  still,  it  maybe. 
The  symptoms  produced  by  its  injection  are  quite  different 
from  those  produced  by  injection  of  thyroid  extract. 


CHAPTER  XXIV. 

DIGESTION. 

The  Object  of  Digestion.  Of  the  various  foodstuffs  swal- 
lowed, some  are  already  in  solution  and  ready  to  dialyze  at 
once  into  the  lymphatics  and  blood-vessels  of  the  alimentary 
canal;  others,  such  as  a  lump  of  sugar,  though  not  dissolved 
when  put  into  the  mouth,  are  readily  soluble  in  the  liquids 
found  in  the  alimentary  canal,  and  need  no  further  digestion. 
In  the  case  of  many  most  important  foodstuffs,  however, 
special  chemical  changes  have  to  be  wrought,  either  with  the 
object  of  converting  insoluble  bodies  into  soluble,  or  non- 
dialyzable  into  dialyzable,  or  both.  The  different  secretions 
poured  into  the  alimentary  tube  act  in  various  ways  upon 
different  foodstuffs,  and  at  last  get  them  into  a  state  in  which 
thev  can  pass  into  the  circulating  medium  and  be  carried  to 
all  parts  of  the  Body. 

The  Saliva.  The  first  solvent  that  the  food  meets  with 
is  the  saliva,  which,  as  found  in  the  mouth,  is  a  mixture  of 
pure  saliva,  formed  in  parotid,  submaxillary,  and  sublingual 
glands,  with  the  mucus  secreted  by  small  glands  of  the  buccal 
mucous  membrane.  This  mixed  saliva  is  a  colorless,  cloudy, 
feebly  alkaline  liquid,  "ropy"  from  the  mucin  present  in  it, 
and  usually  containing  air-bubbles.  Pure  saliva,  as  obtained 
by  putting  a  fine  tube  in  the  duct  of  one  of  the  salivary 
glands,  is  more  fluid  and  contains  no  imprisoned  air. 

I  3imlly  but  little  saliva  is  secreted  ;  the  presence  of  food 
in  the  mouth,  especially  highly  flavored  or  acid  food,  leads 
to  a  more  abundant  flow  :  the  mere  chewing  of  a  tasteless 
inert  substance  will,  however, excite  some  secretion.  The  secre- 
tion thus  brought  about  is  reflex:  the  afferent  fibres  running 
to  the  brain  in  the  glossopharyngeal  and  lingual  nerves,  and 
exciting  there  the  centre  from  which  the  efferent  secretory 
nerve-fibres  Cor  the  glands  arise.  The  centre  may  be  excited 
in  other  ways:  as  by  nausea,  or  through  the  nerves  of  eye  or 
■    when   the  sight  or  smell  of  desirable  food  makes  "  the 

mouth  water." 

361 


362  TEE  III  MAX  BODY. 

The  uses  of  the  saliva  are  for  the  mosl  pari  physical  and 
mechanical.  It  keeps  the  mouth  moist  and  allows  us  to  speak 
with  comfort;  mosl  young  orators  know  the  distress  occa- 
sioned by  the  suppression  of  the  salivary  secretion  through 
nervousness,  and  the  imperfect  efficacy  under  such  circum- 
stances of  the  traditional  glass  of  water  placed  beside  public 
speakers.  The  saliva,  also,  enables  us  to  swallow  dry  food; 
such  a  thing  as  a  cracker  when  chewed  would  give  rise  merely 
to  a  heap  of  dust,  impossible  to  swallow,  were  not  the  mouth 
cavity  kept  moist.  This  fact  used  to  be  taken  advantage  of 
in  the  East  Indian  rice  ordeal  for  the  detection  of  criminals. 
The  guilty  person,  believing  firmly  that  he  cannot  swallow 
the  parched  rice  given  him,  and  fearful  of  detection,  is  apt  to 
have  the  nerve-centres  of  his  salivary  glands  inhibited  or 
paralyzed  by  terror,  and  does  actually  become  unable  to  swal- 
low the  rice;  while  in  those  with  clear  consciences  the  nerv- 
ous system  excites  the  usual  reflex  secretion,  and  the  dry 
food  gives  rise  to  no  difficulty  in  its  deglutition.  The  saliva, 
also,  dissolves  such  bodies  as  salt  and  sugar,  when  they  are 
taken  into  the  mouth  in  solid  form,  and  enables  us  to  taste 
them;  undissolved  substances  are  not  tasted,  a  fact  which  any 
one  can  verify  for  himself  by  wiping  his  tongue  dry  and 
placing  a  fragment  of  sugar  upon  it.  No  sweetness  will  be 
felt  until  a  little  moisture  has  exuded  and  dissolved  part  of 
the  sugar. 

In  addition  to  such  actions  the  saliva,  however,  exerts  a 
chemical  one  on  an  important  foodstuff.  Starch  (although 
it  swells  up  greatly  in  hot  water)  is  insoluble,  and  could  not 
be  absorbed  from  the  alimentary  canal.  The  saliva  contains 
an  enzyme,  ptyalin,  which  has  the  power  of  turning  starch 
into  soluble  substances.  Until  recently  the  chief  product  was 
believed  to  be  grape  sugar  (glucose);  but  it  is  now  ascertained 
that  it  is  maltose,  belonging  to  the  cane-sugar  chemical  series. 
In  the  small  intestine  the  maltose  is  changed  into  glucose  and 
absorbed ;  so  the  chemical  action  of  ptyalin  upon  starch  is  at 
most  but  a  preparatory  one.  In  effecting  the  change  the  ptyalin 
is  not  altered;  a  very  small  amount  of  it  can  convert  a  vast 
amount  of  starch,  and  does  not  seem  to  have  its  activity  im- 
paired in  the  process.  The  starch  is  made  to  combine  with 
the  elements  of  one  or  more  molecules  of  water,  and  the 
ptyalin  is  unchanged. 

This  faculty  of  ptyalin  is  known  as  amylolytic :  and  since 


DIGESTION.  363 

it  is  associated  with  the  taking  up  of  a  molecule  of  water  is 
a  hydrolytic  action.  Ptyalin  is  a  typical  enzyme;  it  differs 
from  the  true  ferments,  such  as  yeast,  in  the  fact  that  it  is 
not  a  living  organism,  and  does  not  multiply  during  the  oc- 
currence of  the  change  which  it  sets  up;  its  activity  belongs 
to  the  obscure  chemical  category  of  contact  actions. 

In  order  that  the  ptyalin  may  act  upon  starch  certain 
conditions  are  essential.  Water  must  be  present,  and  the 
liquid  must  be  neutral  or  feebly  alkaline;  acids  retard,  or  if 
stronger,  entirely  stop  the  process.  The  change  takes  place 
most  quickly  at  about  the  temperature  of  the  human  Body, 
and  is  greatly  checked  by  cold.  Boiling  the  saliva  destroys 
its  ptyalin  and  renders  it  quite  incapable  of  converting  starch. 
Cooked  starch  is  changed  more  rapidly  and  completely  than 
raw. 

Saliva  has  another  important  but  indirect  influence  in 
promoting  digestion.  Weak  alkalies  stimulate  the  mucous 
membrane  of  the  stomach  and  cause  it  to  pour  forth  more 
gastric  juice.  Hence  the  efficacy  of  a  little  carbonate  of  soda, 
taken  before  meals,  in  some  forms  of  dyspepsia.  The  saliva 
by  its  alkalinity  exerts  such  an  action;  and  this  is  one  reason 
why  food  should  be  well  chewed  before  being  swallowed ;  for 
then  its  taste,  and  the  movements  of  the  jaws,  cause  the 
secretion  of  more  saliva. 

Deglutition.  A  mouthful  of  solid  food  is  broken  up  by 
the  teeth,  and  rolled  about  the  mouth  by  the  tongue,  until  it 
i.s  thoroughly  mixed  with  saliva  and  made  into  a  soft  pasty 
muss.  The  muscles  of  the  cheeks  keep  this  from  getting 
between  them  and  the  gums;  persons  with  facial  paralysis 
have,  from  time  to  time,  to  press  out  with  the  finger  food 
which  has  collected  outside  the  gums,  where  it  can  neither  be 
chewed  nor  swallowed.  The  mass  is  finally  sent  on  from  the 
mouth  to  the  stomach  by  the  process  of  deglutition,  which  is 
described  as  occurring  in  three  stages.  The  first  stage  in- 
cludes the  passage  from  the  mouth  into  the  pharynx.  The 
food  being  collected  into  a  heap  on  the  tongue,  the  tip  of 
that  organ  is  placed  against  the  front  of  the  hard  palate,  and 
then  the  rest  of  the  tongue  is  raised  from  before  back,  so  as 
to  press  the  food  mass  between  it  and  the  palate,  and  drive  it 
back  through  the  fauces.  This  portion  of  the  act  of  swallow- 
ing is  voluntary,  or  at  least  i.s  under  the  control  of  the  will, 
although  it  commonly  takes  place  unconsciously.     The  second 


364  THE  HUMAN  BOD  Y. 

stage  of  deglutition  is  that  in  which  the  food  passes  through 
the  pharynx;  it  is  the  most  rapid  part  of  its  progress,  since 
the  pharynx  has  to  be  emptied  quickly  bo  as  to  clear  the 
opening  of  the  air-passages  for  breathing  purposes.  The 
food  mass,  passing  hack  over  the  root  of  the  tongue,  pushes 
down  the  epiglottis;  at  the  same  time  the  larynx  (or  voice- 
box  at  the  top  of  the  windpipe)  is  raised,  so  as  to  meet  it, 
and  thus  the  passage  to  the  lungs  is  closed  ;  muscles  around 
the  aperture  probably  also  contract  and  narrow  the  opening. 
The  raising  of  the  larynx  can  be  readily  felt  by  placing  the 
finger  on  the  large  cartilage  forming  "Adam's  apple"  in  the 
neck,  and  then  swallowing  something.  The  soft  palate  is  at 
the  same  time  raised  and  stretched  horizontally  across  the 
pharynx,  thus  cutting  off  communication  with  its  upper,  or 
respiratory  portion,  leading  to  the  nostrils  and  Eustachian 
tubes.  Finally,  the  isthmus  of  the  fauces  is  closed  as  soon  as 
the  food  has  passed  through,  by  the  contraction  of  the  mus- 
cles on  its  sides  and  the  elevation  of  the  root  of  the  tongue. 
All  passages  out  of  the  pharynx  except  the  gullet  are  thus 
blocked,  and  when  the  pharyngeal  muscles  contract  the  food 
can  be  squeezed  only  into  the  oesophagus.  The  muscular 
movements  concerned  in  this  part  of  deglutition  are  all  re- 
flexly  excited;  food  coming  in  contact  with  the  mucous  mem- 
brane of  the  pharynx  stimulates  afferent  nerve-fibres  in  it; 
these  excite  the  centre  of  deglutition  which  is  placed  in  the 
medulla  oblongata,  and  from  it  efferent  nerve-fibres  proceed 
to  the  muscles  concerned  and  (under  the  co-ordinating  influ- 
ence of  the  centre)  cause  them  to  contract  in  proper  sequence. 
The  pharyngeal  muscles,  although  of  the  striped  variety,  are 
but  little  under  the  control  of  the  will;  it  is  extremely  diffi- 
cult to  go  through  the  movements  of  swallowing  without 
something  (if  only  a  little  saliva)  to  swallow  and  thus  excite 
the  movements  reflexly.  .Many  persons,  after  having  got  the 
mouth  completely  empty  cannot  perform  the  movements  of 
the  second  stage  of  deglutition  at  all.  On  account  of  the  re- 
flex nature  of  the  contractions  of  the  pharynx,  any  food  which 
has  once  entered  it  must  be  swallowed:  the  isthmus  of  the 
fauces  is  a  sort  of  Rubicon;  food  that  has  passed  it  must 
continue  its  course  to  the  stomach,  although  the  swallower 
learnt  immediately  that  he  was  taking  poison.  The  third 
stage  of  deglutition  is  that  in  which  the  food  is  passing  along 
the  gullet,  and  is  comparatively  slow.     Even  liquid  substances 


DIGESTION.  365 

do  not  fall  or  flow  down  this  tube,  but  have  their  passage 
controlled  by  its  muscular  coats,  which  grip  the  successive 
portions  swallowed  and  pass  them  on.  Hence  the  possibility 
of  performing  the  apparently  wonderful  feat  of  drinking  a 
glass  of  water  while  standing  upon  the  head,  often  exhibited 
by  jugglers;  the  onlookers  forget  that  the  same  thing  is  done 
every  day  by  horses,  and  other  animals,  which  drink  with  the 
pharyngeal  end  of  the  gullet  lower  than  the  stomach.  The 
movements  of  the  oesophagus  are  of  the  kind  known  as  ver- 
micular  or  peristaltic.  Its  circular  muscular  fibres  contract 
behind  the  morsel  and  narrow  the  passage  there;  and  the  con- 
striction then  travels  along  to  the  stomach,  pushing  the  food 
in  front  of  it.  Simultaneously  the  longitudinal  fibres,  at  the 
point  where  the  food-mass  is  at  any  moment  and  immediately 
in  front  of  that,  contracting,  shorten  and  widen  the  passage. 
The  Gastric  Juice. — The  food  having  entered  the  stom- 
ach is  subjected  to  the  action  of  the  gastric  juice,  which  is  a 
thin,  colorless  or  pale  yellow  liquid,  of  a  strongly  acid  reac- 
tion. It  contains  as  specific  elements  free  hydrochloric  acid 
(about  .2  per  cent),  and  an  enzyme  called  pepsin  which,  in 
acid  liquids,  has  the  power  of  converting  the  ordinary  non- 
dialyzable  proteids  which  we  eat,  into  closely  allied  bodies, 
some  of  which  are  dialvzable  and  named  peptones.  It  also 
dissolves  solid  proteids,  changing  them  similarly.  Dilute 
acids  will  by  themselves  produce  the  same  changes  in  the 
course  of  several  days,  but  in  the  presence  of  pepsin  and  at 
the  temperature  of  the  Body  the  conversion  is  far  more 
rapid.  In  neutral  or  alkaline  media  the  pepsin  is  inactive; 
and  cold  checks  its  activity.  Boiling  destroys  it.  In  addi- 
tion to  pepsin,  gastric  juice  contains  another  enzyme  (re/rnui) 
which  coagulates  the'caseinogen  of  milk,  as  illustrated  by 
the  use  of  "rennet,"  prepared  from  the  mucous  membrane 
of  the  calf's  digestive  stomach,  in  cheese-making.  The  acid 
of  the  natural  gastric  juice  would,  it  is  true,  precipitate  the 
casein,  but  such  precipitate  is  quite  different  from  the  true 
tyrein,  and  neutralized  gastric  juice  still  possesses  this  power; 
moreover,  boiled  gastric  juice  loses  the  milk-clotting  property, 
and  a  very  little  normal  juice  can  coagulate  a  great  quantity 
of  milk.  The  curdled  condition  of  the  milk  regurgitated  by 
infants  is,  therefore,  not  any  sign  of  a  disordered  state  of  the 
stomach,  as  nurses  commonly  suppose.     It  is  proper  for  milk 


366  THE  HUMAN  BODY. 

to  undergo  this  change,  before  the  pepsin  and  acid  of  the 
gastric  juice  digest  it. 

The  most  important  change  effected  by  the  gastric  juice 
is  that  of  the  proteids.  This  may  be  studied  either  on  natu- 
ral juice  obtained  from  the  stomach  of  an  animal  through  an 
opening  (gastric  fistula)  or  on  an  artificial  juice  prepared  by 
extracting  the  mucous  membrane  of  a  fresh  stomach  with 
glycerine,  and  adding  a  large  quantity  of  dilute  (0.2$)  hydro- 
chloric acid.  If  blood-fibrin  or  boiled  white  of  egg  be  placed 
in  such  a  mixture  and  kept  at  a  temperature  of  about  38°  C. 
(100°  F.)  these  bodies  swell,  become  transparent,  and  soon 
dissolve;  and  all  other  solid  proteids  undergo  similar  changes. 
If  the  solution  be  now  neutralized  a  small  white  precipitate 
of  parapeptone  (which  is  probably  only  ordinary  acid  albu- 
min) is  obtained.  The  filtrate  from  this  gives  no  precipitate 
on  boiling,  but  an  abundant  one  of  albumose  on  the  addition 
of  ammonium  sulphate.  The  filtrate  from  this  precipitate 
yields  an  abundant  precipitate  of  peptone  when  alcohol  is 
added.  Peptone  is  dialyzable,  though  not  so  easily  as  saline 
bodies,  and  in  this  differs  from  albumose  and  parapeptone 
and  all  other  proteids.  The  parapeptone  is  probably  a  bye- 
product  due  to  the  action  of  the  acid  of  the  juice  alone:  the 
albumose  and  peptone  are  true  products  of  peptic  digestion  of 
proteids,  due  to  their  breaking  up  with  concomitant  hy- 
dration, the  peptone  being  the  more  finished  or  complete 
digestive  product.  If  instead  of  solid  proteids  we  use  solu- 
tion of  white  of  egg  or  of  serum  albumin,  the  earlier  stages  of 
the  process  cannot  be  followed  by  the  eye,  but  the  final  prod- 
ucts are  the  same:  the  original  proteid  disappears,  giving 
origin  to  some  parapeptone, to  albumose,  and  to  peptone;  and 
prolonged  artificial  peptic  digestion  causes  no  further  breaking 
up  of  the  albumose  or  peptone.  Peptone  is  very  soluble  in 
water,  and  its  solutions  are  not  coagulated  by  boiling.  A 
very  small  amount  of  pepsin  can,  if  some  acid  be  added  from 
time  to  time,  convert  a  very  large  amount  of  proteid :  it  is  de- 
stroyed by  boiling. 

Gastric  Digestion.  The  process  of  swallowing  is  contin- 
uous, but  in  the  stomach  the  onward  progress  of  the  food  is 
stayed  for  some  time.  The  pyloric  sphincter,  remaining  con- 
tracted, closes  the  aperture  leading  into  the  intestine,  and  the 
irregularly  disposed  muscular  layers  of  the  stomach  keep  its 
semi-liquid  contents  in   constant  movement,  maintaining  a 


DIGESTION.  367 

sort  of  churning  by  which  all  portions  are  brought  into  con- 
tact with  the  mucous  membrane,  and  thoroughly  mixed  with 
the  secretion  of  its  glands.  The  gelatin-yielding  connective 
tissue  of  meats  is  dissolved  away,  and  the  proteid-containing 
fibres,  left  loose,  are  dissolved  and  changed.  The  albuminous 
walls  of  the  fat-cells  are  dissolved  and  their  oily  contents  set 
free;  but  the  gastric  juice  does  not  act  upon  the  latter.  Cer- 
tain mineral  salts  (as  phosphate  of  lime,  of  which  there  is 
always  some  in  bread)  which  are  insoluble  in  water  but  solu- 
ble in  dilute  acids,  are  also  dissolved  in  the  stomach.  On 
the  other  hand,  the  gastric  juice  has  itself  no  action  upon 
starch,  and  since  ptyalin  does  not  act  at  all,  or  only  imper- 
fectly, in  an  acid  medium,  the  activity  of  the  saliva  in  con- 
verting starch  is  stayed  in  the  stomach.  By  the  solution  of 
the  white  fibrous  connective  tissue,  that  disintegration  of  ani- 
mal foods  commenced  by  the  teeth,  is  carried  much  farther 
in  the  stomach,  and  the  food-mass,  mixed  with  much  gastric 
secretion,  becomes  reduced  to  the  consistency  of  a  thick  soup, 
usually  of  a  grayish  color.  In  this  state  it  is  called  chyme. 
Chyme  contains,  after  an  ordinary  meal,  much  peptone,  though 
some  of  this  has  been  already  dialyzed  into  the  gastric  mucous 
membrane  and  carried  off  along  with  other  dissolved  dialyz- 
able  bodies,  such  as  salts  and  sugar.  The  albumose,  fats,  and 
starch  still  remain  in  the  chyme.  After  the  food  has  re- 
mained in  the  stomach  some  time  (one  and  a  half  to  two 
hours)  the  chyme  begins  to  be  passed  071  into  the  intestine 
in  successive  portions.  The  pyloric  sphincter  relaxes  at  in- 
tervals, and  the  rest  of  the  stomach,  contracting  at  the  same 
moment,  injects  a  quantity  of  chyme  into  the  duodenum; 
this  is  repeated  frequently,  the  larger  undigested  fragments 
being  at  first  unable  to  pass  the  orifice.  At  the  end  of  about 
three  or  four  hours  after  a  meal  the  stomach  is  again  quite 
emptied,  the  pyloric  sphincter  finally  relaxing  to  a  greater 
extent  and  allowing  any  larger  indigestible  masses,  which  the 
gastric  juice  cannot  break  down,  to  be  driven  into  the  in- 
testine. 

The  Chyle.  When  the  chyme  passes  into  the  duodenum 
it  finds  preparation  made  for  it.  The  pancreas  is  in  reflex 
connection  with  the  stomach,  and  its  nerves  cause  it  to  com- 
mence  secreting  as  soon  as  food  enters  the  latter;  hence  a 
quantity  of  its  secretion  is  already  accumulated  in  the  intes- 
tine when    food    enters.      The   gall-bladder   is  distended    with 


368  THE  HUMAN  BODY. 

bile,  secreted  since  the  last  meal;  this  passing  down  the 
hepatic  duct  lias  been  turned  back  up  the  cystic  duct  {Dc, 
Fig.  11."))  on  account  of  the  closure  of  the  common  bile-duct. 
The  acid  chyme,  stimulating  nerve-endings  in  the  duodenal 
mucous  membrane,  causes  reflex  contraction  of  the  muscular 
coat  of  the  gall-bladder,  and  a  relaxation  of  the  oritice  of  the 
common  bile-duct;  and  so  a  gush  of  bile  is  poured  out  on  the 
chyme.  From  this  time  on,  both  liver  and  pancreas  continue 
secreting  actively  for  some  hours,  and  pour  their  products 
into  the  intestine.  The  glands  of  Brunner  and  the  crypts 
of  Lieberkuhn  are  also  set  at  work,  but  concerning  their 
physiology  we  know  very  little.  All  of  these  secretions  are 
alkaline,  and  they  suffice  very  soon  to  more  than  neutralize 
the  acidity  of  the  gastric  juice,  and  to  convert  the  acid  chyme 
into  alkaline  chyle,  which,  after  an  ordinary  meal,  will  con- 
tain a  great  variety  of  things:  mucus  derived  from  the  ali- 
mentary canal;  ptyalin  from  the  saliva;  pepsin  from  the 
stomach;  water,  partly  swallowed  and  partly  derived  from 
the  salivary  and  other  secretions;  the  peculiar  constituents  of 
the  bile  and  pancreatic  juice  and  of  the  intestinal  secretions; 
some  undigested  proteids;  unchanged  starch;  oils  from  the 
fats  eaten;  peptones  formed  in  the  stomach  but  not  yet  ab- 
sorbed; albumose;  parapeptone;  possibly  salines  and  sugar 
which  have  also  escaped  absorption  in  the  stomach;  and  in- 
digestible substances  taken  with  the  food. 

The  Pancreatic  Secretion  is  clear,  watery,  alkaline,  and 
much  like  saliva  in  appearance.  The  Germans  call  the  pan- 
creas the  "abdominal  salivary  gland."  In  digestive  prop- 
erties, however,  the  pancreatic  secretion  is  far  more  impor- 
tant than  the  saliva,  or  even  the  gastric  juice.  Starch 
it  changes  as  the  saliva  does,  but  converts  it  into  maltose 
more  quickly  :  and  it  acts  also  on  proteids  and  fats.  It 
is  by  far  the  most  important  of  all  the  digestive  secretions. 
All  proteids  not  already  converted  into  peptone  or  albumose 
are  acted  upon  by  the  pancreatic  juice  even  more  ener- 
getically than  in  the  stomach,  being  not  only  converted  into 
peptone,  but  in  part  further  broken  up,  if  the  digestion  (arti- 
ficial) be  prolonged,  and  converted  into  crystallizable  nitrog- 
enous bodies  which,  unlike  peptone,  retain  no  proteid-like 
characters:  the  chief  of  these  are  leuc'ui  and  tyrosin,  the 
former  allied  chemically  to  the  fatty  acids,  the  other  to  bodies 
of  the  aromatic  series.     In  normal  digestion,  however,  it  is 


DIGESTION.  369 

probable  that  but  little  of  the  proteid  is  broken  up  beyond 
the  peptone  stage,  and  all  of  it  never  is;  an  albumose  is 
formed  as  an  intermediate  product.  The  enzyme  concerned  is 
trypsin  ;  it  is  active  only  in  an  alkaline  or  neutral  medium, 
and  before  dissolving  solid  proteids  does  not  cause  them  to 
swell  and  become  transparent  as  pepsin  does.  Like  the  other 
digestive  ferments,  it  is  most  active  at  about  the  temperature 
of  the  Body,  and  is  destroyed  by  boiling.  On  fats  the  pan- 
creatic secretion  has  a  double  action.  To  a  certain  extent  it 
breaks  them  up,  with  hydration,  into  free  fatty  acids  and 
glycerin;  for  example — 

(CJ8HS50)3  )  o   4-  3H  0  -  3(C»H»°  I  o\  +  C^  I  O 
C3H6       j-u3-r-dttau-o^  Hj-U|  H3fu=' 

1  Stearin       +       3  Water  =         3  Stearic  acid    +         1  Glycerine. 

The  fatty  acid  then  combines  with  some  of  the  alkali  present 
to  make  a  soap,  which  being  soluble  in  water  is  capable  of 
absorption.  Glycerin,  also,  is  soluble  in  water  and  dialyz- 
able.  The  greater  part  of  the  fats  are  not,  however,  so  broken 
up,  but  are  simply  mechanically  separated  into  droplets, 
which  remain  suspended  in  the  chyle  and  give  it  a  whitish 
color,  just  as  the  cream -drops  are  suspended  in  milk,  or  the 
olive-oil  in  mayonnaise  sauce.  This  is  effected  by  the  help  of 
a  quantity  of  albumin  which  exists  dissolved  in  the  pancreatic 
secretion.  In  the  stomach,  the  animal  fats  eaten  have  lost 
their  cell-walls,  and  have  become  melted  by  the  temperature 
to  which  they  were  exposed.  Hence  their  oily  part  floats  free 
in  the  chyme  when  it  enters  the  duodenum.  If  oil  be  shaken 
up  with  water,  the  two  cannot  be  got  to  mix;  immediately 
the  shaking  ceases,  the  oil  floats  up  to  the  top;  but  if  some 
raw  egg  be  added,  a  creamy  mixture  is  readily  formed,  in 
which  the  oil  remains  for  a  long  time  evenly  suspended  in 
the  watery  menstruem.  The  reason  of  this  is  that  each  oil- 
droplet  becomes  surrounded  by  a  delicate  pellicle  of  albumin, 
and  is  thus  prevented  from  fusing  with  its  neighbors  to  make 
large  drops,  which  would  soon  float  to  the  top.  Such  a  mix- 
ture is  called  an  emulsion,  and  the  albumin  of  the  pancreatic 
secretion  emulsifies  the  oils  in  the  chyle,  which  becomes 
white  (for  the  same  reason  as  milk  is  that  color)  because  the 
innumerable  tiny  oil-drops  floating  in  it  reflect  all  the  light 
which  falls  on  its  surface. 

In    brief,  the   pancreatic  secretion    converts   starch    into 


370  THE  HUMAN  BODY. 

maltose  ;  dissolves  proteids  (if  necessary)  and  converts  them 
into  peptones;  emulsifies  fats,  and,  to  a  certain  extent,  breaks 
them  up  into  glycerin  and  fatty  acids;  the  latter  are  then 
saponified  by  the  alkalies  present. 

The  Bile. — Human  bile  when  quite  fresh  is  a  golden 
brown  liquid;  it  becomes  green  when  kept.  As  formed  in 
the  liver  it  contains  hardly  any  mucin,  but  if  it  make  any 
stay  in  the  gall-bladder  it  acquires  much  from  the  lining  mem- 
brane of  that  bag,  and  becomes  slimy  and  "  ropy."'  It  is 
alkaline  in  reaction  and,  besides  coloring  matters  (the  more 
important  of  which,  biliniliiii,  is  probably  a  waste  product 
derived  from  haemoglobin),  contains  mineral  salts  and  water, 
and  the  sodium  salts  of  two  nitrogenized  acids,  taurocholic 
and  glychocholic,  the  former  predominating  in  human  bile. 

Pettenkofefs  Bile  Test.  If  a  small  fragment  of  cane 
sugar  be  added  to  some  bile,  and  then  a  large  quantity  of  strong 
sulphuric  acid,  a  brilliant  purple  color  is  developed,  by  cer- 
tain products  of  the  decomposition  of  the  bile  acids;  the 
physician  can  by  this  test,  in  disease,  detect  their  presence  in 
the  urine  or  other  secretions  of  the  Body.  GmeMn's  I  Hie 
Test.  The  bile-coloring  matters,  treated  with  yellow  nitric 
acid,  go  through  a  series  of  oxidations,  accompanied  with 
changes  of  color  from  yellow-brown  to  green,  then  to  blue, 
violet,  purple,  red,  and  dirty  yellow. 

Bile  has  no  digestive  action  upon  starch  or  proteids.  It 
does  not  break  up  fats,  but  to  a  limited  extent  emulsifies 
them,  though  far  less  perfectly  than  the  pancreatic  secretion. 
It  is  even  doubtful  whether  this  action  is  exerted  in  the  in- 
testines at  all.  In  many  animals,  as  in  man,  the  bile  and 
pancreatic  ducts  open  together  into  the  duodenum,  so  that, 
on  killing  a  dog  during  digestion  and  finding  emulsified  fats 
in  the  chyle,  it  is  impossible  to  say  whether  or  no  the  bile 
had  a  share  in  the  process.  In  the  rabbit,  however,  the  pan- 
creatic duct  opens  into  the  intestine  about  a  foot  farther 
from  the  stomach  than  the  bile-duct,  and  it  is  found  that  if  a 
rabbit  be  killed  after  being  fed  with  oil,  no  milky  chyle  is 
found  down  to  the  point  where  the  pancreatic  duct  opens. 
In  this  animal,  therefore,  the  bile  alone  does  not  emulsify 
fats,  and,  since  the  bile  is  pretty  much  the  same  in  it  and 
other  mammals,  it  probably  does  not  emulsify  fats  in  them 
either.  From  the  inertness  of  bile  with  respect  to  most  food- 
stuffs it  has  been  doubted  whether  it  be  of  any  digestive  use  at 


DIGESTION.  371 

all,  and  whether  it  should  not  be  regarded  merely  as  an  excre- 
tion, poured  into  the  alimentary  canal  to  be  got  rid  of.  But 
there  are  many  reasons  against  such  a  view.  In  the  first  place, 
the  entry  of  the  bile  into  the  upper  end  of  the  small  intestine 
where  it  has  to  traverse  a  course  of  more  than  twenty  feet 
before  getting  out  of  the  Body,  instead  of  its  being  sent  into 
the  rectum,  close  to  the  final  opening  of  the  alimentary  canal, 
makes  it  probable  that  it  has  some  function  to  fulfil  in  the 
intestine.  Moreover,  a  great  part  of  the  bile,  including  prac- 
tically all  the  bile  salts,  poured  into  the  intestines  is  again 
absorbed  from  them  ;  this  seems  to  show  that  part  of  the  bile 
is  secreted  for  some  other  purpose  than  mere  elimination 
from  the  Body.  One  subsidiary  use  is  to  assist,  by  its  alka- 
linity, in  overcoming  the  acidity  of  the  chyme,  and  so  to 
allow  the  trypsin  of  the  pancreatic  secretion  to  act  upon  pro- 
teids.  Constipation  is,  also,  apt  to  occur  in  cases  where  the 
bile-duct  is  temporarily  stopped,  so  that  bile  probably  helps  to 
excite  the  contractions  of  the  muscular  coats  of  the  intestine; 
under  similar  circumstances  putrefactive  decompositions  are 
apt  to  occur  in  the  intestinal  contents.  Apart  from  such  sec- 
ondary influences,however,  the  bile  probably  has  some  influence 
in  promoting  the  absorption  of  fats.  If  one  end  of  a  capillary 
glass  tube,  moistened  with  water,  be  dipped  in  oil,  the  latter  will 
not  ascend  in  it,  or  but  a  short  way;  but  if  the  tube  be  moist- 
ened with  bile,  instead  of  water,  the  oil  will  ascend  higher  in 
it.  So,  too,  oil  passes  through  a  plug  of  porous  clay  kept  moist 
with  bile,  under  a  much  lower  pressure  than  through  one  wet 
with  water.  Hence  bile,  by  soaking  the  epithelial  cells  lining 
the  intestine,  may  facilitate  the  passage  into  the  villi  of  oily 
substances.  At  any  rate,  experiment  shows  that  if  the  bile 
be  prevented  from  entering  the  intestine  of  a  dog,  the  animal 
eats  an  enormous  amount  of  food  compared  with  that 
amount  which  it  needed  previously;  and  that  of  this  food  a 
great  proportion  of  the  fatty  parts  passes  out  of  the  alimen- 
tary canal  anabsorbed.  There  is  no  doubt,  therefore,  that 
the  bile  somehow  aids  in  the  absorption  of  fats,  but  exactly 
how  is  uncertain.  Its  possible  action  in  exciting  the  muscles 
of  the  villi  to  contract  will  be  referred  to  presently. 

The  Intestinal  Secretions  or  Succus  Entericus.  These 
consist  of  the  secretions  of  the  glands  of  Brunner  and  the 
crypts  of  Lieberkuhn.  li  is  difficult  to  obtain  them  pure;;  in- 
deed the  product  of  Brenner's  glands  has  never  been  obtained 


372  THE  HUMAN  BODY. 

unmixed.  That  of  the  crypts  of  Lieberkiihn  is  watery  and 
alkaline  and  poured  out  more  abundantly  during  digestion 
than  at  other  times.  It  has  no  special  action  on  starches, 
most  proteids,  or  on  tats:  but  is  said  to  dissolve  blood  fibrin 
and  convert  it  into  peptone,  and  it  changes  maltose  into 
grape  sugar;  so  that  this  cane  sugar  is  turned  into  a  grape 
sugar  before  being  absorbed.  Mucus  is  also  Eormed  and 
poured  out  abundantly  by  the  epithelium  cells  of  the  intes- 
tinal lining  membrane.  It,  is  more  especially  secreted  during 
fasting,  and  by  its  stickiness  collects  debris  and  keeps  the 
mucous  membrane  clean. 

Intestinal  Digestion.  Having  considered  separately  i  In- 
actions of  the  secretions  which  the  food  meets  with  in  the 
small  intestine  we  may  now  consider  their  combined  effect. 

The  neutralization  of  the  chyme,  followed  by  its  conver- 
sion into  alkaline  chyle,  will  prevent  any  further  action  of 
the  pepsin  on  proteids,  but  will  allow  tin;  ptyalin  of  the 
saliva  (the  activity  of  which  was  stopped  by  the  acidity  of  the 
gastric  juice)  to  recommence  its  action  upon  starch.  More- 
over, in  the  stomach  there  is  produced,  alongside  of  the  albu- 
mose  and  true  peptone,  the  parapeptone,  which  agrees  very 
closely  with  syntonin  in  its  properties,  and  this  passes  into 
the  duodenum  in  the  chyme.  As  soon  as  the  bile  meets 
the  chyme  it  precipitates  the  parapeptone,  and  this  carries 
down  with  it  any  peptones  which,  having  escaped  absorption 
in  the  stomach,  may  be  present;  it  also  precipitates  the  pep- 
sin. In  consequence,  one  finds  in  an  animal  killed  during 
digestion,  a  granular  precipitate  over  the  villi,  and  in  the 
folds  between  the  valvular  conniventes  of  the  duodenum. 
This  is  redissolved  by  the  pancreatic  secretion,  which  also 
changes  into  peptone  the  proteids  (usually  a  considerable  pro- 
portion of  those  eaten  at  a  meal)  which  have  passed  through 
the  stomach  unchanged,  or  as  albumose  or  parapeptone.  The 
conversion  of  starch  into  maltose  will  go  on  very  rapidly  under 
the  influence  of  the  pancreatic  secretion.  Fats  will  be  split 
up  and  saponified  to  a  certain  extent,  but  a  far  larger  pro- 
portion will  be  emulsified  and  give  the  chyle  a-  whitish  appear- 
ance. Later  cane  sugar,  which  may  have  escaped  absorption  in 
the  stomach,  and  maltose  will  be  converted  into  grape  sugar 
and  absorbed,  along  with  such  salines  as  may,  also,  have  hith- 
erto escaped.     Elastic  tissue  from  animal  substances  eaten, 


DIGESTION.  373 

cellulose  from  plants,  and  mucin  from  the  secretions  of  the 
alimentary  tract,  will  all  remain  unchanged. 

Absorption  from  the  Small  Intestine.     The  chyme  leav- 
ing the  stomach  is  a  semi-liquid  mass  which,  mixed  in  the 
duodenum  with  considerable  quantities  of  pancreatic  secre- 
tion and   bile,  is   further  diluted.     Thenceforth  it  gets  the 
intestinal  secretion  added  to    it,    but   the    absorption   more 
than    counterbalancing    the    addition    of   liquid,   the   food- 
mass  becomes  more  and  more  solid  as  it  approaches  the  ileo- 
colic valve.     At  the  same  time  it  becomes  poorer  in  nutritive 
constituents,   these    being  gradually  removed  from  it  in  its 
progress;  most  dialyze  through  the  epithelium  into  the  sub- 
jacent  blood    and   lymphatic   vessels,    and   are   carried   off. 
Those  passing  into  the  blood  capillaries  are  taken  by  the  por- 
tal vein  to  the  liver;  while  those  entering  the  lacteals  are 
carried  into  the  left  jugular  vein  by  the  thoracic  duct.     As 
to  which  foodstuffs  go  one  road  and  which  the  other,  there  is 
still  much  doubt;  sugars  probably  go  by  the  portal  system, 
while  the  fats,  mainly,  if  not  entirely,  go  through  the  lacteals! 
How  the  fats  are  absorbed  is  not  clear,  since  oils  will  not  dia- 
lyze through  membranes,  such  as  that  lining  the  intestine, 
moistened  with  watery  liquids.     Most  of  them,  nevertheless' 
get  into  the  lacteals  as  oils  and  not  as  soluble  soaps;  for  one 
finds  these  vessels,  in  a  digesting  animal,  filled   with  white 
milky  chyle;  while  at  other  periods  their  contents  are  watery 
and  colorless  like  the  lymph  elsewhere  in  the  Body.     The 
little  fat-drops  of  the  emulsion  formed  in  the  intestine,  go 
through  the  epithelial  cells  and  not  between  them,  for  during 
digestion  these  cells  are  loaded  with  oil-droplets;  as  their 
free  ends  are  striated  and  probably  devoid  of  any  definite 
cell-wall,  it  is  possible  that  the  intestinal  movements  squeeze 
oil-drops  into  them,  but  the  cells  may  play  a  more  active  part. 
The  striation  of  the  border  is  due  to  closely-set  rods  which 
are  known  to  be  able  to  change  their  form,  and  it  is  possible 
tli at  they  actively  seize  oil-droplets  and  other  minute  solid 
food    particles.     The  cell  passes  the  fat  to   its  deeper  end 
and,  thence,  out  into  the  subjacent  lymphoid  tissue.     It  is 
probable   that   here   certain     amoeboid  cells  of  the  adenoid 
tissue  pick  it  up,  and  carry  it  into  the  central  lacteal  of  a  villus, 
where  they  break  up  and  set  it  free.     In  the  villus  there  an. 
:<"  'I'"  anatomical  arrangements  for  a  mechanism  which  shall 
actively   .suck    substances   info   it.      Each    is   more   or  less 


374  THE  ill  MA. \  BODY. 

elastic,  and  moreover,  its  capillary  network  when  filled  with 
blood  will  distend  it.  If  its  plain  muscular  layer  contracts 
and  compresses  it,  causing  its  central  lacteal  to  empty  into 
vessels  lying  deeper  in  the  intestinal  wall,  the  villus  will 
actively  expand  again  so  soon  as  its  muscles  relax.  In  so 
doing  it  cannot  till  its  lacteals  from  the  deeper  vessels  on 
account  of  the  valves  in  the  latter,  and.  accordingly,  must 
tend  to  draw  into  itself  materials  from  the  intestines;  much 
like  a  sponge  re-expanding  in  water,  after  having  been 
squeezed  dry.  The  liquid  thus  sucked  up  may  draw  oil-drops 
with  it,  into  the  free  ends  of  the  cells  and  between  them  ;  and 
by  repetitions  of  the  process  it  is  possible  that  considerable 
quantities  of  liquid,  with  suspended  oil-drops,  might  be  car- 
ried into  the  epithelial  cells  covering  a  villus.  The  bile 
moistening  the  surfaoe  of  the  villus  may  facilitate  the  passage 
of  oil,  and  it  is  also  said  to  stimulate  the  contractions  of  the 
villi;  if  so,  its  efficacy  in  promoting  the  absorption  of  fats 
will  be  explained,  in  spite  of  its  chemical  inertness  with  re- 
spect to  those  bodies.  There  is  also  reason  to  believe  that  a 
good  deal  of  the  emulsified  fat  is  also  directly  picked  up  by 
amoeboid  corpuscles,  which  push  their  way  between  the 
epithelial  cells  and  thrusting  processes  into  the  intestine,  pick 
up  oil-droplets,  and  then  travel  back  and  convey  their  load 
to  the  lacteal. 

The  path  taken  by  peptones  is  uncertain.  They  seem  to 
be  very  rapidly  converted  into  proteids  (?  serum  albumin)  after 
absorption  as  they  cannot  be  found,  or  only  traces  of  them, 
in  the  thoracic  duct  or  the  portal  vein  blood  of  a  digesting 
animal.  Moreover,  peptones  directly  injected  into  the  blood 
are  poisonous.  Probably  they  are  seized  upon  and  trans- 
formed by  the  cells  of  the  lymphoid  tissue. 

Digestion  in  the  Large  Intestine.  The  contractions  of 
the  small  intestine  drive  on  its  continually  diminishing  con- 
tents until  they  reach  the  ileo-colic  valve,  through  which 
they  are  ultimately  pressed.  As  a  rule,  when  the  mass  enters 
the  large  intestine  its  nutritive  portions  have  been  almost 
entirely  absorbed,  and  it  consists  merely  of  some  water,  with 
the  indigestible  portion  of  the  food  and  of  the  secretions  of 
the  alimentary  canal.  It  contains  cellulose,  elastic  tissue, 
mucin,  and  somewhat-altered  bile  pigments;  some  fat  if  a 
large  quantity  has  been  eaten;  and  some  starch,  if  raw  vege- 
tables have  formed  part  of  the  diet.     In  its  progress  through 


DIGESTION.  375 

the  large  intestine  it  loses  more  water,  and  the  digestion  of 
starch  and  the  absorption  of  fats  is  continued.  Finally  the 
residue,  with  some  excretory  matters  added  to  it  in  the  large 
intestine,  collects  in  the  sigmoid  flexure  of  the  colon  and  in 
the  rectum,  and  is  sent  out  of  the  Body  from  the  latter. 

The  Digestion  of  an  Ordinary  Meal.  We  may  best  sum 
up  the  facts  stated  in  this  chapter  by  considering  the  diges- 
tion of  a  common  meal;  say  a  breakfast  consisting  of  bread 
and  butter,  beefsteak,  potatoes  and  milk.  Many  of  these 
substances  contain  several  alimentary  principles,  and,  since 
these  are  digested  in  different  ways  and  in  different  parts  of 
the  alimentary  tract,  the  first  thing  to  be  done  is  to  consider 
what  are  the  proximate  constituents  of  each.  We  thus  sepa- 
rate the  materials  of  the  breakfast  as  in  table  on  next  page. 

From  such  a  meal  we  may  first  separate  the  elastin,  cellu- 
lose, and  calcium  sulphate,  as  indigestible  and  passed  out  of 
the  Body  in  the  same  state  and  in  the  same  quantity  as  they 
entered  it.  Then  come  the  salines  which  need  no  special 
digestion,  and,  taken  either  in  solution  or  dissolved  in  the 
saliva  or  gastric  juice,  are  absorbed  from  the  mouth,  stomach, 
and  intestines  without  further  change.  Cane  and  grape 
sugars  experience  the  same  lot,  except  that  any  cane  sugar 
or  maltose  reaching  the  intestines  before  absorption  is 
changed  into  grape  sugar  by  the  succus  ente?'icus.  Calcium 
phosphate  will  be  dissolved  by  the  free  acid  in  the  stomach, 
yielding  calcium  chloride,  which  will  be  absorbed  there  or  in 
the  intestine.  Starch  will  be  partially  converted  into  maltose 
during  mastication  and  deglutition,  and  it  is  possible  that 
some  of  this  sugar  may  be  absorbed  from  the  stomach. 
A  great  part  of  the  starch  will,  however,  be  passed  on 
into  the  intestine  unchanged,  since  the  action  of.  saliva  is 
suspended  in  the  stomach;  and  its  conversion  will  be  com- 
pleted by  the  pancreatic  secretion,  and  perhaps  by  the  ptyalin, 
though  this  is  probably  destroyed  in  the  stomach  by  the  gastric 
juice;  but  in  any  case  the  starch  will  only  have  been  changed 
to  maltose,  and  will  need  further  digestive  treatment.  The 
various  proteids  will  be  partially  dissolved  in  the  stomach 
and  converted  into  peptone,  which  will  in  part  be  absorbed 
there;  the  residue,  with  the  undigested  proteids,  will  be 
passed  on  to  the  intestines.  There  the  bile  will  precipitate 
the  peptones  and  parapeptones  and,  with  the  pancreatic 
accretion,  render  the  chyme  alkaline,  and  so  stop  the  activity 


376 


THE  11 V MAS    HuhY. 


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CO 

b       i       '- 

■J 

m 

&. 
S 
5 

H 

ot> 

a 
o 
o 

u. 

0 

S 

< 
o 

s 

o 

2 

c  - 

SO    EQ 

X    - 

-/.  IE 

Calcium 

phosphate, 

( lalcium 
sulphate. 

( 'alciuni 
phosphate, 

Iron 

phosphate. 

IS  c 

-   CO 

:  eg 

go  ■* 

ga 

Common  sail 
and  others. 

( 'ominon  sail 
and  others. 

Potassium 

phosphate, 

( lommon  salt 
and  others. 

<  lommon  sail 

and  others, 
especially 

phosphates. 

OS 

Water. 
Water. 
Water. 

Water 

OJ 

-** 

i      .1 

03                        4- 

1       s 

1      i 

t3                a 
a>               r 
M                PC 

■a 

01 

"S 

OJ 

*  — 

si 

o  2 

! 

a 

c 

1* 

i 

c 
< 

1 

1 

DIGESTION.  377 

of  the  gastric  pepsin.  The  pancreatic  secretion  will,  how- 
ever, redissolve  the  precipitated  peptone,  and  the  unchanged 
proteids  and  parapeptone  and  the  albumose,  and  turn  the 
three  last  into  peptone,  breaking  up  part  of  this  into  leucin 
and  tyrosin;  these  will  be  absorbed  as  they  pass  along  the 
small  intestine;  a  small  quantity  perhaps  passing  into  the 
large  intestine,  to  be  taken  up  there.  The  fats  will  remain 
unchanged  until  they  enter  the  small  intestine,  except  that 
the  proteid  cell- walls  of  the  adipose  tissue  of  the  beefsteak 
will  be  dissolved  away.  In  the  small  intestine  some  of  these 
little  oil  masses  will  be  in  part  saponified,  but  most  will  be 
emulsified  and  taken  up  into  the  lacteals  in  that  condition. 
Gelatin,  from  the  white  fibrous  tissue  of  the  beefsteak,  will 
undergo  changes  in  the  stomach  and  intestine,  and  be  dis- 
solved and  absorbed. 

The  substances  leaving  the  alimentary  canal  after  such  a 
meal  would  be,  primarily,  the  indigestible  cellulose  and 
elastin,  and  some  water.  But  there  might  also  be  some  unab- 
sorbed  fats,  starch,  and  salts.  To  these  would  be  added,  in 
the  alimentary  canal,  mucin,  some  of  the  ferments  of  the  di- 
gestive secretions,  some  slightly  altered  bile  pigments,  and 
other  bodies  excreted  by  the  large  intestine. 

Dyspepsia  is  the  common  name  of  a  number  of  diseased 
conditions  attended  with  loss  of  appetite  or  troublesome 
digestion.  Being  often  unattended  with  acute  pain,  and  if 
it  kills  at  all  doing  so  very  slowly,  it  is  pre-eminently  suited 
for  treatment  by  domestic  quackery.  In  reality,  however, 
the  immediate  cause  of  the  symptoms,  and  the  treatment 
called  for,  may  vary  widely;  and  their  detection  and  the 
choice  of  the  proper  remedial  agents  often  call  for  more  than 
ordinary  medical  skill.  A  few  of  the  more  common  forms 
of  dyspepsia  may  be  mentioned  here,  with  their  proximate 
causes,  not  in  order  to  enable  people  to  undertake  the  rash 
experiment  of  dosing  themselves,  but  to  show  how  wide  a 
chance  there  is  for  any  unskilled  treatment  to  miss  its  end, 
and  do  more  harm  than  good. 

Appetite  is  primarily  due  to  a  condition  of  the  mucous 
membrane  of  the  stomach  which,  in  health,  comes  on  after  a 
short  fast,  and  stimulates  its  sensory  nerves;  and  loss  of  appe- 
tite may  be  dm-  to  cither  of  several  causes.  The  stomach 
may  be  apathetic  and  Lack  its  normal  sensibility,  so  that  the 
empty  condition  does  not  act,  as  it  normally  does,  as  a  suffi- 


378  THE  11  UMAX  BODY. 

cient  excitant.  When  food  is  taken  it  is  a  further  stimulus 
and  may  be  enough;  in  Buch  cases  ••appetite  comes  with  eat- 
ing." A  bitter  before  a  meal  is  useful  as  an  appetizer  to 
patients  of  this  sort.  On  the  other  hand,  the  stomach  may 
be  too  sensitive,  and  a  voracious  appetite  be  felt  before  a 
meal,  which  is  replaced  by  nausea,  or  even  vomiting,  as  soon 

as  a  few  mouthfuls  have  been  swallowed;  tl xtra  stimulus 

of  the  food  then  overstimulates  the  too  irritable  stomach, 
just  as  a  draught  of  mustard  and  warm  water  will  a  healthy 
one.  The  proper  treatment  in  such  cases  is  a  soothing  one. 
When  food  is  taken  it  ought  to  stimulate  the  sensory  gastric 
nerves,  so  as  to  excite  the  reflex  centres  for  the  secretory 
nerves,  and  for  the  dilatation  of  the  blood-vessels  of  the 
organ;  if  it  does  not,  the  gastric  juice  will  be  imperfectly 
secreted.  In  such  cases  one  may  stimulate  the  secretory 
nerves  by  weak  alkalies,  as  certain  mineral  waters  or  a  little 
carbonate  of  soda,  before  meals;  or  give  drugs,  as  strychnine, 
which  increase  the  irritability  of  reflex  nerve-centres.  The 
vascular  dilatation  may  be  helped  by  warm  drinks,  and  this  is 
probably  the  rationale  of  the  glass  of  hot  water  after  eating 
which  has  often  been  found  useful;  the  usual  small  cup  of 
hot  coffee  after  dinner  is  a  more  agreeable  form  of  the  same 
aid  to  digestion.  In  states  of  general  debility,  when  the 
stomach  is  too  feeble  to  secrete  under  any  stimulation,  the 
administration  of  weak  acids  and  artificially  prepared  pepsin 
is  needed,  to  supply  gastric  juice  from  outside,  until  the  im- 
proved digestion  strengthens  the  stomach  up  to  the  point  of 
being  able  to  do  its  own  work. 

Enough  has  probably  been  said  to  show  that  dyspepsia  is 
not  a  disease,  but  a  symptom  accompanying  many  pathologi- 
cal conditions,  requiring  special  knowledge  for  their  treat- 
ment. From  its  nature— depriving  the  Body  of  its  proper 
nourishment — it  tends  to  intensify  itself,  and  so  should  never 
be  neglected;  a  stitch  in  time  saves  nine. 

The  Movements  of  the  Intestines.  When  the  abdomen 
of  a  living  anaesthetized  animal  is  opened,  especially  during 
digestion,  contractions  are  seen  slowly  travelling  along  the 
bowels,  which  have  in  consequence  somewhat  the  appearance 
of  a  writhing  mass  of  worms,  hence  the  name  vermicular 
often  given  to  these  movements:  they  are  also  called  peri- 
staltic. On  observing  a  portion  of  the  gut  a  narrowing  due 
to  contraction  of  its  circular  muscular  coat  will  be  seen  to 


DIGESTION.  379 

pass  slowly  along  it,  normally  in  a  direction  towards  the 
rectum;  these  contractions  push  before  them  part  of  the  con- 
tents of  the  intestine.  The  simultaneous  contractions  of  the 
outer  longitudinal  layer  of  the  muscular  coat  are  not  so 
marked  or  so  easily  directly  observable.  If  the  bowels  be 
entirely  removed  from  the  body  of  the  animal  the  movements 
go  on  for  some  time,  so  they  are  obviously  not  directly  de- 
pendent on  extrinsic  nerves.  They  are  probably  primarily 
due  to  a  slight  automaticity  of  the  muscle  itself,  which  as  in 
the  case  of  the  heart  (Chap.  XVII)  is  favored  by  distension,  but 
they  may  be  due  to  nerve  impulses  arising  in  the  cells  of  the 
plexus  of  Auerbach.  As  in  the  case  of  the  heart  these  move- 
ments are  under  control  of  extrinsic  nerve-fibres,  originating 
in  the  cerebro-spinal  centre,  and  these  fibres  are  excitor  and 
depressor.  Exactly  contrary  to  that  which  we  find  in  the 
case  of  the  heart,  the  fibres  reaching  the  intestines  through 
the  pneumogastrics  are  excitor,  causing  more  powerful  con- 
tractions, and  the  fibres  coming  from  the  sympathetic  through 
the  splanchnics  (where  they  are  mixed  with  but  quite  dis- 
tinct from  the  vaso-constrictor  fibres)  are  inhibitory.  Stimu- 
lation of  the  splanchnic  nerves  will  bring  actively  contract- 
ing intestines  to  rest.  The  influence  of  the  central  nervous 
system  on  the  motions  of  the  bowel  is  shown  by  the  contrac- 
tions caused  by  fright  or  other  strong  emotions,  illustrated 
by  the  Hebrew  phrase  "bowels  moved  with  compassion." 
Deficiency  of  arterial  blood  excites  powerful  intestinal  con- 
tractions. The  various  purgative  medicines  act  in  very  differ- 
ent ways  ;  some  directly  on  the  intestinal  neuro-muscular 
apparatus;  some  on  the  extrinsic  nerve  centres  concerned; 
some  (as  Epsom  salts)  mainly  by  causing  a  great  secretion  of 
liquid  into  the  bowel  and  so  distending  it. 


CHAPTER  XXV. 

THE  RESPIRATORY  MECHANISM. 

Definitions.  The  blood  as  it  flows  from  the  right  ventri- 
cle of  the  heart,  through  the  lungs,  to  the  left  auricle,  loses 
carbon  dioxide  and  gains  oxygen.  In  the  systemic  circula- 
tion exactly  the  reverse  changes  take  place,  oxygen  leaving 
the  blood  to  supply  the  living  tissues;  and  carbon  dioxide,  gen- 
erated in  them,  passing  back  into  the  blood  capillaries.  The 
oxvgen  loss  and  carbon  dioxide  gain  are  associated  with  a 
change  in  the  color  of  the  blood  from  bright  scarlet  to  purple 
red,  or  from  arterial  to  venous;  and  the  opposite  changes  in 
the  lungs  restore  to  the  dark  blood  its  bright  tint.  The  whole 
set  of  processes  through  which  blood  becomes  venous  in  the 
systemic  circulation  and  arterial  in  the  pulmonary — in 
other  words  the  processes  concerned  in  the  gaseous  reception, 
distribution  and  elimination  of  the  Body — constitute  the 
function  of  respiration;  so  much  of  this  as  is  concerned  in 
the  interchanges  between  the  blood  and  air  being  known  as 
external  respiration  ;  while  the  interchanges  occurring  in 
the  systemic  capillaries,  and  the  processes  in  general  by 
which  oxygen  is  fixed  and  carbon  dioxide  formed  by  the  liv- 
ing tissues,  are  known  as  internal  respiration.  When  the 
term  respiration  is  used  alone,  without  any  limiting  adjective, 
the  external  respiration  only,  is  commonly  meant. 

Respiratory  Organs.  The  blood  being  kept  poor  in  oxy- 
gen and  rich  in  carbon  dioxide  by  the  action  of  the  living 
tissues,  a  certain  amount  of  gaseous  interchange  will  nearly 
always  take  place  when  it  comes  into  close  proximity  to  the 
surrounding  medium:  whether  this  lie  the  atmosphere  itself 
or  water  containing  air  in  solution.  When  an  animal  is 
small  there  are  often  no  special  organs  for  its  external  res- 
piration, its  general  surface  being  sufficient  (especially  in 
aquatic  animals  with  a  moist  skin)  to  permit  of  all  the  gas- 
eous exchange  that  is  necessary.  In  the  simplest  creatures, 
indeed,  there  is  even  no  blood,  the  cell  or  cells  composing 

380 


THE  RESPIRATORY  MECHANISM.  381 

them  taking  up  for  themselves  from  their  environment  the 
oxygen  which  they  need,  and  passing  out  into  it  their  car- 
bon dioxide  waste;  in  other  words,  there  is  no  differen- 
tiation of  the  external  and  internal  resjnrations.  When, 
however,  an  animal  is  larger  many  of  its  cells  are  so  far  from 
a  free  surface  that  they  cannot  transact  this  give-and-take 
with  the  surrounding  medium  directly,  and  the  blood,  or 
some  liquid  representing  it  in  this  respect,  serves  as  a  mid- 
dleman between  the  living  tissues  and  the  external  oxygen; 
and  then  one  usually  finds  special  respiratory  organs  devel- 
oped, to  which  the  blood  is  brought  to  make  good  its  oxygen 
loss  and  get  rid  of  its  excess  of  carbon  dioxide.  In  aquatic 
animals  such  organs  take  commonly  the  form  of  gills;  tbese 
are  protrusions  of  the  body  over  which  a  constant  current  of 
water,  containing  oxygen  in  solution,  is  kept  up;  and  in 
which  blood  capillaries  form  a  close  network  immediately  be- 
neath the  surface.  In  air-breathing  animals  a  different  ar- 
rangement is  usually  found.  In  some,  as  frogs,  it  is  true,  the 
skin  is  always  moist  and  serves  as  an  important  respiratory 
organ,  large  quantities  of  venous  blood  being  sent  to  it  for 
aeration.  But  for  the  occurrence  of  the  necessary  gaseous 
diffusion,  the  skin  must  be  kept  very  moist,  and  this,  in  a 
terrestrial  animal,  necessitates  a  great  amount  of  secretion  by 
the  cutaneous  glands  to  compensate  for  evaporation;  accord- 
ingly in  most  land  animals  the  air  is  carried  into  the  body 
through  tubes  with  narrow  external  orifices  and  so  the  drying 
up  of  the  breathing  surfaces  is  greatly  diminished;  just  as 
water  in  a  bottle  with  a  narrow  neck  will  evaporate  much  more 
slowly  than  the  same  amount  exposed  in  an  open  dish.  In 
insects  (as  bees,  butterflies,  and  beetles)  the  air  is  carried  by 
tubes  which  split  up  into  extremely  fine  branches  and  ramify 
all  through  the  body,  even  down  to  the  individual  tissue  ele- 
ments, which  thus  carry  on  their  gaseous  exchanges  without 
the  intervention  of  blood.  But  in  the  great  majority  of 
air-breathing  animals  the  arrangement  is  different;  the  air- 
tubes  leading  from  the  exterior  of  the  body  do  not  subdivide 
into  branches  which  ramify  all  through  it,  but  open  into  one 
or  more  large  sacs  to  which  the  venous  blood  is  brought,  and 
in  whose  walls  it,  Hows  through  a  close  capillary  network. 
Such  respiratory  sacs  are  called  lungs,  and  it,  is  a  highly  de- 
veloped form  of  them  which  is  employed  in  the  Human 
Body. 

The  Air-Passages  and  Lungs.     In  our  own  Bodies  some 


382 


THE  HUMAN  BODY. 


small  amount  of  respiration  is  carried  on  in   the  alimentary 

canal,  the  air  swallowed  with  food 
or  saliva  undergoing  gaseous  ex- 
changes with  the  blood  in  the  gas- 
tric and  intestinal  mucous  mem- 
branes. The  amount  of  oxygen 
thus  obtained  by  the  blood  is 
however  very  trivial,  as  is  that 
absorbed  through  the  skin,  cov- 
ered as  it  is  by  its  dry  horny  non- 
vascular epidermis.  All  the  really 
essential  gaseous  interchanges  be- 
tween the  Body  and  the  atmos- 
phere take  place  in  the  lungs,  two 
large  sacs  (lu,  Fig.  1)  lying  in  the 
thoracic  cavity,  one  on  each  side 
of  the  heart.  To  these  sacs  the 
air  is  conveyed  through  a  series  of 
passages.  Entering  the  pharynx 
through  the  nostrils  or  mouth, 
it  passes  out  of  this  by  the  open- 
ing leading  into  the  larynx,  or 
voice-box  (a,  Fig.  122),  lying  in 
the  upper  part  of  the  neck  (the  communication  of  the  two 
is  seen  in  Fig.  10T) ;  from  the  larynx  passes  back  the  trachea 
or  windpipe,  b,  which,  after  entering  the  chest  cavity,  divides 
into  the  right  and  left  bronchi,  d,  e.  Each  bronchus  divides 
up  into  smaller  and  smaller  branches,  called  bronchial  tubes, 
within  the  lung  on  its  own  side;  and  the  smallest  bronchial 
tubes  end  in  sacculated  dilatations,  the  infundibula  of  the 
lungs,  the  sacculations  (Fig.  124)  being  the  alveoli:  the  word 
"  cell  "  being  here  used  in  its  prim-  *■ 
itive  sense  of  a  small  cavity,  and 
not  in  its  later  technical  significa- 
tion of  a  morphological  unit  of 
the  Body.  On  the  walls  of  the 
air-cells  the  pulmonary  capillaries 
ramify,  and  it  is  in  them  that  the 
interchanges  of  the  external  res- 
piration take  place. 

Structure  of  the  Trachea  and 
Bronchi.  The  windpipe  may 
readily  be  felt  in  the  middle  line  of  the  neck,  a  little  below 


lungs  and  air-  / 
passages  seen  from  the  front.  On 
the  left  of  the  figure  the  pulmo- 
nary tissue  has  been  dissected 
away  to  show  the  ramifications  of 
the  bronchial  tubes,  a,  larynx  ; 
6,  trachea  ;  d,  right  bronchus. 
The  left  bronchus  is  seen  entering 
the  root  of  its  lung. 


Fig.  123. — A  small  bronchial  tube, 
«,  dividing  into  its  terminal  branch- 
es, c  ;  these  havepouched  or  saccu- 
lated walls  and  end  in  the  saccu- 
lated infundibula,  b. 


THE  RESPIRATORY  MECHANISM. 


383 


Adam's  apple,  as  a  rigid  cylindrical  mass.  It  consists  funda- 
mentally of  a  fibrous  tube  in  which  cartilages  are  imbedded, 
so  as  to  keep  it  from  collapsing;  and  is  lined  internally  by  a 
mucous  membrane  covered  by  several  layers  of  epithelium 
cells,  of  which  tbe  superficial  is  ciliated.  The  elastic  car- 
tilages imbedded  in  its  walls  are  imperfect  rings,  each  some- 
what the  shape  of  a  horse  shoe,  and  tbe  deficient  part  of 
each  ring  being  turned  backwards,  it  comes  to  pass  that  the 
deeper  or  dorsal  side  of  the  windpipe  has  no  hard  parts  in  it. 
Against  this  side  the  gullet  lies,  and  the  absence  there  of  the 
cartilages  no  doubt  facilitates  swallowing.  The  bronchi  re- 
semble the  windpipe  in  structure. 

The  Structure  of  the  Lungs.  These  consist  of  the  bron- 
chial tubes  and  their  terminal  dilatations;  numerous  blood- 
vessels, nerves  and  lymphatics;  and  an  abundance  of  connec- 
tive tissue,  rich  in  elastic  fibres,  binding  all  together.  The 
bronchial  tubes  ramify  in  a  tree-like  manner  (Fig.  122).  In 
structure  the  larger  ones  resemble  the  trachea,  except  that 
the  cartilage  rings  are  not  regularly  arranged  so  as  to  have 
their  open  parts  all  turned  one  way.  As  the  tubes  become 
smaller  their  constituents  thin  away;  the  cartilages  become 
less  frequent  and  finally  disappear;  the  epithelium  is  re- 
duced to  a  single  layer  of  cells  which,  though  still  ciliated, 
are  much  shorter  than  the  columnar  superficial  cell-layer  of 
the  larger  tubes.  The  terminal 
alveoli  (a,  a,  Fig.  124),  and  the 
air-cells,  h,  which  open  into  them, 
have  walls  composed  mainly  of 
elastic  tissue  and  lined  by  a 
single  layer  of  flat,  non-ciliated 
epithelium,  immediately  beneath 
which  is  a  very  close  network 
of  capillary  blood-vessels.  The 
air  entering  by  the  bronchial  tube 
is  thus  only  separated  from  the 
blood  by  the  thin  capillary  wall 
and  the  thin  epithelium,  both  of 
which  are  moist,  and  well  adapted 

,  .  „.       .           '  Fia.  124.— Two  infuixlibula  of  the 

to   permit  gaseous  dllliision.  lung  much  ma&ninVd     6,6.theair- 

_,        _ ,,  J                   -nil  •       cells,   or   hollow    protrusions  of  Our 

The  Pleura.        J\ach     lung  IS    alveolus,   opening   into    its   central 

covered,  excepl  at  one  point,  by  ttiuai'tube "'"  branche8  of  a 

an  elastic  serous  membrane  which  adheres  tightly  to  it  and 


384  THE  1 1 1. MAX  BODY. 

is  called  the  pleura;  thai  poinl  al  which  the  pleura  is 
wanting  is  called  the  root  of  the  lung  and  is  on  its 
median  side:  it  is  there  thai  its  bronchus,  blood-vessels  and 
nerves  enter  it.  At  the  root  of  the  lung  the  pleura  turns 
back  and  lines  the  inside  of  the  chesl  cavity,  as  represented 
by  the  dotted  line  in  the  diagram  Fig.  :!.  The  part  of  the 
pleura  attached  to  each  Lung  is  its  visceral,  and  thai  attached 
to  the  chest-wall  its  parietal  layer.  Bach  pleura  thus  forms 
a  closed  sac  surrounding  a  pleural  cavity,  in  which,  during 
health,  there  are  found  a  few  drops  of  lymph,  keeping  its 
surfaces  moist.  This  lessens  friction  between  the  two  layers 
during  the  movements  of  the  chest-walls  and  the  lungs;  for 
although,  to  insure  distinctness,  the  visceral  and  parietal 
layers  of  the  pleura  are  represented  in  the  diagram  as  not  in 
contact,  that  is  not  the  natural  condition  of  things;  the  lungs 
are  in  life  distended  so  that  the  visceral  pleura  rubs  against 
the  parietal,  and  the  pleural  cavity  is  practically  obliterated. 
This  is  due  to  the  pressure  of  the  atmosphere  exerted  through 
the  air-passages  on  the  interior  of  the  lungs.  The  lungs  are 
extremely  elastic  and  distensible,  and  when  the  chest  cavity 
is  perforated  each  shrivels  up  just  as  an  indian-rubber  blad- 
der does  when  its  neck  is  opened;  the  reason  being  that  then 
the  air  presses  on  the  outside  of  each  with  as  much  force  as 
it  does  on  the  inside.  These  two  pressures  neutralizing  one 
another,  there  is  nothing  to  overcome  the  tendency  of  the 
lungs  to  collapse.  So  long  as  the  chest-walls  are  whole,  how- 
ever, the  lungs  remain  distended.  The  pleural  sac  is  air-tight 
and  contains  no  air,  and  the  pressure  of  the  air  around  the 
Body  is  borne  by  the  rigid  walls  of  the  chest  and  prevented 
from  reaching  the  lungs;  consequently  no  atmospheric  pres- 
sure is  exerted  on  their  outside.  On  their  interior,  however, 
the  atmosphere  presses  with  its  full  weight, 
equal  to  about  90  centigrams  on  a  square 
centimeter  (14.5  lbs.  on  the  square  inch),  and 
this  is  far  more  than  sufficient  to  dis- 
tend the  lungs  so  as  to  make  them  com- 
pletely fill  all  the  parts  of  the  thoracic  cav- 
ity not  occupied  by  other  organs.  Suppose 
mS^i'SVZ  -'  (Fig.  L25)  to  be  a  bottle  closed  air-tight 
sure  relationships  of  ky   a  C01fe  through    which   two  tubes  pass, 

the  lungs  in   tne   tno-      J  o  * 

rax.  011e  of  which.  I>.  lends  into  an   elastic  bag, 

<7,  and  the  other,  c,  provided  with  a  stop-cock,  opens  freely 


THE  RESPIRATORY  MECHANISM.  385 

below  into  the  bottle.  When  the  stop-cock,  c,  is  open 
the  air  will  enter  the  bottle  and  press  there  on  the  out- 
side of  the  bag,  as  well  as  on  its  inside  through  b.  The  bag 
will  therefore  collapse,  as  the  lungs  do  when  the  chest  cavity 
is  opened.  But  if  some  air  be  sucked  out  through  c  the  pres- 
sure of  that  remaining  in  the  bottle  will  diminish,  and  of  that 
inside  thebag  will  be  unchanged,  and  the  bag  will  thus  be  blown 
op,  because  the  atmospheric  pressure  on  its  interior  will  not  be 
balanced  by  that  on  its  exterior.  At  last,  when  all  the  air  is 
sucked  out  of  the  bottle  and  the  stop-cock  on  c  closed,  the 
bag.  if  sufficiently  distensible,  will  be  expanded  so  as  to  com- 
pletely till  the  bottle  and  press  against  its  inside,  and  the 
state  of  things  will  then  answer  to  that  naturally  found  in 
the  chest.  If  the  bottle  were  now  increased  in  size  without 
letting  air  into  it,  the  bag  would  expand  still  more,  so  as  to 
fill  it,  and  in  so  doing  would  receive  air  from  outside  through 
b;  and  if  the  bottle  then  returned  to  its  original  size,  its 
walls  would  press  on  the  bag  and  cause  it  to  shrink  and 
expel  some  of  its  air  through  b.  Exactly  the  same  must  of 
course  happen,  under  similar  circumstances,  in  the  chest,  the 
windpipe  answering  to  the  tube  b  through  which  air  enters 
or  leaves  the  elastic  sac. 

The  Respiratory  Movements.  The  air  taken  into  the 
lungs  soon  becomes  laden  in  them  with  carbon  dioxide,  and 
at  the  same  time  loses  much  of  its  oxygen;  these  interchanges 
take  place  mainly  in  the  deep  recesses  of  the  alveoli,  far  from 
the  exterior  and  only  communicating  with  it  through  a  long 
tract  of  narrow  tubes.  The  alveolar  air,  thus  become  unfit 
to  any  longer  convert  venous  blood  into  arterial,  could  only 
very  slowly  be  renewed  by  gaseous  diffusion  with  the  atmos- 
phere through  the  long  air-passages — not  nearly  fast  enough 
for  the  requirements  of  the  Body,  as  one  learns  by  the  sensa- 
tion of  suffocation  which  follows  holding  the  breath  for  a 
short  time  with  mouth  and  larynx  open.  Consequently  co- 
operating  with  the  lungs  is  a  respiratory  mechanism,  by 
which  the  air  within  them  is  periodically  mixed  with  fresh 
air  taken  from  the  outside,  and  also  the  air  in  the  alveoli  is 
stirred  up  bo  as  to  bring  fresh  layers  of  it  in  contact  with  the 
walls  of  the  air-cells.  This  mixing  is  brought  about  by  the 
breathing  movements,  consisting  of  regularly  alternating  in- 
spirations, 'luring  which  the  chesl  cavity  is  enlarged  and 
fresh  air  enters  the  lungs,  and  expirations,  in  which  fche  cav- 


386  run:  human  hody. 

ity  is  diminished  and  air  expelled  from  the  lungs.  When  the 
chest  is  enlarged  the  air  the  lungs  contain  immediately  dis- 
tends them  so  as  to  ±111  the  larger  space  ;  in  so  doing  it  be- 
comes rarefied  and  less  dense  than  the  external  air;  and  since 
gases  flow  from  points  of  greater  to  those  of  less  pressure, 
some  outside  air  at  once  flows  in  by  the  air-passages  and 
enters  the  lungs.  In  expiration  the  reverse  takes  place.  The 
chest  cavity,  diminishing,  presses  on  the  lungs  and  makes  the 
air  inside  them  denser  than  the  external  air,  and  so  some 
passes  out  until  an  equilibrium  of  pressure  is  restored.  The 
chest,  in  fact,  acts  very  much  like  a  bellows.  When  the  bel- 
lows are  opened  air  enters  in 
consequence  of  the  rarefaction 
of  that  in  the  interior,  which 
is  expanding  to  fill  the  larger 
space;  and  when  the  bellows 
_     ,M    _.  .    ...    .    .   M         are  closed  again  it  is  expelled. 

Fig.  126.— Diagram  to  illustrate  the  en-  n  i 

try  of  air  to  the  lungs  when  the  thoracic   To     make      the     bellows     quite 
cavity  enlarges.  * 

like  the  lungs  we  must,  how- 
ever, as  in  Fig.  126,  have  only  one  opening  in  them,  that  of 
the  nozzle,  for  both  the  entry  and  exit  of  the  air;  and  this 
opening  should  lead,  not  directly  into  the  bellows  cavity,  but 
into  an  elastic  bag  lying  in  it,  and  tied  to  the  inner  end  of 
the  nozzle-pipe.  This  sac  would  represent  the  lungs  and  the 
space  between  its  outside  and  the  inside  of  the  bellows,  the 
pleural  cavities. 

We  have  next  to  see  how  the  expansion  and  contraction 
of  the  chest  cavity  are  brought  about. 

The  Structure  of  the  Thorax.  The  thoracic  cavity  has 
a  conical  form  determined  by  the  shape  of  its  skeleton  (Fig. 
127),  its  narrower  end  being  turned  upwards.  Dorsally,  ven- 
trally,  and  on  the  sides,  it  is  supported  by  the  rigid  frame- 
work afforded  by  the  thoracic  vertebras,  the  breast-bone,  and 
the  ribs.  Between  and  over  these  lie  muscles,  and  the 
whole  is  covered  in,  air-tight,  by  the  skin  externally,  and  the 
parietal  layers  of  the  pleuras  inside.  Above,  its  aperture  is 
closed  by  muscles  and  by  various  organs  passing  between  the 
thorax  and  the  neck:  and  below  it  is  bounded  by  the  dia- 
phragm, which  forms  a  movable  bottom  to  the,  otherwise, 
tolerably  rigid  box.  In  inspiration  this  box  is  increased  in 
all  its  diameters — dorso-ventrally,  laterally,  and  from  above 
down. 


THE  RESPIRATORY  MECHANISM. 


387 


The  Vertical  Enlargement  of  the  Thorax.  This  is 
brought  about  by  the  contraction  of  the  diaphragm  which 
(Figs.  1  and  128)  is  a  thin  muscular  sheet,  with  a  fibrous 
membrane,  serving  as  a  tendon,  in  its  centre.  In  rest,  the 
diaphragm  is  dome-shaped,  its  concavity  being  turned  towards 
the  abdomen.  From  the  tendon  on  the  crown  of  the  dome 
striped  muscular  fibres  radiate,  downwards  and  outwards,  to 
all  sides;  and  are  fixed  by  their  inferior  ends  to  the  lower 
ribs,  the  breast-bone,  and  the  vertebral  column.  In  expiration 
the  lower  lateral  portions  of  the  diaphragm  lie  close  against 
the  chest-walls,  no  lung  intervening  between  them.  In  in- 
spiration the  muscular  fibres,  shortening,  flatten  the  dome 


Fio.  127.— The  skeleton  of  the  thorax,    a,  g,  vertebral  column;  b,  first  rib;  e, 
clavicle;  d,  third  rib;  i,  glenoid  fossa. 

and  enlarge  the  thoracic  cavity  at  the  expense  of  the  ab- 
dominal; and  at  the  same  time  its  lateral  portions  are  pulled 
away  from  the  chest-walls,  leaving  a  space  into  which  the 
lower  ends  of  the  lungs  expand.  The  contraction  of  the 
diaphragm  thus  increases  greatly  the  size  of  the  thorax  cham- 
ber by  adding  to  its  lowest  and  widest  part. 

The  Dorso-Ventral  Enlargement  of  the  Thorax.  The 
ribs  on  the  whole  slope  downwards  from  the  vertebral 
column  to  the  breast-bone,  the  slope  being  most  marked 
in    the    lower    ones.      During    inspiration    the    breast-bone 


388 


THE  HUMAN  BODY 


and  the  sternal  ends  of  the  ribs  attached  to  it  are  raised, 
and  so  the  distance  between  the   sternum  and   the  vertebral 


Fig.  128.— The  diaphragm  seen  from  helow. 

column  is  increased.  That  this  must  be  so  will  readily  be 
seen  on  considering  the  diagram  Fig.  129,  where  ab  repre- 
sents the  vertebral  column,  c  and  d  two 
ribs,  and  st  the  sternum.  The  continu- 
ous lines  represent  the  natural  position 
of  the  ribs  at  rest  in  expiration,  and  the 
dotted  lines  the  position  in  inspiration. 
It  is  clear  that  when  their  lower  ends 
are  raised,  so  as  to  make  the  bars  lie  in 
a  more  horizontal  plane,  the  sternum  is 
pushed  away  from  the  spine,  and  so  the 
chest  cavity  is  increased  dorso-ventrally. 
The  inspiratory  elevation  of  the  ribs  is 

Fig.  l-'O.-Diagram  illus-  .  *  * 

trating  the  dorso-ventrai  mainly  due  to  the  action  of  the  scalene 

increase  in  the  diameter  of  ,  ,  T     .     .  .    ,  7  ..,-, 

the  thorax  when  the  ribs  and  external  intercostal  muscles,  liie 
scalene  muscles,  three  on  each  side,  arise 
from  the  cervical  vertebra?,  and  are  inserted  into  the  upper 
ribs.  The  external  intercostals  (Fig.  113,  A)  lie  between  the 
ribs  and  extend  from  the  vertebral  column  to  the  costal  carti- 
lages; their  fibres  slope  downwards  and  forwards.  During 
an  inspiration  the  scalenes  contract  and  fix  the  upper  ribs 
firmly;  then  the  external  intercostals  shorten  and  each  raises 
the  rib  below  it.  The  muscle,  in  fact,  tends  to  pull  together 
the  pair  of  ribs  between  which  it  lies,  but  as  the  upper  one  of 
these  is  held  tight  by  the  scalenes  and  other  muscles  above, 


THE  RESPIRATORY  MECHANISM. 


389 


the  result  is  that  the  lower  rib  is  pulled  up,  and  not  the  upper 
down.  In  this  way  the  lower  ribs  are  raised  much  more  than 
the  upper,  for  the  whole  external  intercostal  muscles  on  each 
side  may  be  regarded  as  one  great  muscle  with  many  bellies, 
each  belly  separated  from  the  next  by  a  tendon,  represented 
by  the  rib.  When  the  whole  muscular  sheet  is  fixed  above 
and  contracts,  it  is  clear  that  its  lower  end  will  be  raised  more 
than  any  intermediate  point,  since  there  is  a  greater  length 
of  contracting  muscle  above  it.  The  elevation  of  the  ribs 
tends  to  diminish  the  vertical  diameter  of  the  chest;  this  is 


Fig.  130— Portions  of  four  ribs  of  a  dog  with  the  muscles  between  them,  a,  a, 
ventral  ends  of  the  ribs,  joining  at  c  the  rib  cartilages,  6,  which  are  fixed  to  carti- 
laginous portions,  il.  of  the  sternum.  A.  external  intercostal  muscle,  ceasing  be- 
tweer  the  rib  cartilages,  where  the  internal  intercostal,  B.  is  seen.  Between  the 
middle  two  ribs  the  external  Intercostal  muscle  has  been  dissected  away,  so  as  to 
display  the  internal  which  was  covered  by  it. 

more  than  compensated  for  by  the  simultaneous  descent  of 
the  diaphragm. 

The  Lateral  Enlargement  of  the  Chest  is  mainly  due  to 
the  diaphragm,  which,  when  it  contracts,  adds  to  the  lowest 
and  widest  part  of  the  conical  chest  cavity.  Some  small 
widening  is,  however,  brought  about  by  a  rotation  of  some  of 
the  middle;  ribs  which,  as  they  arc  raised,  roll  round  a,  little 
at     their    vertebral     articulations    and    twist     their    cartilages. 

Bach  rib  is  curved  and.il'  the   hone-;  he  examined    in   their 
natural  position  in  a  skeleton,  it  will   he  seen  that,  the  most 


390  77/ A'  HUMAN  BODY. 

curved  part  lies  below  the  level  of  a  straight  line  drawn  from 
the  vertebral  to  the  sternal  attachmenl  of  the  hone.  By  the 
rotation  of  the  rib,  during  inspiration,  this  curved  pari  is 
raised  and  turned  out,  and  the  chest  widened.  The  mech- 
anism can  be  understood  by  clasping  the  hands  opposite  the 
lower  end  of  the  sternum  and  a  few  inches  in  front  of  it, 
with  the  elbows  bent  and  pointing  downwards.  Each  arm 
will  then  answer,  in  an  exaggerated  way,  to  a  curved  rib,  and 
the  clasped  hands  to  the  breast-hone.  If  the  hands  be  sim- 
ply raised  a  few  inches  by  movement  at  the  shoulder-joints 
only,  they  will  be  separated  farther  from  the  front  of  the 
Body,  and  rib  elevation  and  the  consequent  dorso-ventral  en- 
largement of  the  cavity  surrounded  will  he  represented.  But 
if,  simultaneously,  the  arms  be  rotated  at  the  shoulder-joints 
so  as  to  raise  the  elbows  and  turn  them  out  a  little,  it  will  be 
seen  that  the  space  surrounded  by  the  two  arms  is  consider- 
ably increased  from  side  to  side,  as  the  chest  cavity  is  in  in- 
spiration by  the  similar  elevation  of  the  most  curved  part  or 
"angle"  of  the  middle  ribs. 

Expiration.  To  produce  an  inspiration  requires  consid- 
erable muscular  effort.  The  ribs  and  sternum  have  to  be 
raised;  the  elastic  rib  cartilages  bent  and  somewdiat  twisted; 
the  abdominal  viscera  pushed  down;  and  the  abdominal  wall 
pushed  out  to  make  room  for  them.  In  expiration,  on  the 
contrary,  but  little,  if  any,  muscular  effort  is  needed.  As 
soon  as  the  muscles  which  have  raised  the  ribs  and  sternum 
relax,  these  tend  to  return  to  their  natural  unconstrained 
position,  and  the  rib  cartilages,  also,  to  untwist  themselves 
and  bring  the  ribs  back  to  their  position  of  rest;  the  elastic 
abdominal  wall  presses  the  contained  viscera  against  the 
under  side  of  the  diaphragm,  and  pushes  that  up  again  as 
soon  as  its  muscular  fibres  cease  contracting.  By  these  means 
the  chest  cavity  is  restored  to  its  original  capacity  and  the 
air  sent  out  of  the  lungs,  rather  by  the  elasticity  of  the  parts 
which  were  stretched  or  twisted  in  inspiration,  than  by  any 
special  expiratory  muscles. 

Forced  Respiration.  When  a  very  deep  breath  is  drawn 
or  expelled,  or  when  there  is  some  impediment  to  the  entry 
or  exit  of  the  air,  a  great  many  muscles  take  part  in  produc- 
ing the  respiratory  movements;  and  expiration  then  becomes, 
in  part,  an  actively  muscular  act.  The  main  expiratory  mus- 
cles are  the  internal  intercostals  which  lie  beneath  the  exter- 


THE  RESPIRATORY  MECHANISM.  391 

nal  between  each  pair  of  ribs  (Fig.  130  B),  and  have  an  oppo- 
site direction,  their  fibres  running  upwards  and  forwards.  In 
forced  expiration  the  lower  ribs  are  fixed  or  pulled  down  by 
muscles  running  in  the  abdominal  wall  from  tbe  pelvis  to 
them  and  to  the  breast-bone.  The  internal  intercostals,  con- 
tracting, pull  down  the  upper  ribs  and  the  sternum,  and  so 
diminish  the  thoracic  cavity  dorso-ventrally.  At  the  same 
time,  the  contracted  abdominal  muscles  press  the  walls  of 
that  cavity  against  the  viscera  within  it,  and  pushing  these 
up  forcibly  against  the  diaphragm  make  it  very  convex 
towards  the  chest,  and  so  diminish  the  latter  in  its  vertical 
diameter.  In  very  violent  expiration  many  other  muscles 
may  co-operate,  tending  to  fix  points  on  which  those  muscles 
which  can  directly  diminish  the  thoracic  cavity,  pull.  In 
violent  inspiration,  also,  many  extra  muscles  are  called  into 
play.  The  neck  is  held  rigid  to  give  the  scalenes  a  firm  at- 
tachment; the  shoulder-joint  is  held  fixed  and  muscles  going 
from  it  to  the  chest-wall,  and  commonly  serving  to  move  the 
arm,  are  then  used  to  elevate  the  ribs;  the  head  is  held  firm 
on  the  vertebral  column  by  the  muscles  going  between  the 
two,  and  then  other  muscles,  which  pass  from  the  collar-bone 
and  sternum  to  the  skull,  are  used  to  pull  up  the  former. 
The  muscles  which  are  thus  called  into  play  in  labored  but 
not  in  quiet  breathing  are  called  extraordinary  muscles  of 
respiration. 

The  Respiratory  Sounds.  The  entry  and  exit  of  air 
are  accompanied  by  respiratory  sounds  or  murmurs,  which 
can  be  heard  on  applying  the  ear  to  the  chest  wall.  The 
character  of  these  sounds  is  different  and  characteristic  over 
the  trachea,  the  larger  bronchial  tubes,  and  portions  of  lung 
from  which  large  bronchial  tubes  are  absent.  They  are  vari- 
ously modified  in  pulmonary  affections,  and  hence  the  value 
of  auscultation  of  the  lungs  in  assisting  the  physician  to 
form  a  diagnosis. 

The  Capacity  of  the  Lungs.  Since  the  chest  cavity 
never  even  approximately  collapses,  the  lungs  are  never  com- 
pletely emptied  of  air:  the  space  they  have  to  occupy  is 
larger  in  inspiration  than  during  expiration,  but  is  always 
considerable,  bo  that  after  a  forced  expiration  they  still  con- 
tain a  large  amounl  of  air  which  can  only  be  expelled  from 
them  by  opening  the  pleural  cavities;  then  they  entirely  col- 
lapse, just  as  the  bag  in   Fig.  L25  would  it'  the  hot  tie  inclosing 


392  THE  HUMAN  BODY. 

it  were  broken.  The  capacity  of  the  chest,  and  therefore  of 
the  lungs,  varies  much  in  different  individuals,  but  in  a  man 
of  medium  height  there  remain  in  the  lungs  after  the  most 

violent  possible  expiration,  about  1640  cub.  cent.  (100  cub. 
inches)  of  air,  called  the  residual  air.  After  an  ordinary 
expiration  there  will  be  in  addition  to  this  about  as  much 
more  supplemental  air  ;  the  residual  and  supplemental  to- 
gether forming  the  stationary  air,  which  remains  in  the 
chest  during  quiet  breathing.  In  an  ordinary  inspiration  500 
cub.  cent.  (30  cub.  inches)  of  tidal  air  are  taken  in,  and 
about  the  same  amount  is  expelled  in  natural  expiration. 
By  a  forced  inspiration  about  1G00  cub.  cent.  (98  cub.  inches) 
of  complemental  air  can  be  added  to  the  tidal  air.  After  a 
forced  inspiration  therefore  the  chest  will  contain  1640  -4- 
1640  +  500  -f  1G00  =  5380  cubic  centimeters  (328  cubic 
inches)  of  air.  The  amount  which  can  be  taken  in  by  the 
most  violent  possible  inspiration  after  the  strongest  possible 
expiration,  that  is,  the  supplemental,  tidal,  and  complemental 
air  together,  is  known  as  the  vital  capacity.  For  a  healthy 
man  1.7  meters  (5  feet  8  inches)  high  it  is  about  3700  cub. 
cent.  (225  cub.  inches)  and  increases  CO  cub.  cent,  for  each 
additional  centimeter  of  stature;  or  about  9  cubic  inches  for 
each  inch  of  height. 

The  Quantity  of  Air  Breathed  Daily.  Knowing  the 
quantity  of  air  taken  in  at  each  breath  and  expelled  again 
(after  more  or  less  thorough  admixture  with  the  stationary  air) 
we  have  only  to  know,  in  addition,  the  rate  at  which  the 
breathing  movements  occur,  to  be  able  to  calculate  how 
much  air  passes  through  the  lungs  in  twenty-four  hours. 
The  average  number  of  respirations  in  a  minute  is  found  by 
counting  on  persons  sitting  quietly,  and  not  knowing  that 
their  breathing  rate  is  under  observation,  to  be  fifteen  in  a 
minute.  In  each  respiration  half  a  liter  (30  cubic  inches)  of 
air  is  concerned;  therefore  0.5  X  15  X  GO  X  24  =  10,800 
liters  (375  cubic  feet)  is  the  quantity  of  air  breathed  under 
ordinary  circumstances  by  each  person  in  a  day. 

Hygienic  Remarks.  Since  the  diaphragm  when  it  con- 
tracts pushes  down  the  abdominal  viscera  beneath  it.  these 
have  to  make  room  for  themselves  by  pushing  out  the  soft 
front  of  the  abdomen  which,  accordingly,  protrudes  when  the 
diaphragm  descends.  Hence  breathing  by  the  diaphragm, 
being  indicated,  on  the  exterior  by  movements  of  the  abdo- 


THE  RESPIRATORY  MECHANISM.  393 

men,  is  often  called  "  abdominal  respiration,"  as  di-stinguished 
from  breathing  by  the  ribs,  called  "  costal  "  or  "  chest  breath- 
ing.'' In  both  sexes  the  diaphragmatic  breathing  is  the 
most  important,  but,  as  a  rule,  men  and  children  use  the  ribs 
less  than  adult  women.  Since  both  abdomen  and  chest  alter- 
nately expand  and  contract  in  healthy  breathing,  anything 
which  impedes  their  free  movement  is  to  be  avoided;  and  the 
tight  lacing  which  used  to  be  thought  elegant  a  few  years 
back,  and  is  still  indulged  in  by  some  who  think  a  distorted 
form  beautiful,  seriously  impedes  one  of  the  most  important 
functions  of  the  Body,  leading,  if  nothing  worse,  to  shortness 
of  breath  and  an  incapacity  for  muscular  exertion.  In  ex- 
treme cases  of  tight  lacing  some  organs  are  often  directly 
injured,  weals  of  fibrous  tissue  being,  for  example,  not  unfre- 
quently  found  developed  on  the  liver,  from  the  pressure  of 
the  lower  ribs  forced  against  it  by  a  tight  corset. 

The  Aspiration  of  the  Thorax.  As  already  pointed  out, 
the  external  air  cannot  press  directly  upon  the  contents  of 
the  thoracic  cavity,  on  account  of  the  rigid  framework  which 
supports  its  walls;  it  still,  however,  presses  on  them  indi- 
rectly through  the  lungs.  Pushing  on  the  interior  of  these 
with  a  pressure  equal  to  that  exerted  on  the  same  area  by  a 
column  of  mercury  700  mm.  (30  inches)  high,  it  distends 
them  and  forces  them  against  the  inside  of  the  chest-walls, 
the  heart,  the  great  thoracic  blood-vessels,  the  thoracic-duct, 
and  the  other  contents  of  the  chest-cavity.  This  pressure  is 
not  equal  to  that  of  the  external  air,  since  some  of  the  total 
air-pressure  on  the  inside  of  the  lungs  is  used  up  in  overcom- 
ing their  elasticity,  and  it  is  only  the  residue  which  pushes 
them  against  the  things  outside  them.  In  expiration  this 
residue  is  equal  to  that  exerted  by  a  column  of  mercury  754 
mm.  (29.8  inches)  high.  On  most  parts  of  the  Body  the  at- 
mospheric pressure  acts,  however,  with  full  force.  Pressing 
on  a  limb  it  pushes  the  skin  against  the  soft  parts  beneath, 
and  tbese  compress  the  blood  and  lymph  vessels  among  them; 
and  the  yielding  abdominal  walls  do  not,  like  the  rigid  tho- 
racic walls,  carry  the  atmospheric  pressure  themselves,  but 
transmit  it  to  the  contents  of  the  cavity.  It  thus  comes  to 
pass  that  the  blood  and  lymph  in  most  parts  of  the  Body  are 
under  a  higher  atmospheric  pressure  than  they  are  exposed 
to  in  the  chest,  and  consequently  these  liquids  tend  to  How 
into  the  thorax,  until   the  extra  distention   of  the  vessels  in 


394  THE  HUMAN  BODY. 

which  they  there  accumulate  compensates  for  the  less  exter- 
nal pressure  to  which  those  vessels  are  exposed.  An  equilib- 
rium would  thus  very  soon  be  brought  about  were  it  not  for 
the  respiratory  movements,  in  consequence  of  which  the 
intra-thoracic  pressure  is  alternately  increased  and  dimin- 
ished, and  the  thorax  comes  to  act  as  a  sort  of  suction-pump 
on  the  contents  of  the  vessels  of  the  Body  outside  it;  thus 
the  respiratory  movements  influence  the  circulation  of  the 
blood  and  the  flow  of  the  lymph. 

Influence  of  the  Respiratory  Movements  upon  the  Cir- 
culation. Suppose  the  chest  in  a  condition  of  normal  expira- 
tion and  the  external  pressure  on  the  blood  in  the  blood-ves- 
sels within  it  and  in  the  heart,  to  have  come,  in  the  manner 
pointed  out  in  the  last  paragraph,  into  equilibrium  with  the 
atmospheric  pressure  exerted  on  the  blood-vessels  of  the  neck 
and  abdomen.  If  an  inspiration  now  occurs,  the  chest  cavity 
being  enlarged  the  pressure  on  all  of  its  contents  will  be  di- 
minished. In  consequence,  air  enters  the  lungs  from  the 
windpipe,  and  blood  enters  the  venae  cavaa  and  the  right  au- 
ricle of  the  heart.  Thus  not  only  the  lungs,  but  the  right 
side  of  the  heart,  and  the  intra-thoracic  portions  of  the  sys- 
temic veins  leading  to  it,  are  expanded  during  an  inspiration; 
but  the  lungs  being  much  the  most  distensible  take  far  the 
greatest  part  in  filling  up  the  increased  space.  The  left  side 
of  the  heart  is  not  much  influenced  as  it  is  filled  from  the 
pulmonary  veins;  and  the  whole  vessels  of  the  lesser  circula- 
tion lying  within  the  chest,  and  being  all  affected  in  the 
same  way  at  the  same  time,  the  blood-flow  in  them  is  not  di- 
rectly influenced  by  the  aspiration  of  the  thorax.  Distention 
of  the  lungs  seems,  however,  to  diminish  the  capacity  of  their 
vessels,  and  so  to  a  certain  extent  the  flow  is  influenced;  as 
the  lungs  expand  blood  is  forced  out  of  their  vessels  into  the 
left  auricle,  and  when  they  again  contract  their  vessels  fill 
up  from  the  right  ventricle.     The  pressure  on  the  thoracic 

aorta  being  diminished  in  inspiration,  bl 1  tends  to  flow  back 

into  it  from  the  abdominal  portion  of  the  vessel,  but  cannot 
enter  the  heart  on  account  of  the  semilunar  valves;  and  the 
back-flow  does  not  in  any  case  equal  the  onflow  due  to  the 
beat  of  the  heart;  so  what  happens  in  the  aorta  is  but  a 
slight  slowing  of  the  current.  The  general  result  of  all  this 
is  that  the  circulation  is  considerably  assisted.  When  the 
next  expiration  occurs,  and  the  pressure  in  the  thorax  again 


THE  RESPIRATORY  MECHANISM.  395 

rises,  air  and  blood  botli  tend  to  be  expelled  from  tbe  cavity. 
The  aorta  thus  regains  what  it  lost  during  inspiration;  the 
pressure  on  it  is  increased  and  it  empties  itself  faster  into  its 
abdominal  portion.  The  semilunar  valves  having  prevented 
any  regurgitation  into  the  heart,  there  is  neither  gain  nor 
loss  so  far  as  it  is  concerned.  With  the  systemic  intra-tho- 
racic  veins,  however,  this  is  not  the  case;  the  extra  blood  en- 
tering them  has  already  in  great  part  gone  on  beyond  the 
tricuspid  valve,  and  cannot  flow  back  during  expiration;  and 
the  pressure  in  the  auricle  being  constantly  kept  low  by  its 
emptying  into  the  ventricle,  the  increased  pressure  on  the 
venae  cavae  tends  rather  to  send  blood  on  into  the  heart,  than 
back  into  the  extra-thoracic  veins.  Moreover,  whatever 
blood  tends  to  take  the  latter  course  cannot  do  it  effectually 
since,  although  the  venae  cavae  themselves  contain  no  valves, 
the  more  distant  veins  which  open  into  them  do.  Conse- 
quently, whatever  extra  blood  has,  to  use  the  common  phrase, 
been  '•'  sucked"  into  the  intra-thoracic  vena?  cavae  in  inspira- 
tion and  has  not  been  sent  already  on  into  the  right  ventricle 
before  expiration  occurs,  is,  on  account  of  the  venous  valves, 
imprisoned  in  the  cavae  under  an  increased  pressure  during 
expiration;  and  this  tends  to  make  it  flow  faster  into  the  au- 
ricle during  the  diastole  of  the  latter.  How  much  the  alter- 
nating respiratory  movements  assist  the  venous  flow  is  shown 
by  the  dilation  of  the  veins  of  the  head  and  neck  which  oc- 
curs when  a  person  is  holding  his  breath;  and  the  blackness 
for  the  face,  from  distention  of  the  veins  and  stagnation  of 
the  capillary  flow,  which  occurs  during  a  prolonged  fit  of 
coughing,  which  is  a  series  of  expiratory  efforts  without  any 
inspirations. 

On  the  whole  the  influence  of  the  respiratory  movements 
on  the  blood-flow  is  such  as  to  favor  it  in  inspiration  and  to 
impede  it  during  expiration.  This  influence  very  often  shows 
itself  on  tracings  of  arterial  pressure  taken  as  described  in 
Chap.  XVIII.  Such  tracings  usually  show  in  addition  to  the 
pulse  waves,  slower  and  greater  rises  and  falls  of  pressure 
which  have  the  same  rhythm  as  the  respiration.  In  general, 
the  rise  of  pressure  in  these  respiratory  waves  of  blood-pres- 
sure ie  synchronous  with  inspiration  and  the  fall  with  expira- 
tion, but  not  exactly.  The  changes  manifest  themselves  on 
the  blood-pressure  curve  a  little  later  than  the  commencement 
of  the  thoracic  movement  which  leads  to  them;  the  rise  be- 


396 


THE  HUMAN  BODY. 


ginning  ;i  little  after  the  beginning  of  inspiration,  the  fall  a 
little  Later  than  the  commencement  of  expiration. 

In  still  another  way  the  aspiration  of  the  thorax  assists 
the  heart.  The  heart  and  lungs  are  both  distensible,  though 
in  different  degrees,  and  each  is  stretched  in  the  chest  some- 
what beyond  its  natural  size;  the  one  by  the  atmospheric 
pressure  directly,  the  other  by  that  pressure  indirectly  ex- 
erted through  the  blood  exposed  to  it  in  the  extra-thoracic 
veins.  Supposing,  therefore,  the  heart  suddenly  to  shrink,  it 
would  leave  more  space  in  the  chest  to  be  filled  by  the  lungs; 
these  must  accordingly,  at  each  cardiac  systole,  expand  a  lit- 
tle to  fill  the  extra  room,  just  as  they  do  when  the  space 
around  them  is  otherwise  enlarged,  as  during  an  inspiration. 
The  elasticity  of  the  lungs,  however,  causes  them  to  resist 
this  distention  and  oppose  the  cardiac  systole.  The  matter 
may  be  made  clear  by  an  arrangement  like  that  in  Fig.  131. 
A  is  an  air-tight  vessel  with  a  tube,  e,  provided  with  a  stop- 
cock, leading  from  it;  b  is  a  highly  distensible  elastic  bag  in 
free  communication  through  d  with  the 
exterior;  and  c,  representing  the  heart, 
is  a  less  extensible  sac,  from  which  a 
tube  leads  and  dips  under  water  in  the 
vessel  B.  If  air  be  pumped  out  through 
e  both  bags  will  dilate,  b  filling  with  air, 
and  c  with  water  driven  up  by  atmos- 
pheric pressure.  Ultimately,  if  suffi- 
ciently extensible,  they  would  fill  the 
whole  space,  the  thinner- walled,  b,  occu- 
pying most  of  it.  If  then  the  stop-cock 
be  closed,  things  will  remain  in  equilib- 
rium, each  bag  striving  to  collapse  and 
so  exerting  a  pull  on  the  other,  for  if  b 
shrinks  c  must  expand  and  vice  versa. 
If  c  suddenly  shrink,  as  the  heart  does  in  its  systole,  b  will 
dilate;  but  as  soon  as  the  systole  of  c  ceases,  b  will  shrink 
again  and  pull  c  out  to  its  previous  size.  In  the  same  way, 
after  the  cardiac  systole,  when  the  heart-walls  relax,  the  lungs 
pull  them  out  again  and  dilate  the  organ.  The  contracting 
heart  thus  expends  some  of  its  work  in  overcoming  the  elas- 
ticity of  the  lungs,  which  opposes  their  expansion  to  fill  the 
space  left  by  the  smaller  heart;  but  during  the  diastole  of 
the  heart  this  work  is  utilized  to  pull  out  its  walls  again,  and 


^p£ 


B 


Fig.  131.— Diagram  il- 
lustrating i  he  influence  of 
aspiration  of  the  thorax 
on  the  circulation  of  the 
blood. 


THE  RESPIRATORY  MECHANISM.  397 

draw  blood  into  it.  Since  the  normal  heart  has  muscular 
power,  and  to  spare,  for  its  systole,  this  arrangement,  by 
which  some  of  the  work  then  spent  is  stored  away  to  assist 
the  diastole,  which  cannot  be  directly  performed  by  cardiac 
muscles,  is  of  service  to  it  on  the  whole.  It  is  a  physiological 
though  not  a  mechanical  advantage;  no  work  power  is 
gained,  but  what  there  is,  is  better  distributed. 

Influence  of  the  Respiration  on  the  Lymph-Flow. 
During  inspiration,  when  intra-thoracic  pressure  is  lowered, 
lymph  is  pressed  into  the  thoracic  duct  from  the  abdominal 
lymphatics.  In  expiration,  when  thoracic  pressure  rises 
again,  the  extra  lymph  cannot  flow  back  on  account  of  the 
valves  in  the  lymphatic  vessels,  and  it  is  consequently  driven 
on  to  the  cervical  ending  of  the  thoracic  duct.  The  breath- 
ing movements  thus  pump  the  lymph  on. 


CHAPTER    XXVI. 
THE   CHEMISTRY   OF  RESPIRATION. 

Nature  of  the  Problems.  The  study  of  the  respiratory 
process  from  a  chemical  standpoint  has  for  its  object  to  dis- 
cover what  are,  in  kind  and  extent,  the  interchanges  between 
the  air  in  the  lungs  and  the  blood  in  the  pulmonary  capilla- 
ries; and  the  nature  and  amount  of  the  corresponding  gaseous 
changes  between  the  living  tissues,  and  the  blood  in  the  sys- 
temic capillaries.  Neglecting  some  oxygen  used  up  otherwise 
than  in  forming  carbon  dioxide,  and  some  carbon  dioxide  elim- 
inated by  other  organs  than  the  lungs,  these  processes  in  the 
long-run  balance,  the  blood  losing  as  much  carbon  dioxide  gas 
in  the  lungs  as  it  gains  elsewhere,  and  gaining  as  much  oxygen 
in  the  lungs  as  it  loses  in  the  systemic  capillaries.  To  compre- 
hend the  matter  it  is  necessary  to  know  the  physical  and  chemical 
conditions  of  these  gases  in  the  lungs,  in  the  blood,  and  in  the 
tissues  generally;  for  only  so  can  we  understand  how  it  is  that 
in  different  localities  of  the  Body  such  exactly  contrary  pro- 
cesses occur.  So  far  as  the  problems  connected  with  the 
external  respiration  are  concerned  our  knowledge  is  tolerably 
complete;  but  as  regards  the  internal  respiration,  taking 
place  all  through  the  Body,  much  has  yet  to  be  learnt; 
we  know  that  a  muscle  at  work  gives  more  carbon  dioxide 
to  the  blood  than  one  at  rest  and  takes  more  oxygen  from 
it,  but  how  much  of  the  one  it  gives  and  of  the  other  it 
takes  is  only  known  approximately;  as  are  the  conditions 
iinder  which  this  greater  interchange  during  the  activity 
of  the  muscular  tissue  is  effected:  and  concerning  nearly 
all  the  other  issues  we  know  even  less  than  about  muscle. 
In  fact,  as  regards  the  Body  as  a  whole,  it  is  compara- 
tively easy  to  find  how  great  its  gaseous  interchanges  with 
the   air  are    during   work  and   rest,   waking  and   sleeping, 

398 


THE  CHEMISTRY  OF  RESPIRATION.  399 

while  fasting  or  digesting,  and  so  on  ;  but  when  it  comes  to 
be  decided  what  organs  are  concerned  in  each  case  in  pro- 
ducing the  greater  or  less  exchange,  and  how  much  of  the 
whole  is  due  to  each  of  them,  the  question  is  one  far  more 
difficult  to  settle  and  still  very  far  from  completely  answered. 

The  Changes  Produced  in  Air  by  being  once  Breathed. 
These  are  fourfold — changes  in  its  temperature,  in  its  mois- 
ture, in  its  chemical  composition,  and  its  volume. 

The  air  taken  into  the  lungs  is  nearly  always  cooler  than 
that  expired,  which  has  a  temperature  of  about  36°  C.  (97° 
F.).  The  temperature  of  a  room  is  usually  less  than  21°  C. 
(70°  F.).  The  warmer  the  inspired  air  the  less,  of  course,  the 
heat  which  is  lost  to  the  Body  in  the  breathing  process;  its 
average  amount  is  calculated  as  about  equal  to  50  calories  in 
twenty-four  hours;  a  calorie  being  as  much  heat  as  will  raise 
the  temperature  of  one  kilogram  (2.2  lbs.)  of  water  one  degree 
centigrade  (1.8°  F.). 

The  inspired  air  always  contains  more  or  less  water  vapor, 
but  is  rarely  saturated;  that  is,  rarely  contains  so  much  but  it 
can  take  up  more  without  showing  it  as  mist;  the  warmer  air  is, 
the  more  water  vapor  it  requires  to  saturate  it.  The  expired 
air  is  nearly  saturated  for  the  temperature  at  which  it  leaves 
the  Body,  as  is  readily  shown  by  the  water  deposited  when  it 
is  slightly  cooled,  as  when  a  mirror  is  breathed  upon;  or  by 
the  clouds  seen  issuing  from  the  nostrils  on  a  frosty  day, 
these  being  due  to  the  fact  that  the  air,  as  soon  as  it  is  cooled, 
cannot  hold  all  the  water  vapor  which  it  took  up  when 
warmed  in  the  Body.  Air,  therefore,  when  breathed  once, 
gains  water  vapor  and  carries  it  off  from  the  lungs;  the 
actual  amount  being  subject  to  variation  with  the  tempera- 
ture and  saturation  of  the  inspired  air:  the  cooler  and  drier 
th.it  is,  the  more  water  will  it  gain  when  breathed.  On  an 
average  the  amount  thus  carried  off  in  twenty-four  hours  is 
about  250  grams  (f)  ounces).  To  evaporate  this  water  in  the 
lungs  itn  amount  of  heat  is  required,  which  disappears  for 
this  purpose  in  the  Body,  to  reappear  again  outside  it  when 
the  water  vapor  condenses.  The  amount  of  heat  taken  off  in 
this  way  during  the  day  is  abont  I4S  calories.  The  total  daily 
loss  of  heat  from  tie'  Body  through  the  lungs  is  therefore 
198  calories,  50  in  warming  the  inspired  air  and  148  in  the 
evaporation  of  water. 

The    most    important    changes    brought    about    in    the 


400  THE  HUMAN  BODY 

breathed  air   are  those   iu    its   chemical  composition.     Pure 
air  when  completely  dried  consists  in  each  100  parts  of — 

By  Volume.  by  Weight. 

Oxygen 80.8  23 

Nitrogen 79.2  77 

Ordinary  atmospheric  air  contains  in  addition  4  volumes 
of  carbon  dioxide  in  10,000,  or  0.04  in  100,  a  quantity  which, 
for  practical  purposes,  may  be  neglected.  When  breathed 
once,  such  air  gains  rather  .more  than  4  volumes  in  100  of 
carbon  dioxide,  and  loses  rather  more  than  5  of  oxygen. 
More  accurately,  100  volumes  of  expired  air  after  drying  give 
98.9  volumes,  which  consist  of — 

Oxygen 15.4 

Nitrogen 79.2 

Carbon  dioxide 4.3 

The  expired  air  also  contains  volatile  organic  substances 
in  quantities  too  minute  for  chemical  analysis,  but  readily 
detected  by  the  nose  upon  coming  into  a  close  room  in  which 
a  number  of  persons  have  been  collected. 

Since  10,800  litres  (375  cubic  feet)  of  air  are  breathed  in 
twenty-four  hours  and  lose  5.4  per  cent  of  oxygen,  the  total 
quantity  of  this  gas  taken  up  in  the  lungs  daily  is  10,800  X 
5.4  ^  100  =  583.2  litres  (20.4  cubic  feet).  One  litre  of 
oxygen  measured  at  0°  0.  (32°  F.)  and  under  a  pressure  equal 
to  one  atmosphere,  weighs  1.43  grams,  so  the  total  weight  of 
oxygen  taken  up  by  the  lungs  daily  is  583.2  X  1.43  =  833.9 
grams.  Or,  using  inches  and  grains  as  standards,  44.5  cubic 
inches  of  oxygen  at  the  above  temperature  and  pressure 
weigh  almost  exactly  16  grains,  so  the  20.4  cubic  feet  ab- 
sorbed in  the  lungs  daily  weigh  20.4  X  1T28  -=-  44.5  X  16  = 
12,818  grains. 

The  amount  of  carbon  dioxide  excreted  from  the  lungs 
being  4.3  per  cent  of  the  volume  of  the  air  breathed  daily,  is 
10,800  X  4.3  -f-  100  =  464.4  litres  (16.25  cubic  feet)  measured 
at  the  normal  temperature  and  pressure.  This  volume 
weighs  910  grams,  or  14,10.")  grains. 

If  the  expired  air  be  measured  as  it  leaves  the  Body  its 
bulk  will  be  found  greater  than  that  of  the  inspired  air,  since 
it  not  only  has  water  vapor  added  to  it,  but  is  expanded  in 
consequence  of  its  higher  temperature.  If,  however,  it  be 
dried  and  reduced  to  the  same  temperature  as  the  inspired 


THE  CHEMISTRY  OF  RESPIRATION.  401 

air  its  volume  will  be  found  diminished,  since  it  has  lost  5.4 
volumes  per  cent  of  oxygen  and  gained  only  4.3  of  carbon 
dioxide.  In  round  numbers,  100  volumes  of  dry  inspired  air 
at  zero,  give  99  volumes  of  dry  expired  air  measured  at  the 
same  temperature  and  pressure. 

Ventilation.  Since  at  every  breath  some  oxygen  is  taken 
from  the  air  and  some  carbon  dioxide  given  to  it,  were  the 
atmosphere  around  a  living  man  not  renewed  be  would,  at 
last,  be  unable  to  get  from  the  air  the  oxygen  he  required;  he 
would  die  of  oxygen  starvation  or  be  suffocated,  as  such  a 
mode  of  death  is  called,  as  surely,  though  not  quite  so  fast,  as 
if  he  were  put  under  the  receiver  of  an  air-pump  and  all  the 
air  around  him  removed.  Hence  the  necessity  of  ventilation 
to  supply  fresh  air  in  place  of  that  breathed,  and  clearly  the 
amount  of  fresh  air  requisite  must  be  determined  by  the 
number  of  persons  collected  in  a  room;  the  supply  which 
would  be  ample  for  one  person  would  be  insufficient  for  two. 
Moreover  fires,  gas,  and  oil  lamps,  all  use  up  the  oxygen  of 
the  air  and  give  carbon  dioxide  to  it,  and  hence  calculation 
must  be  made  for  them  in  arranging  for  the  ventilation  of  a 
building  in  which  they  are  to  be  employed. 

In  order  that  air  be  unwholesome  to  breathe,  it  is  by  no 
means  necessary  that  it  have  lost  so  much  of  its  oxygen  as  to 
make  it  difficult  for  the  Body  to  get  what  it  wants  of  that 
gas.  The  evil  results  of  insufficient  air-supply  are  rarely,  if 
ever,  due  to  that  cause  even  in  the  worst-ventilated  room  for, 
as  we  shall  see  hereafter,  the  blood  is  able  to  take  what 
oxygen  it  wants  from  air  containing  comparatively  little  of 
that  gas.  The  headache  and  drowsiness  which  come  on  from 
sitting  in  a  badly  ventilated  room,  and  the  want  of  energy 
and  general  ill-health  which  result  from  permanently  living  in 
•such,  are  dependent  on  a  slow  poisoning  of  the  Body  by  the 
reabsorption  of  the  things  eliminated  from  the  lungs  in 
previous  expirations.  What  these  are  is  not  accurately 
known;  they  doubtless  belong  to  those  volatile  bodies  men- 
tioned above,  as  carried  off  in  minute  quantities  in  each 
breath;  since  observation  shows  that  the  air  becomes  injuri- 
ous long  before  the  amount  of  carbon  dioxide  in  it  is  suffi- 
cient to  do  any  harm.  Breathing  air  containing  one  or  two 
per  cent  of  thai  gas  produced  by  ordinary  chemical  methods 
does  no  particular  injury,  but  breathing  air  containing  one 
per  cent  of  it  produced  by  respiration  is  decidedly  injurious, 


402  THE  HUMAN  BODY. 

because  of  the  other  things  seut  out  of  the  lungs  at  the  same 
time.  Carbon  dioxide  itself,  at  least  in  any  such  percentage 
as  is  commonly  found  in  a  room,  is  not  poisonous,  as  used  to 
be  believed,  but,  since  it  is  tolerably  easily  estimated  in  air, 
while  the  actually  injurious  substances  evolved  in  breathing 
are  not,  the  purity  or  foulness  of  the  air  in  a  room  is  usually 
determined  by  tin  ding  the  percentage  of  carbon  dioxide 
in  it:  it  must  be  borne  in  mind  that  to  mean  much  this 
carbon  dioxide  must  have  been  produced  by  breathing;  the 
amount  of  it  found  is  in  itself  no  guide  to  the  quantity 
of  really  important  injurious  substances  present.  Of  course 
when  a  great  deal  of  carbon  dioxide  is  present  the  air  is 
irrespirable:  as  for  example  sometimes  at  the  bottom  of 
wells  or  brewing-vats. 

In  one  minute  .5  X  15  =  7.5  liters  (0.254  cubic  feet)  of 
air  are  breathed  and  this  is  vitiated  with  carbon  dioxide 
to  the  extent  of  rather  more  than  four  per  cent;  mixed 
with  three  times  its  volume  of  external  air,  it  would  give 
thirty  liters  (a  little  over  one  cubic  foot)  vitiated  to  the 
extent  of  one  per  cent,  and  such  air  is  not  respirable  for 
any  length  of  time  with  safety.  The  result  of  breathing  it 
for  an  evening  is  headache  and  general  malaise;  of  breath- 
ing it  for  weeks  or  months  a  lowered  tone  of  the  whole  Body 
— less  power  of  work,  physical  or  mental,  and  less  power  of 
resisting  disease;  the  ill  effects  may  not  show  themselves  at 
once,  and  may  accordingly  be  overlooked,  or  considered  scien- 
tific fancies,  by  the  careless;  but  they  are  nevertheless  there 
ready  to  manifest  themselves.  In  order  to  have  air  to  breathe 
in  a  fairly  pure  state  every  man  should  get  for  his  own 
allowance  at  least  23,000  liters  of  space  to  begin  with 
(about  800  cubic  feet)  and  the  arrangements  for  ventilation 
should,  at  the  very  least,  renew  this  at  the  rate  of  30  litres 
(one  cubic  foot)  per  minute.  The  nose  is,  however,  the  best 
guide,  and  it  is  found  that  at  least  five  times  this  supply  of 
fresh  air  is  necessary  to  keep  free  from  odor  a  small  room 
inhabited  by  one  adult.  In  the  more  recently  constructed 
hospitals,  as  a  result  of  experience,  twice  the  above  minimum 
cubic  space  is  allowed  for  each  bed  in  a  ward,  and  the  re- 
placement of  the  old  air  at  a  far  more  rapid  rate  is  also 
provided  for. 

Ventilation  does  not  necessarily  imply  draughts  of  cold 
air,  as  is  too  often  supposed.     In  warming  by  indirect  radia- 


THE   CHEMISTRY  OF  RESPIRATION.  403 

tion  it  may  readily  be  secured  by  arranging,  in  addition  to 
the  registers  from  which  the  warmed  air  reaches  the  room, 
proper  openings  at  the  opposite  side,  by  which  the  old  air 
may  pass  ofE  to  make  room  for  the  fresh.  An  open  fire  in  a 
room  will  always  keep  up  a  current  of  air  through  it,  and  is 
the  healthiest,  though  not  the  most  economical,  method  of 
warming  an  apartment. 

Stoves  in  a  room,  unless  constantly  supplied  with  fresh 
air  from  without,  dry  its  air  to  an  unwholesome  extent.  If 
no  appliance  for  providing  this  supply  exists  in  a  room,  it 
can  usually  he  got,  without  a  draught,  by  fixing  a  board  about 
four  inches  wide  under  the  lower  sash  and  shutting  the  win- 
dow down  on  it.  Fresh  air  then  comes  in  by  the  opening 
between  the  two  sashes  and  in  a  current  directed  upwards, 
which  gradually  diffuses  itself  over  the  room  without  being 
felt  as  a  draught  at  any  one  point.  In  the  method  of  heating 
by  direct  radiation,  the  apparatus  employed  provides  of  itself 
no  means  of  drawing  fresh  air  into  a  room,  as  the  draught  up 
the  chimney  of  an  open  fireplace  or  of  a  stove  does;  and 
therefore  special  inlet  and  outlet  openings  are  very  necessary. 
Since  few  doors  and  windows,  fortunately,  fit  quite  tight, 
fresh  air  gets  even  into  closed  rooms,  in  tolerable  abundance 
for  one  or  two  inhabitants,  if  there  be  outlets  for  the  air 
already  in  them. 

Changes  undergone  by  the  Blood  in  the  Lungs.  These 
are  the  exact  reverse  of  those  undergone  by  the  breathed  air 
— what  the  air  gains  the  blood  loses,  and  vice  versa.  Con- 
sequently, the  blood  loses  heat,  and  water,  and  carbon  dioxide 
in  the  pulmonary  capillaries;  and  gains  oxygen.  These 
gains  and  losses  are  accompanied  by  a  change  of  color  from 
the  dark  purple  which  the  blood  exhibits  in  the  pulmonary 
artery,  to  the  bright  scarlet  it  possesses  in  the  pulmonary 
veins. 

The  dependence  of  this  color  change  upon  the  access  of 
fresh  air  to  the  lungs  while  the  blood  is  flowing  through 
them,  can  be  readily  demonstrated.  If  a  rabbit  be  rendered 
unconscious  by  chloroform,  and  its  chest  be  opened,  after  a 
pair  of  bellows  has  been  connected  with  its  windpipe,  it  is 
seen  that,  so  long  as  the  bellows  are  worked  to  keep  up  arti- 
ficial respiration,  the  blood  in  the  right  side  of  the  heart  (as 
seen  through  the  thin  auricle)  and  that  in  the  pulmonary 
artery,  is  dark   colored,  while  that   in  the  pulmonary  veins 


404  Till;  HUMAN  BODY. 

and  the  left  auricle  is  bright  red.  Let,  however,  the  artificial 
respiration  be  stopped  for  a  few  seconds  and,  consequently, 
the  renewal  of  the  air  in  the  lungs  (since  an  animal  cannot 
breathe  for  itself  when  its  chest  is  opened),  and  very  soon  the 
blood  returns  to  the  left  auricle  as  dark  as  it  left  the  right. 
In  a  very  short  time  symptoms  of  suffocation  show  them- 
selves and  the  animal  dies,  unless  the  bellows  be  again  set  at 
work. 

The  Blood  Gases.  If  fresh  blood  be  rapidly  exposed  to 
as  complete  a  vacuum  as  can  be  obtained,  it  gives  off  certain 
gases,  known  as  the  gases  of  tlie  blood.  These  are  the  same 
in  kind,  but  differ  in  proportion,  in  venous  and  arterial 
blood;  there  being  more  carbon  dioxide  and  less  oxygen  ob- 
tainable from  the  venous  blood  going  to  the  lungs  by  the 
pulmonary  artery,  than  from  the  arterial  blood  coming  back 
to  the  heart  by  the  pulmonary  veins.  The  gases  given  off  by 
venous  and  arterial  blood,  measured  under  the  normal  pres- 
sure and  at  the  normal  temperature,  amount  to  from  58  to  62 
volumes  for  every  100  volumes  of  blood,  and  in  the  two  cases 
are  about  as  follows — 

Venous  Blood.  Arterial  Blood. 

Oxygen 10  20 

Carbon  dioxide 46  40 

Nitrogen 2  2 

It  is  important  to  bear  in  mind  that  while  arterial  blood 
contains  some  carbon  dioxide  that  can  be  removed  by  the 
air-pump,  venous  blood  also  contains  some  oxygen  removable 
in  the  same  way;  so  that  the  difference  between  the  two  \a 
only  one  of  degree.  When  an  animal  is  killed  by  suffocation, 
however,  the  last  trace  of  oxygen  which  can  be  yielded  up  in 
a  vacuum  disappears  from  the  blood  before  the  heart  ceases 
to  beat.  All  the  blood  of  such  an  animal  is  what  might  be 
called  suffocation  blood,  and  has  a  far  darker  color  than 
ordinary  venous  blood. 

The  Cause  of  the  Bright  Color  of  Arterial  Blood.  The 
color  of  the  blood  depends  on  its  red  corpuscles,  since  pure 
blood  plasma  or  blood  serum  is  colorless,  or  at  most  a  very 
faint  straw  yellow.  Hence  the  color  change  which  the  blood 
experiences  in  circulating  through  the  lungs  must  be  due  to 
some  change  in  its  red  corpuscles.  Now,  minute  solid  bodies 
suspended  in  a  liquid  reflect  more  light  when  they  are  more 
dense,  other  things  being  equal;  and  the  first  thing  that  sug- 


THE  CHEMISTRY  OF  RESPIRATION.  405 

gests  itself  as  the  cause  of  the  change  in  color  of  the  blood  is 
that  its  red  corpuscles  have  shrunk  in  the  pulmonary  circula- 
tion, and  so  reflect  more  light  and  give  the  blood  a  brighter 
look.  This  idea  gains  some  support  from  the  fact  that,  as 
seen  under  the  microscope,  the  red  blood  corpuscles  of  some 
animals,  as  the  frog,  do  expand  somewhat  when  exposed  to 
carbon  dioxide  gas  and  shrink  up  a  little  in  oxygen.  But 
that  this  is  not  the  chief  cause  of  the  color  change  is  readily 
proved.  By  diluting  blood  with  water  the  coloring  matter  of 
the  red  corpuscles  can  be  made  to  pass  out  of  them  and  go 
into  solution  in  the  plasma,  and  it  is  found  that  such  a 
solution,  in  which  there  can  be  no  question  as  to  the  reflect- 
ing powers  of  colored  solid  bodies  suspended  in  it,  is  brighter 
red  when  supplied  with  oxygen  than  when  deprived  of  that 
gas.  This  suggests  that  the  coloring  matter  or  haemoglobin 
of  the  red  corjmscles  combines  with  oxygen  to  form  a  scarlet 
compound,  and  when  deprived  of  that  gas  has  a  darker  and 
more  purple  color;  and  other  experiments  confirm  this. 
Haemoglobin  combined  with  oxygen  is  known  as  oxyhemo- 
globin, and  it  is  on  its  predominance  that  the  color  of  arterial 
blood  depends.  Haemoglobin  uncombined  with  oxygen, 
sometimes  named  reduced  hmmoglobin,  predominates  in 
venous  blood,  and  is  the  only  kind  found  in  the  blood  of  a 
suffocated  mammal. 

The  Laws  Governing  the  Absorption  of  Gases  by  a 
Liquid.  In  order  to  understand  the  condition  of  the  gases 
in  the  blood  liquid  it  is  necessary  to  recall  the  general  laws 
in  accordance  with  which  liquids  absorb  gases.  They  are  as 
follows  : 

1.  A  given  volume  of  a  liquid  at  a  definite  temperature  if 
it  absorbs  any  of  a  gas  to  which  it  is  exposed,  and  yet  does 
not  combine  chemically  with  it,  takes  up  a  definite  volume 
of  the  gas.  If  the  gas  be  compressed  the  liquid  will  still,  at 
the  same  temperature,  take  up  the  same  volume  as  before, 
but  now  it  takes  up  a  greater  weight;  and  a  weight  exactly 
as  much  greater  as  the  pressure  is  greater,  since  one  volume 
of  a  gas  under  any  pressure  contains  exactly  twice  as  much 
of  the  gas  by  weight  as  the  same  volume  under  half  the  pres- 
sure; and  so  on.  A  liter  or  a  quart  of  water,  for  example, 
exposed  to  the  air  will  dissolve  a  certain  amount  of  oxygen. 
If  the  air  (and  therefore  the  oxygen  in  it)  be  compressed  to 
one  fourth  its  bulk  then  the  water  will  dissolve  exactly  the 


406  THE  HUMAN  BODY. 

same  volume  of  oxygen  as  before,  but  this  volume  of  the 
compressed  gas  will  contain  exactly  four  times  as  much 
oxygen  as  did  the  same  volume  of  the  gas  under  the  original 
pressure;  and  if,  now,  the  pressure  be  again  diminished  the 
oxygen  will  be  given  off  exactly  in  proportion  as  its  pressure 
on  the  surface  of  the  water  decreases.  Finally,  when  a  com- 
plete vacuum  is  formed  above  the  surface  of  the  water,  it  will 
be  found  that  the  latter  has  given  off  all  its  dissolved  oxygen. 
This  law,  that  the  quantity  of  a  gas  dissolved  by  a  liquid 
varies  directly  as  the  pressure  of  that  gas  on  the  surface  of 
the  liquid  is  known  as  Dalton's  law. 

2.  The  amount  of  a  gas  dissolved  by  a  liquid  depends,  not 
on  the  total  pressure  exerted  by  all  the  gases  pressing  on  its 
surface,  but  on  the  fraction  of  the  total  pressure  which  is 
exerted  by  the  particular  gas  in  question.  For  example,  the 
average  atmospheric  pressure  is  equal  to  that  of  a  column 
of  mercury  760  mm.  (30  inches)  high.  But  100  volumes  of 
air  contain  approximately  80  volumes  of  nitrogen  and  20  of 
oxygen :  therefore  \  of  the  total  pressure  is  due  to  oxygen 
and  |  to  nitrogen  :  and  the  amount  of  oxygen  absorbed  by 
water  is  just  the  same  as  if  all  the  nitrogen  were  removed 
from  the  air  and  its  total  pressure  therefore  reduced  to  \  of 
760  mm.  (30  inches)  of  mercury ;  that  is,  to  152  mm.  (6  inches) 
of  mercury  pressure.  It  is  only  the  fraction  of  the  total 
pressure  exerted  by  the  oxygen  itself  which  affects  the 
quantity  absorbed  by  water  at  any  given  temperature.  So, 
too,  of  all  the  atmospheric  pressure  -f  is  clue  to  nitrogen,  and 
all  the  oxygen  might  be  removed  from  the  air  without  affect- 
ing the  quantity  of  nitrogen  which  would  be  absorbed  from 
it  by  a  given  volume  of  water.  The  atmospheric  pressure 
would  then  be  f  of  760  mm.  of  mercury,  or  608  mm.  (24 
inches),  but  it  would  all  be  due  to  nitrogen  gas — and  be 
exactly  equal  to  the  fraction  of  the  total  pressure  due  to  that 
gas  before  the  oxygen  was  removed  from  the  air.  When 
several  gases  are  mixed  together  the  fraction  of  the  total  pres- 
sure exerted  by  each  one  is  known  as  the  partial  pressure  of 
that  gas;  and  it  is  this  partial  pressure  which  determines  the 
amount  of  each  individual  gas  dissolved  by  a  liquid.  If  a 
liquid  exposed  to  the  air  for  some  time  had  taken  up  all  the 
oxygen  and  nitrogen  it  could  at  the  partial  pressures  of 
those  gases  in  the  air,  and  were  then  put  in  an  atmosphere 
in  which  the  oxygen  had  all  been  replaced  by  nitrogen,  it 


THE  CHEMISTRY  OF  RESPIRATION.  407 

would  uow  give  off  all  its  oxygen,  since,  although  the  total 
gaseous  pressure  on  it  was  the  same,  no  part  of  it  was  any 
longer  due  to  oxygeu;  and  at  the  same  time  it  would  take 
up  one  fifth  more  nitrogen,  since  the  whole  gaseous  pressure 
on  its  surface  was  now  due  to  that  gas,  while  before  only  four 
fifths  of  the  total  was  exerted  by  it.  If,  on  the  contrary,  the 
liquid  were  exposed  to  pure  hydrogen  under  a  pressure  of  one 
atmosphere  it  would  give  off  all  its  previously  dissolved  oxygen 
and  nitrogen,  since  none  of  the  pressure  on  its  surface  would 
now  be  due  to  those  gases;  and  would  take  up  as  much 
hydrogen  as  corresponded  to  a  pressure  of  that  gas  equal  to 
760  mm.  of  mercury  (30  inches). 

3.  A  liquid  may  be  such  as  to  combine  chemically  with  a 
gas.  Then  the  amount  of  the  gas  absorbed  is  independent 
of  the  partial  pressure  of  the  gas  on  the  surface  of  the  liquid. 
The  quantity  absorbed  will  depend  upon  how  much  the 
liquid  can  combine  with.  Or,  a  liquid  may  partly  be  com- 
posed of  things  which  simply  dissolve  a  gas  and  partly  of 
things  which  chemically  combine  with  it.  Then  the  amount 
of  the  gas  taken  up  under  a  given  partial  pressure  will  de- 
pend on  two  things;  a  certain  portion,  that  merely  dissolved, 
will  vary  with  the  pressure  of  the  gas  in  question;  but  an- 
other portion,  that  chemically  combined,  will  remain  the 
same  under  different  pressures.  The  amount  of  this  second 
portion  depends  only  on  the  amount  of  the  substance  in  the 
liquid  which  can  chemically  combine  with  it,  and  is  totally 
independent  of  the  partial  pressure  of  the  gas. 

4.  Bodies  are  known  which  chemically  combine  with 
certain  gases  when  the  partial  pressure  of  these  is  consider- 
able, forming  compounds  which  break  up,  or  dissociate, 
liberating  the  gas,  when  its  partial  pressure  falls  below  a 
certain  limit.  Oxygen  forms  such  a  compound  with  haemo- 
globin. 

5.  A  membrane,  moistened  by  a  liquid  in  which  a  gas  is 
soluble,  does  not  essentially  alter  the  laws  of  absorption,  by 
a  liquid  on  one  side  of  it  of  a  gas  present  on  its  other  side, 
whether  the  absorption  be  due  to  mere  solution  or  to  chem- 
ical combinations  or  to  both. 

The  Absorption  of  Oxygen  by  the  Blood.  Applying 
the  physical  and  chemical  facts  stated  in  the  preceding 
paragraph  to  the  blood,  we  find  that  the  blood  contains  (1) 
plasma,  which  simply  dissolves  oxygen,  and  (2)  hwmoglobin, 


408  THE  HUMAN  BODY. 

.which  combines  with  it  under  some  partial  pressures  of  that 

.  i.ui  gives  it  up  under  lower. 

Blood  plasma  or,  what  comes  to  the  same  thing,  fresh 
serum,  exposed  to  the  air,  takes  up  no  more  oxygen  than  so 
much  water:  about  0.56  volumes  of  the  gas  for  every  100  of 
the  liquid,  at  a  temperature  of  20°  C.  At  the  temperature 
of  the  Body  the  volume  absorbed  would  he  still  less.  This 
quantity  obeys  Dalton's  law. 

If  fresh  whipped  blood  be  employed,  the  quantity  of  oxy- 
gen taken  up  is  much  greater;  this  extra  quantity  must  be 
taken  up  by  the  red  corpuscles  (in  possessing  which  whipped 
blood  alone  differs  from  blood  serum)  and  it  does  not  obey 
Dalton's  law.  If  the  partial  pressure  of  oxygen  on  the  sur- 
face of  the  whipped  blood  be  doubled,  only  as  much  more 
oxygen  will  be  taken  up  as  corresponds  to  that  dissolved  in 
the  serum;  and  if  the  partial  pressure  of  oxygen  on  its  sur- 
face be  reduced  to  one  half,  only  a  very  small  amount  of 
oxygen  (one  half  of  that  dissolved  by  the  serum)  will  be  given 
off.  All  the  much  larger  quantity  taken  up  by  the  red  corpus- 
cles will  be  unaffected  and  must  therefore  be  chemically  com- 
bined writh  something  in  them.  Since  90  per  cent  of  their 
dry  weight  is  haemoglobin,  and  this  body  when  prepared 
pure  is  found  capable  of  combining  with  oxygen,  there  is 
no  doubt  that  it  is  the  haemoglobin  in  the  circulating  blood 
which  carries  around  most  of  its  oxygen.  The  red  corpuscles 
are  so  many  little  packages  in  which  oxygen  is  stowed  away. 

The  compound  formed  between  oxygen  and  haemoglobin 
is,  however,  a  very  feeble  one;  the  two  easily  separate,  and 
always  do  so  when  the  oxygen  pressure  in  the  liquid  or  gas 
to  which  the  oxyhaemoglobin  is  exposed  falls  below  25  mil- 
limeters of  mercury.  Hence,  in  an  air-pump,  the  blood  only 
gives  off  some  of  its  small  portion  of  merely  dissolved  oxygen, 
until  the  pressure  falls  to  about  \  of  an  atmosphere,  that  is 
to  Jf^  =  125  mm.  (5  inches)  of  mercury,  of  which  total 
pressure  one  fifth  (25  millimeters  or  1  inch)  is  due  to  the 
oxygen  present.  As  soon  as  this  limit  is  passed  the  haemo- 
globin gives  up  its  oxygen  with  a  rush. 

Consequences  of  the  Peculiar  Way  in  which  the  Oxy- 
gen of  the  Blood  is  Held.  The  first,  and  most  important, 
is  that  the  blood  can  take  up  far  more  oxygen  in  the  lungs 
than  would  otherwise  be  possible.  Since  blood  serum  ex- 
posed to  pure  oxygen  takes  up  only  3  volumes  for  100,  blood 


THE  CHEMISTRY  OF  RESPIRATION.  409 

exposed  to  the  air  would  take  up  one  fifth  only  of  that  amount 
at  ordinary  temperatures,  and  still  less  at  the  temperature  of 
the  Body,  were  it  not  for  its  haemoglobin.  In  the  lungs  even 
less  would  be  taken  up,  since  the  air  in  the  air-cells  of  those 
organs  is  poorer  in  oxygen  than  the  external  air;  and  conse- 
quently the  partial  pressure  of  that  gas  in  it  is  lower.  The 
tidal  air  taken  in  at  each  breath  serves  merely  to  renew 
directly  the  air  in  the  big  bronchi ;  the  deeper  we  examine 
the  pulmonary  air  the  less  oxygen  and  more  carbon  dioxide 
Avould  be  found;  in  the  layers  farthest  from  the  exterior  and 
only  renewed  by  diffusion  with  the  air  of  the  large  bronchi, 
it  is  estimated  that  the  oxygen  only  exists  in  such  quantity 
that  its  partial  pressure  is  equal  to  130  millimeters  of  mer- 
cury, instead  of  152  as  in'  ordinary  air.  In  the  second  place, 
on  account  of  the  way  in  which  haemoglobin  combines  with 
oxygen,  the  quantity  of  that  gas  taken  up  by  the  blood  is 
independent  of  such  variations  of  its  partial  pressure  in  the 
atmosphere  as  we  are  subjected  to  in  daily  life.  At  the  top 
of  a  high  mountain,  for  example,  the  atmospheric  pressure 
is  greatly  diminished,  but  still  mountaineers  can  breath 
freely  and  get  all  the  oxygen  they  want;  the  distress  felt  for 
a  time  by  persons  unused  to  living  in  high  altitudes  is  due 
mainly  to  circulatory  disturbances  resulting  from  the  low 
atmospheric  pressure.  So  long  as  the  partial  pressure  of  that 
gas  in  the  lung  air-cells  is  above  25  millimeters  of  mercurv, 
the  amount  of  it  taken  up  by  the  blood  depends  on  how  much 
haemoglobin  there  is  in  that  liquid  and  not  on  how  much 
oxygen  there  is  in  the  air.  So,  too,  breathing  pure  oxygen 
under  a  pressure  of  one  atmosphere,  or  air  compressed  to 
one  half  or  a  fourth  its  normal  bulk,  does  not  increase  the 
quantity  of  oxygen  absorbed  by  the  blood,  apart  from  the  small 
extra  quantity  dissolved  by  the  plasma.  The  widespread  state- 
ments as  to  the  exhilaration  caused  by  breathing  pure  oxygen 
are  erroneous,  being  founded  on  experiments  made  with  im- 
pure gas. 

The  General  Oxygen  Interchanges  in  the  Blood.  Sup- 
pose we  have  a  quantity  of  arterial  blood  in  the  aorta.  This, 
fresh  from  the  lungs,  will  have  its  haemoglobin  almost  fully 
combined  with  oxygen  and  in  the  state  of  oxyhsemoglobin. 
In  the  blood  plasma  some  more  oxygen  will  be  dissolved,  viz., 
eo  much  as  answers  to  a  pressure  of  that  gas  equal  to  130 
mm.  (5.2  inches)  of  mercury,  which  is  the  partial  pressure  of 


410  THE  HUMAN  BODY. 

oxygen  in  the  pulmonary  air-cells.  This  tension  of  the  gas 
in  the  plasma  will  be  more  than  sufficient  to  keep  the  haemo- 
globin from  giving  off  its  oxygen.  Suppose  the  blood  now 
enters  the  capillaries  of  a  muscle.  In  the  liquid  moistening 
this  organ  the  oxygen  tension  is  almost  nil,  since  the  tissue 
elements  are  steadily  taking  the  gas  up  from  the  lymph 
around  them.  Consequently,  through  the  capillary  walls, 
the  plasma  will  give  off  oxygen  until  the  tension  of  that  gas 
in  it  falls  below  25  millimeters  of  mercury.  Immediately 
some  of  the  oxyhemoglobin  is  decomposed,  and  the  oxygen 
liberated  is  dissolved  in  the  plasma,  and  from  there  next 
passed  on  to  the  lymph  outside;  and  so  the  tension  in  the 
plasma  is  once  more  lowered  and  more  oxyhemoglobin  decom- 
posed. This  goes  on  so  long  as  the  blood  is  in  the  capillaries 
of  the  muscle,  or  at  any  rate  so  long  as  the  muscular  fibres 
keep  on  taking  oxygen  from  the  lymph  bathing  them;  if 
they  cease  to  do  so  of  course  the  tension  of  that  gas  in  the 
lymph  will  soon  come  to  equal  that  in  the  plasma:  the  latter 
will  therefore  cease  to  yield  oxygen  to  the  former;  and  so 
maintain  its  tension  (by  the  oxygen  received  from  the  last 
decomposed  oxyhemoglobin)  at  a  point  which  will  prevent 
the  liberation  of  any  more  oxygen  from  such  red  corpuscles 
as  have  not  yet  given  all  of  theirs  up.  The  blood  will  now  go 
on  as  ordinary  venous  blood  into  the  veins  of  the  muscle 
and  so  back  to  the  lungs.  It  will  consist  of  (1)  plasma  with 
oxygen  dissolved  in  it  at  a  tension  of  about  25  millimeters 
(1  inch)  of  mercury.  (2)  A  number  of  red  corpuscles  con- 
taining reduced  hemoglobin.  (3)  A  number  of  red  corpus- 
cles containing  oxyhemoglobin.  Or  perhaps  all  of  the  red 
corpuscles  will  contain  some  reduced  and  some  oxidized 
hemoglobin.  The  relative  proportion  of  reduced  and  un- 
reduced hemoglobin  will  depend  on  how  active  the  muscle 
had  been ;  if  it  worked  while  the  blood  flowed  through  it,  it  will 
have  used  up  more  oxygen,  and  the  blood  leaving  it  will  con- 
sequently be  more  venous,  than  if  it  rested.  This  venous 
blood,  returning  to  the  heart,  is  sent  on  to  the  pulmonary 
capillaries.  Here,  the  partial  pressure  of  oxygen  in  the  air- 
cells  being  130  mm.  (5.2  inches)  and  that  in  the  blood 
plasma  much  less,  oxygen  will  be  taken  up  by  the  latter,  and 
the  tension  of  that  gas  in  the  plasma  tend  to  be  raised  above 
the  limit  at  which  hemoglobin  combines  with  it.  Hence,  as 
fast  as  the  plasma  gets  oxygen  those  red  corpuscles  which 


THE  CHEMISTRY  OF  RESPIRATION.  411 

contain  any  reduced  haemoglobin  rob  it,  and  so  its  oxygen 
tension  is  kept  down  below  that  in  the  air-cells  until  all  the 
haemoglobin  is  satisfied.  Then  the  oxygen  tension  of  the 
plasma  rises  to  that  of  the  gas  in  the  air-cells;  no  more 
oxygen  is  absorbed,  and  the  blood  returns  to  the  left  auricle 
of  tbe  heart  in  the  same  condition,  so  far  as  oxygen  is  con- 
cerned, as  when  we  commenced  to  follow  it. 

The  Carbon  Dioxide  of  the  Blood.  The  same  general 
laws  apply  to  this  as  to  the  blood  oxygen.  The  gas  is  partly 
merely  dissolved  and  partly  in  a  loose  chemical  combination 
much  like  that  of  oxygen  with  hsemoglobin,  but  the  body 
with  which  it  combines  probably  exists  in  the  plasma  more 
than  in  the  red  corpuscles;  what  it  may  be  is  not  certainly 
known.  Besides  this,  some  more  carbon  dioxide  is  stably 
combined  and  is  only  given  off  on  the  addition  of  a  stronger 
acid.  The  partial  pressure  of  carbon  dioxide  in  the  pulmo- 
nary air-cells  is  about  40  mm.  (1.6  inches)  of  mercury.  There- 
fore the  tension  of  that  gas  in  the  pulmonary  capillaries 
must  be  more  than  this.  On  the  other  hand  its  tension  in 
arterial  blood  must  be  less  than  that  in  the  lymph  around 
the  tissues;  otherwise  it  could  not  enter  the  blood  in  the 
systemic  circulation,  which  it  does,  as  proved  by  the  fact 
that  100  vols,  of  venous  blood  give  off  46  of  this  gas,  and  100 
vols,  of  arterial  only  40. 

The  nitrogen  dissolved  in  the  blood  is,  so  far  as  we  know, 
quite  unimportant. 

Internal  Respiration.  As  to  the  amount  of  oxygen  used 
by  each  tissue  and  the  quantity  of  carbon  dioxide  produced 
by  it  we  know  but  little;  the  following  points  seem,  however, 
tolerably  certain: 

1.  The  amount  of  carbon  dioxide  produced  in  an  organ 
in  a  given  time  bears  no  constant  ratio  to  the  amount  of 
oxygen  taken  up  by  it  simultaneously.  This  is  certainly 
true  of  muscle,  for  experiment  shows  that  muscular  work 
if  really  severe  leads  to  an  elimination  of  carbon  dioxide 
containing  more  oxygen  than  the  total  oxygen  taken  up  from 
tli''  lunge  at  the  same  time.  The  balance  is  of  course  made 
up  in  subsequent  periods  of  rest,  when  more  free  oxygen  is 
taken  up  than  is  eliminated  in  combination  during  the  same 
time.  Moreover,  a  frog's  muscle  excised  from  the  body  and 
put,  in  an  atmosphere  containing  no  oxygen  and  made  there 
to  contract,  will   evolve   with   each   contraction  considerable 


412  THE  HUMAN  BODY. 

quantities  of  carbon  dioxide — although  from  the  conditions 
of  the  experiment  it  can  receive  from  outside  no  uncombined 
oxygen,  and  other  experiments  show  that  it  contains  none. 
Hence  the  living  muscular  fibre  must  contain  a  substance 
which  is  decomposed  during  activity  and  yields  carbon 
dioxide  as  one  product  of  decomposition;  and  this  quite  in- 
dependent of  any  simultaneous  direct  oxidation. 

2.  What  is  true  of  muscle  is  probably  true  of  most  of  the 
tissues.  During  rest  they  take  up  oxygen  and  fix  it  in  the 
form  of  complex  compounds,  bodies  which,  like  nitroglycer- 
ine, are  readily  decomposed  into  simpler,  and  in  such  decom- 
positions liberate  energy  which  is  used  by  the  working  tissue. 
One  product  of  the  decomposition  is  the  highly  oxidized 
carbon  dioxide,  and  this  is  eliminated;  other  products  are 
less  oxidized,  and  possibly  are  not  eliminated  but  built  up 
again,  with  fresh  oxygen  taken  from  the  blood  and  fresh 
carbon  from  the  food,  into  the  decomposable  substance. 

3.  During  the  day  a  man  gives  off  from  his  lungs  more 
oxygen  in  carbon  dioxide,  than  he  takes  up  by  the  same 
organs  from  the  air.  During  the  night  the  reverse  is  the 
case.  This,  however,  has  nothing  to  do  with  the  alternating 
periods  of  light  and  darkness,  as  it  has  in  the  case  of  a  green 
plant,  which  in  the  light  evolves  more  oxygen  than  it  con- 
sumes, and  in  the  dark  the  contrary.  It  depends,  rather,  on 
the  fact  that  during  the  day  more  muscular  effort  is  exerted 
than  at  night,  and  the  meals  are  then  taken  and  digested. 
The  activity  of  the  muscles  and  the  digestive  glands  is  de- 
pendent on  processes  which  give  rise  to  a  large  production  of 
carbon  dioxide  and,  during  the  night,  when  both  are  at  rest, 
more  oxygen  is  taken  up  than  is  contained  in  the  carbon 
dioxide  eliminated.  If  a  man  works  and  takes  his  meals  at 
night,  and  sleeps  in  the  day,  the  usual  ratios  of  his  gaseous 
exchanges  with  the  exterior  are  entirely  reversed. 

4.  The  amount  of  work  that  a  man's  organs  do,  is  not 
dependent  on  the  amount  of  oxygen  supplied  to  them,  but 
the  amount  of  oxygeu  used  by  him  depends  on  how  much  he 
uses  his  organs.  The  quantity  of  oxygen  supplied  must  of 
course  always  be,  at  least,  that  required  to  prevent  suffoca- 
tion; but  an  excess  above  this  limit  will  not  make  the  tissues 
work.  Just  as  a  man  must  have  a  certain  amount  of  food 
to  keep  him  alive,  so  he  must  have  a  certain  amount  of 
oxv^en;  but  as  extra  food  will  not  make  his  tissues  or  him 


THE  CHEMISTRY  OF  RESPIRATION.  413 

(who  is  physiologically  the  sum  of  all  his  tissues)  work, 
apart  from  some  stimulus  to  exertion,  so  it  is  with  oxygen. 
Highly  arterialized  blood,  or  an  abnormal  amount  of  blood, 
flowing  through  an  organ  will  not  arouse  it  to  activity;  the 
working  organ,  muscle,  or  gland,  for  example,  usually  gets 
enough  more  blood  to  supply  its  extra  needs — just  as  a  healthy 
man  who  works  will  have  a  better  appetite  than  an  idle  one; 
but  as  taking  more  food  by  an  idle  man  will  not  of  itself 
make  him  more  energetic,  so  neither  will  sending  more  arterial 
blood  through  an  organ  excite  it  to  activity. 

5.  The  preceding  statement  is  confirmed  by  experiments 
which  show  that  an  animal  uses  no  more  oxygen  in  an  hour 
when  made  to  breathe  that  gas  in  a  pure  state,  than  when 
allowed  to  breathe  ordinary  air.  In  other  words,  the  amount 
of  oxygen  an  animal  uses  (provided  it  gets  the  minimum 
necessary  for  health)  is  dependent  only  on  how  much  it  usea 
its  tissues.  These  (the  rest  in  most  cases  subject  to  a  certain 
amount  of  control  from  the  nervous)  determine  their  own 
activity,  and  this,  in  turn,  how  much  oxygen  shall  be  used  in 
the  systemic  circulation  and  restored  in  the  pulmonary.  In 
other  words,  the  physiological  work  of  an  animal,  which  of 
course  is  largely  dependent  upon  how  external  forces  act  upon 
it,  determines  how  much  oxygen  it  uses  daily;  and  not  the 
supply  of  oxygen  how  much  its  tissue  activity  shall  be,  unless 
the  supply  sinks  below  the  starvation  limit. 


CHAPTER   XXVII. 

THE    NERVOUS    FACTORS    OF    THE    RESPIRATORY 
MECHANISM.     ASPHYXIA. 

The  Respiratory  Centre.  The  respiratory  movements 
are  to  a  certain  extent  under  the  control  of  the  will;  we  can 
breathe  faster  or  slower,  shallower  or  more  deeply,  as  we 
wish,  and  can  also  "  hold  the  breath  "  for  some  time — but  the 
voluntary  control  thus  exerted  is  limited  in  extent;  no  one 
can  commit  suicide  by  holding  his  breath.  In  ordinary  quiet 
breathing  the  movements  are  quite  involuntary;  they  go  on 
perfectly  without  the  least  attention  on  our  part,  and,  not 
only  in  sleep,  but  during  the  unconsciousness  of  fainting  or 
of  an  apoplectic  fit.  The  natural  breathing  movements  are 
therefore  either  reflex  or  automatic. 

The  muscles  concerned  in  producing  the  changes  in  the 
chest  which  lead  to  the  entry  or  exit  of  air  aro  of  the  ordinary 
striped  kind;  and  these,  as  we  have  seen,  only  contract  in  the_ 
Body  under  the  influence  of  the  nerves  going  to  them;JJhe~ 
nerves  of  the  diaphragm  are  the  two  phrenic  nerves,  one  for 
each  side  of  it;  the  external  intercostal  muscles  are  supplied 
by  certain  brunches  of  the  thoracic  spinal  nerves,  called  the 
intercostal  nerves.  If  the  phrenic  nerves  be  cut  the  diaphragm 
ceases  its  contractions,  and  a  similar  paralysis  of  the  external 
intercostals  follows  section  of  the  intercostal  nerves. 

Since  the  inspiratory  muscles  only  act  when  stimulated 
by  nervous  impulses  reaching  them,  we  have  next  to  seek 
where  these  impulses  originate;  and  experiment  shows  that 
it  is  in  the  medulla  oblongata.  All  the  brain  of  a  cat  or  a 
rabbit  in  front  of  the  medulla  can  be  removed,  and  it  will 
still  go  on  breathing;  and  children  are  sometimes  born  with 
the  medulla  oblongata  only,  the  rest  of  the  brain  being  un- 
developed, and  yet  they  breathe  for  a  time.  If,  on  the 
other  hand,  the  spinal  cord  be  divided  immediately  below 
the  medulla  of  an  animal,  all  breathing  movements  of  the 

414 


THE  RESPIRATORY  MECHANISM.  415 

chest  cease  at  once.  TVe_conclude,  therefore,  that  the  nerv- 
ous impulses  calling  forth  contractions  of  the  respiratory 
muscles  arise  in  the  medulla  oblongata,  and  travel  down  the 
spinal  cord  and  thence  out  along  the  phrenic  and  intercostal 
nerves.  This  is  confirmed  by  the  fact  that  if  the  spinal  cord 
be  cut  across  below  the  origin  of  the  fourth  pair  of  cervical 
spinal  nerves  (from  which  the  phrenics  mainly  arise)  but 
above  the  first  thoracic  spinal  nerves,  the  respiratory  move- 
ments of  the  diaphragm  continue,  but  those  of  the  intercostal 
muscles  cease;  this  phenomenon  has  sometimes  been  observed 
on  men  so  stabbed  in  the  back  as  to  divide  the  spinal  cord  in 
the  region  indicated.  Finally,  that  the  nervous  impulses  ex- 
citing the  inspiratory  muscles  originate  in  the  medulla,  is 
proved  by  the  fact  that  if  a  small  portion  of  that  organ,  the 
so-called  vital  point,  be  destroyed,  all  the  respiratory  move- 
ments cease  at  once  and  forever,  although  all  the  rest  of  the 
brain  and  spinal  cord  may  be  left  uninjured.  This  part  of 
the  medulla  is  known  as  the  resjnratory  centre.  The  im- 
pulses proceeding  from  it  probably  do  not  pass  directly  to 
the  motor  nerve-fibres  concerned,  but  first  to  subsidiary 
centres  in  the  cord,  from  which  pnyperly  co-ordinated  impulses 
are  sent  to  the  muscles  concerned.  Occasionally  in  young 
animals,  especially  after  a  small  dose  of  strychnia  has  been 
administered,  a  few  respiratory  movements  are  seen  after 
section  of  the  cord  high  up  in  the  neck.  But  the  broad 
general  fact  remains,  that  in  the  normal  working  of  the  Body 
the  spinal  respiratory  centres  only  send  out  respiration-caus- 
ing  impulses  when  excited  by  impulses  descending  to  them 
from  the  main  respiratory  centre  in  the  medulla. 

In  the  above  statements,  attention  has  been  chiefly  con- 
fined to  the  diaphragm  and  the  intercostal  muscles;  but 
what  is  said  of  them  is  true  of  the  respiratory  innervation  of 
all  other  breathing  muscles,  whether  expiratory  or  inspira- 
tory, normal  or  extraordinary. 

Is  the  Respiratory  Centre  Reflex?  Since  this  centre 
goes  on  working  independently  of  the  will,  we  have  next  to 
inquire  is  it  a  reflex  centre  or  not  ?  are  the  efferent  discharges 
it  sends  along  the  respiratory  nerves  due  to  afferent  impulses 
reaching  it  by  centripetal  nerve-fibres?  or  does  it  originate 
efferent  nervous  impulses  independently  of  excitation  through 
afferent  nerves? 

We  know,  in  the  first  place,  that  the  respiratory  centre  is 


416  THE  HUMAN  BODY. 

largely  under  reflex  control;  a  dash  of  cold  water  on  the 
skin,  the  irritation  of  the  nasal  mucous  membrane  by  snuff, 
or  of  the  larynx  by  ;i  foreign  body,  will  each  cause  a  modifi- 
cation in  the  respiratory  movements — a  long  indrawn  breath, 
a  sneeze,  or  a  cough.  But,  although  thus  very  subject  to 
influences  reaching  it  by  afferent  nerves,  the  respiratory 
centre  seems  essentially  independent  of  such.  In  many  ani- 
mals, as  rabbits  (and  in  some  men),  marked  breathing  move- 
ments take  place  in  the  nostrils,  which  dilate  during  inspira- 
tion; and  when  the  spinal  cord  of  a  rabbit  is  cut  close  to  the 
medulla,  thus  cutting  off  all  afferent  nervous  impulses  to  the 
respiratory  centre  except  such  as  may  reach  it  through  cranial 
nerves,  the  respiratory  movements  of  the  nostrils  still  con- 
tinue until  death.  The  movements  of  the  ribs  and  dia- 
phragm of  course  cease,  and  so  the  animal  dies  very  soon 
unless  artificial  respiration  be  maintained.  Moreover,  if 
after  cutting  the  spinal  cord  as  above  described,  the  chief 
sensory  cranial  nerves  be  divided,  so  as  to  cut  off  the  respira- 
tory centre  from  almost  all  possible  afferent  nervous  im- 
pulses, the  regular  breathing  movements  of  the  nostrils  con- 
tinue. It  is,  therefore,  nearly  certain  that  the  activity  of  the 
respiratory  centre,  however  much  it  may  be  capable  of  modi- 
fication through  sensory  nerves,  is  essentially  independent  of 
them;  in  other  words  the  normal  respiratory  movements  are 
not  reflex. 

What  it  is  that  Excites  the  Respiratory  Centre.  The 
thing  that,  above  all  others,  influences  the  respiratory  centre 
is  the  greater  or  less  venosity  of  the  blood  flowing  through  it. 
If  this  blood  be  very  rich  in  oxygen  and  comparatively  poor 
in  carbon  dioxide  the  respiratory  centre  acts  but  feebly,  and 
the  respirations  are  shallow.  If,  on  the  other  hand,  this 
blood  be  highly  venous  the  respiratory  movements  are  more 
rapid  than  normal,  and  are  forced,  the  extraordinary  muscles 
of  respiration  being  called  into  play;  this  state  of  violent 
labored  respiration,  due  to  deficient  aeration  of  the  blood  is 
called  dyspnoea.  Normal  quiet  breathing  is  eupneea.  If 
active  artificial  respiration  be  kept  up  on  an  animal  for  a 
short  time,  it  is  found,  on  its  cessation,  that  the  creature 
(dog  or  rabbit)  makes  no  attempt  to  breathe  for  a  period 
which  may  extend  to  oiie  and  a  half  minutes.  This  breath- 
less condition,  in  which  an  animal  with  no  hindrance  opposed 
to  its  breathing  makes  no  respiratory  movement,  is  apnwa. 


THE  RESPIRATORY  MECHANISM.  417 

Apnoea  used  to  be  ascribed  solely  to  an  overloading  of  the 
blood  with  oxygen,  but  the  haemoglobin  of  the  blood  leaving 
the  lungs  is  normally  so  nearly  saturated  with  that  gas  that 
this  explanation  is  not  sufficient.  The  apnoeic  state  is  in 
part  due  no  doubt  to  the  high  j>ercentage  of  oxygen  in  the 
air-cells  of  the  lungs,  brought  about  by  the  active  artificial 
ventilation.  The  blood,  as  it  flows  through  the  lungs,  is  thus 
able  to  supply  itself  with  oxygen  for  some  time  without  any 
renewal  of  the  air  within  them.  But  even  this  is  not  the 
whole  matter,  for  an  animal  made  apnoeic  will  often  continue 
so  after  its  arterial  blood  has  become  distinctly  venous  in 
color;  and  an  animal  may,  if  its  pneumogastric  nerves  be 
intact,  be  rendered  apnoeic  f  or  almTirt-irme  by  rapid  insuffla- 
tion of  its  lungs  with  an  indifferent  gas.  In  fact,  there  is 
evidence  that  distention  of  the  lungs  tends  to  inhibit  the 
sending  out  of  impulses  to  the  inspiratory  muscles,  the 
afferent  fibres  exerting  this  inhibitory  action  on  the  centre 
taking  their  course  in  the  pulmonary  branches  of  the  pneu- 
mogastric; and  this  inhibition  plays  a  part  in  the  production 
of  apneea.  It  should  be  noted  that  by  apncea  physicians 
usually  mean  only  extreme  dyspnoea. 

How  venous  blood  causes  great  excitation  of  the  respira- 
tory centre  is  not  certainly  known.  We  may  make  the 
following  provisional  hypothesis:  the  chemical  changes 
occurring  in  the  respiratory  centre  produce  a  substance 
which  stimulates  its  nerve-cells;  when  the  blood  is  richly 
oxygenated  this  substance  is  oxidized  as  fast  as  it  is  formed, 
and  the  centre  is  7iot  excited;  but  when  the  blood  is  poor  in 
oxygen,  the  stimulating  body  accumulates  and  the  respiratory 
discharges  become  powerful.  Under  normal  circumstances 
the  oxygen  is  not  kept  up  to  the  point  of  entirely  removing 
this  exciting  substance,  and  the  centre  is  stimulated  so  as  to 
produce  the  natural  breathing  movements.  That  the  stiniu- 
lant  acts  upon  the  respiratory  centre  itself,  and  not  upon 
other  organs  of  the  Body  and  through  their  sensory  nerves 
upon  the  medulla,  is  proved  by  experiments  which  show  that 
the  circulation  of  venous  blood  through  the  trunk  and  limbs 
of  an  animal,  while  its  respiratory  centre  is  supplied  with 
arterial  blood,  does  not  produce  dyspnoea. 

Why  are  the  Respiratory  Discharges  Rhythmic  ?  Every 
complete  respiratory  net  consists  of  an  inspiration,  an  expira- 
tion and   a  pause;  ami   then   follows  the  inspiration  of  the 


418  TEE  HUMAN  BODY. 

next  act.  In  natural  quiet  breathing  there  is  no  essential 
difference  between  the  expiration  and  the  pause.  The  in- 
spiration is  the  only  active  part;  the  expiration  and  the 
pause  are  dependent  on  muscular  inactivity  and,  there- 
fore, on  the  cessation  of  the  discharge  of  nervous  impulses 
from  the  respiratory  centre.  But  then,  we  may  ask,  if  in 
accordance  with  the  hypothesis  made  in  the  last  paragraph. 
the  respiratory  centre  is  constantly  being  excited,  why  is  it 
not  always  discharging  ?  why  does  it  only  send  out  nervous 
impulses  at  intervals?  This  epiestion,  which  is  essentially 
the  same  as  that  why  the  heart  beats  rhythmically,  belongs 
to  the  higher  regions  of  Physiology  and  can  only  at  jn-esent 
be  hypothetically  answered.  Let  us  consider,  for  a  moment, 
ordinary  mechanical  circumstances  under  which  a  steady 
supply  is  turned  into  an  intermittent  discharge.  Suppose  a 
tube  closed  water-tight  below  by  a  hinged  bottom,  which  is 
kept  shut  by  a  spring.  If  a  steady  stream  of  water  is  poured 
into  the  tube  from  above,  the  water  will  rise  until  its  weight 
is  able  to  overcome  the  pressure  of  the  spring,  and  the  bottom 
will  then  be  forced  down  and  some  water  flow  out.  The 
spring  will  then  press  the  bottom  up  again,  and  the  water 
accumulate  until  its  weight  again  forces  open  the  bottom  of 
the  tube,  and  there  is  another  outrush;  and  so  on.  By 
opposing  a  certain  resistance  to  the  exit  we  could  thus  turn 
a  steady  inflow  into  a  rhythmic  outflow.  Or,  take  the  case 
of  a  tube  with  one  end  immersed  in  water  and  a  steady 
stream  of  air  blown  into  its  other  end.  The  air  will  emerge 
from  the  immersed  end,  not  in  a  steady  current,  but  in  a 
series  of  bubbles.  Its  pressure  in  the  tube  must  rise  until 
it  is  able  to  overcome  the  cohesive  force  of  the  water,  and 
then  a  bubble  bursts  forth;  after  this  the  air  has  again  to 
get  up  the  requisite  pressure  in  the  tube  before  another 
bubble  is  ejected;  and  so  the  continuous  supply  is  trans- 
formed into  an  intermittent  delivery.  Pl^siologists  sup- 
pose something  of  the  same  kind  to  occur  in  the  respiratory 
centre.  Its  nerve-cells  are  always,  under  usual  circum- 
tances,  being  excited;  but,  to  discharge  a  nervous  impulse 
along  the  efferent  respiratory  nerves,  they  have  to  overcome 
a  certain  resistance.  The  nervous  impulses  have  to  accumu- 
late, or  "  gain  a  head,"  before  they  travel  out  from  the 
centre,  and,  after  their  discharge,  time  is  required  to  attain 
once  more  the  necessary  level  of  irruption  before  a  fresh  in- 


THE  RESPIRATORY  MECHANISM.  419 

nervation  is  sent  to  the  muscles.  This  method  of  account- 
ing for  the  respiratory  rhythm  is  known  as  the  "  resistance 
theory."  If  not  altogether  satisfactory  it  is  at  least  far 
preferable  to  the  older  mode  of  considering  the  question 
solved  by  assuming  a  rhythmic  character  or  property  of  the 
respiratory  centre.  It  gives  a  definite  hypothesis,  which 
accords  with  what  is  known  of  general  natural  laws  outside 
of  the  Body,  and  the  validity  of  which  can  be  subjected  to 
experiment:  and  so  serves  very  well  to  show  how  scientific 
differs  from  pre-scientific,  or  mediaeval,  physiology.  The 
latter  was  content  with  observing  things  in  the  Body  and 
considered  it  explained  a  phenomenon  when  it  gave  it  a 
name.  Now  we  call  a  phenomenon  explained,  when  we  have 
found  to  what  general  category  of  natural  laws  it  can  be 
reduced  as  a  special  example;  and  this  reducing  a  special 
case  to  a  particular  manifestation  of  some  one  or  more 
general  properties  of  matter  already  known  is,  of  course,  all 
that  we  ever  mean  when  we  say  we  explain  anything.  We 
explain  the  fall  of  an  apple  and  the  rise  of  the  tides  by 
referring  them  to  the  class  of  general  results  of  the  law  of 
gravitation ;  but  the  why  of  the  law  of  gravitation  we  do  not 
know  at  all;  it  is  merely  a  fact  which  we  have  found  out. 
So  with  regard  to  Physiology;  we  are  working  scientifically 
when  we  try  to  reduce  the  activities  of  the  living  Body  to 
special  instances  of  mechanical,  physical,  or  chemical  laws 
otherwise  known  to  us,  and  unscientifically  when  we  lose 
sight  of  that  aim.  Certain  vital  phenomena,  as  those  of 
blood-pressure,  we  can  thus  explain,  as  much  as  we  can  ex- 
plain anything;  others,  as  the  rhythm  of  the  respiratory 
movements,  we  can  provisionally  explain,  although  not  yet 
certain  that  our  explanation  is  the  right  one;  and  still 
others,  as  the  phenomena  of  consciousness,  we  cannot  explain 
at  all,  and  possibly  never  shall,  by  referring  them  to  general 
properties  of  matter,  since  they  may  be  associated  only  with 
that  particular  kind  of  matter  called  protoplasm,  and  per- 
haps only  with  some  varieties  of  it. 

The  Relation  of  the  Pneumogastrie  Nerves  to  the  Re- 
spiratory Centre.  We  have  next  to  consider  if  any  phenom- 
ena presented  by  the  living  Body  give  support  to  the  resist- 
ance theory  of  the  respiratory  rhythm.  A  very  important 
collateral  prop  to  it  is  given  by  the  relation  of  the  pneumo- 
gastrie nerves  to  the  rate  and  force  of  the  respiratory  move- 


420  THE  HUMAN  BODY. 

merits.  These  nerves  give  branches  to  the  larynx,  the  wind- 
pipe] and  the  lungs,  and  might  therefore  be  suspected  to 
have  something  to  do  with  breathing.  Indeed  at  one  time  it 
was  maintained  that  the  breathing  movements  were  purely 
reflex,  the  afferent  fibres  running  in  the  pneumogastrics 
from  the  lungs  to  the  respiratory  centre.  That  the  vagi  are 
not  concerned  in  influencing  the  respiratory  muscles  directly 
is  shown  by  the  fact  that  all  of  these  muscles  (except  certain 
small  ones  in  the  larynx)  contract  as  usual  in  breathing  after 
both  pneumogastric  nerves  have  been  divided.  Still,  the 
section  of  both  nerves  has  a  considerable  influence  on  the 
respiratory  movements;  they  become  slower  and  deeper. 
We  may  understand  this  by  supposing  that  the  resistance  to 
the  discharges  of  the  respiratory  centre  is  liable  to  variation. 
It  may  be  increased,  and  then  the  discharges  will  be  fewer 
and  larger;  or  diminished,  and  then  they  will  be  more  fre- 
quent but  each  one  less  powerful.  If  the  spring,  in  the 
illustration  used  in  the  preceding  paragraph,  be  made  stronger, 
while  the  inflow  of  water  to  the  tube  remains  the  same,  the 
outflows  will  be  less  frequent  but  each  one  greater;  and  vice 
versa.  The  effect  of  section  of  the  pneumogastric  trunk 
may,  therefore,  be  explained  if  we  suppose  that,  normally,  it 
carries  up,  from  its  lung  branches,  nervous  impulses  which 
diminish  the  resistance  to  the  discharges  of  the  respiratory 
centre;  when  the  nerves  are  cut  these  helping  impulses  are 
lost  to  the  centre,  and  its  impulses  must  gather  more  head 
before  they  break  out,  but  will  be  greater  when  they  do. 
This  view  is  confirmed  by  the  fact  that  stimulation  of  the 
central  ends  of  the  divided  pneumogastrics,  if  weak,  brings 
back  the  respirations  to  their  normal  rate  and  force;  if 
stronger  makes  them  more  rapid  and  shallower;  and  when 
stronger  still,  abolishes  the  respiratory  rhythm  altogether, 
with  the  inspiratory  muscles  in  a  steady  state  of  feeble  con- 
traction. That  is  to  say,  the  resistance  to  the  discharges  of 
the  centre  being  entirely  taken  away  (which  is  equivalent  to 
the  total  removal  of  the  spring  in  our  example),  the  centre 
sends  out  uninterrupted  and  non-rhythmic  stimuli  to  the 
inspiratory  muscles. 

The  pneumogastric  nerve  gives  two  branches  to  the 
larynx;  known  respectively  as  the  superior  and  inferior  (re- 
current) laryngeal  nerves;  the  action  of  these  on  the  respira- 
tory centre  is  opposite  to  that  of  the  fibres  from  the  lungs 


THE  RESPIRATORY  MECHANISM.  421 

coming  np  in  the  main  pneumogastric  trunk.  If  the  supe- 
rior laryngeal  branch  be -divided  and  its  central  end  stimu- 
lated, the  respirations  become  less  frequent  but  each  one 
more  powerful;  hence  this  nerve  appears  to  contain  fibres 
which  increase  the  resistance  to  inspiratory  discharges  from 
the  respiratory  centre.  The  same,  but  to  a  less  degree,  is  true 
of  the  inferior  laryngeal  branch.  Both  are  inhibitory  fibres 
so  far  as  inspiration  is  concerned;  whereas  the  main  vagus 
stem  when  its  central  end  is  electrically  stimulated  is  acceler- 
ator or  augmentoFi— - ■* 

The  Expiratory  Centre.  Hitherto  we  have  considered 
breathing  as  due  to  the  rhythmically  alternating  activity  and 
rest  of  an  inspiratory  centre — and  such  is  the  case  in  normal 
quiet  breathing,  in  which  the  expirations  are  passive.  But 
in  dyspnoea  expiration  is  a  muscular  act,  and  so  there  must 
be  a  section  of  the  respiratory  centre  controlling  the  expira- 
tory muscles,  and  we  may  regard  the  whole  centre  as  consist- 
ing really  of  two;  an  inspiratory  and  expiratory.  The  latter 
part  of  the  respiratory  centre,  however,  is  less  irritable  than 
the  inspiratory  part,  and  hence  when  the  blood  is  in  a  normal 
state  of  aeration  never  gets  stimulated  up  to  the  discharging 
point.  In  dyspnoea  the  stimulus  becomes  sufficient  to  cause 
it  also  to  discharge,  but  only  after  the  more  irritable  inspira- 
tory centre;  hence  the  expiration  follows  the  inspiration. 
This  alternation  of  activity  is,  moreover,  promoted  by  the  fact 
that  the  pneumogastric  nerve-fibres  coming  up  from  the 
lungs  are  of  two  kinds.  The  predominant  sort  are  the 
accelerator  set  already  referred  to,  which  favor  discharge  of 
the  inspiratory  centre,  and  perhaps  also  increase  the  resist- 
ance to  the  expiratory  discharge.  This  set  is  excited  when 
the  lungs  diminish  in  bulk,  as  in  expiration;  and  when  the 
whole  nerve  is  stimulated  electrically  they  usually  get  the 
better  of  the  other  set,  which  carry  up  to  the  medulla  im- 
pulses which  inhibit  inspiratory  discharges.  This  set  is 
stimulated  by  expansion  of  the  lungs,  even  in  quiet  breath- 
ing: and  they  play  a  part  in  producing  the  phenomenon 
of  apn&a.  When  the  distention  of  the  lungs  is  con- 
siderable these  fibres  not  only  check  inspiration  but  favor 
expiratory  movements.  Hence,  every  expansion  of  the  lungs 
(inspiration)  tends  to  promote  an  expiration,  and  every  col- 
lapse of  the  lungs  (expiration)  tends  to  produce  an  inspira- 


422  THE  HUMAN  BODY. 

tion;  and  so,  through  the  pneumogastric  nerves,  the  respira- 
tory mechanism  is  largely  self-regulating. 

Asphyxia.  Asphyxia  is  death  from  suffocation,  or  want 
of  oxygen  by  the  tissues.  Jt  may  be  brought  about  in 
various  ways;  as  by  strangulation,  which  prevents  the  entry 
of  air  into  the  lungs;  or  by  exposure  in  an  atmosphere  con- 
taining no  oxygen;  or  by  putting  an  animal  in  a  vacuum; 
or  by  making  it  breathe  air  containing  a  gas  which  has  a 
stronger  affinity  for  haemoglobin  than  oxygen  has,  and  which, 
therefore,  turns  the  oxygen  out  of  the  red  corpuscles  and 
takes  its  place.  The  gases  which  do  the  latter  are  very  in- 
teresting since  they  serve  to  prove  conclusively  that  the  Body 
can  only  live  by  the  oxygen  carried  round  by  the  haemoglobin 
of  the  red  corpuscles;  that  amount  dissolved  in  the  blood 
plasma  being  insufficient  for  its  needs.  Of  such  gases  carbon 
monoxide  is  the  most  important  and  best  studied ;  in  the  fre- 
quent French  mode  of  committing  suicide  by  stopping  up  all 
the  ventilation  holes  of  a  room  and  burning  charcoal  in  it,  it 
is  poisoning  by  carbon  monoxide  which  causes  death. 

The  Relations  of  Carbon  Monoxide  to  Haemoglobin. 
If  aerated  whipped  blood,  or  a  solution  of  oxyhyaemoglobin, 
be  exposed  to  a  gaseous  mixture  containing  carbon  monoxide, 
the  liquid  will  absorb  the  latter  gas  and  give  off  oxygen. 
The  amount  of  carbon  monoxide  taken  up  will  (apart  from 
a  small  amount  dissolved  in  the  plasma)  be  independent  of 
the  partial  pressure  of  that  gas  in  the  gaseous  mixture  to 
which  the  blood  is  exposed ;  the  quantity  absorbed  depends 
on  the  quantity  of  haemoglobin  in  the  liquid,  and  is  replaced 
by  an  equal  volume  of  oxygen  liberated.  This  equivalence  of 
volume,  of  itself,  proves  that  the  phenomenon  is  due  to  the 
chemical  replacement  of  oxygen  in  some  compound,  by  the 
carbon  monoxide;  for  if  the  carbon  monoxide  were  merely 
dissolved  in  the  liquid  in  proportion  to  its  partial  pressure  on 
the  surface,  it  would  turn  out  no  oxygen;  the  quantity  of 
dissolved  gases  held  by  a  liquid  being  dependent  only  on  the 
partial  pressure  of  each  individual  gas  on  its  surface,  and 
unaffected  by  that  of  all  others.  During  the  taking  up  of 
carbon  monoxide  the  blood  changes  color  in  a  way  that  can 
be  recognized  by  a  practised  eye;  it  becomes  cherry-red  in- 
stead of  scarlet.  This  shows  that  some  new  chemical  com- 
pound has  been  formed  in  it;  examination  with  the  spectro- 
scope confirms  this,  and  shows  the  color  change  to  be  due  to 


THE  RESPIRATORY  MECHANISM.  423 

the  formation  of  carbon-monoxide  haemoglobin  which  has  a 
different  color  from  oxyhemoglobin.  A  dilute  solution  of 
reduced  haemoglobin  absorbs  all  the  rays  of  light  in  one 
region  about  the  green  of  the  solar  spectrum,  and  so  pro- 
duces there  a  dark  band;  a  thin  layer  of  the  blood  of  an 
asphyxiated  animal  does  the  same.  Dilute  solution  of  oxy- 
hemoglobin absorbs  the  rays  in  two  narrow  regions  of  the 
solar  spectrum  at  the  confines  of  the  yellow  and  green,  and 
arterial  blood  does  the  same.  Dilute  solution  of  carbon- 
monoxide  hemoglobin,  or  blood  which  has  been  exposed  to 
this  gas,  also  absorbs  the  light  in  two  narrow  bands  of  the 
..olar  spectrum;  but  these  are  a  little  nearer  the  blue  end  of 
the  spectrum  than  the  absorption  bands  of  oxyhemoglobin. 
Pure  blood  serum  saturated  with  oxygen  gas  or  with  carbon 
monoxide  does  not  specially  absorb  any  part  of  the  spectrum; 
therefore  the  absorptions  when  hemoglobin  is  present  must 
be  due  to  chemical  compounds  of  those  gases  with  that  body. 

Since  carbon-monoxide  hemoglobin  has  a  bright-red  color, 
we  find,  in  the  Bodies  of  persons  poisoned  by  that  gae,  the 
blood  all  through  the  Body  cherry-red;  the  tissues  being 
unable  to  take  carbon  monoxide  from  hemoglobin  in  the 
systemic  circulation.  Hence  the  curious  fact  that,  while 
death  is  really  due  to  asphyxia,  the  blood  is  almost  the  color 
of  arterial  blood,  instead  of  very  dark  purple,  as  in  ordinary 
cases  of  death  by  suffocation.  Experiments  with  animals 
show  that  in  poisoning  by  carbon  monoxide  persistent  ex- 
posure of  the  blood  to  oxygen,  by  means  of  artificial  respira- 
tion, will  cause  the  poisonous  gas  to  be  slowly  replaced  again 
by  oxygen;  hence  if  the  heart  has  not  yet  quite  stopped 
beating,  artificial  respiration,  kept  up  patiently,  should  be 
employed  in  the  case  of  poisoning  by  carbon  monoxide  unless 
transfusion  of  blood  be  possible. 

The  Phenomena  of  Asphyxia.  As  soon  as  the  oxygen 
in  the  blood  falls  below  the  normal  amount  the  breathing 
becomes  hurried  and  deeper,  and  the  extraordinary  muscles 
of  respiration  are  called  into  activity.  The  dyspnoea  be- 
comes more  and  more  marked,  and  this  is  especially  the  case 
with  the  expirations  which,  almost  or  quite  passively  per- 
formed in  natural  breathing,  become  violently  muscular.  At 
last  Dearly  all  the  muscles  in  the  Body  are  set  at  work;  the 
rhythmic  character  of  the  respiratory  acts  is  lost,  and  general 
convulsions  occur,  but,  on  the  whole,  the  contractions  of  the 


424  THE  HUMAN  BODY. 

expiratory  muscles  are  more  violent  than  those  of  the  inspira- 
tory. Thus  undue  want  of  oxygen  at  first  merely  brings 
about  an  increased  activity  of  the  respiratory  centre,  and 
especially  of  its  expiratory  division  which  is  not  excited  in 
normal  breathing.  Then  it  stimulates  other  portions  (the 
convulsive  centre)  of  the  medulla  oblongata  also,  and  gives 
rise  to  violent  and  irregular  muscular  spasms.  That  the 
convulsions  are  due  to  excitation  of  nerve-centres  in  the 
medulla  (and  not,  as  might  be  supposed,  to  poisoning  of  the 
muscles  or  of  the  fore  parts  of  the  brain  by  the  extremely 
venous  blood)  is  shown  of  the  facts  (1)  that  they  do  not 
occur  in  the  trunk  of  an  animal  when  the  spinal  cord  has 
been  divided  in  the  neck  so  as  to  cut  off  the  muscles  from 
the  medulla;  and  (2)  that  they  still  occur  if  (the  spinal  cord 
remaining  undivided)  all  the  parts  of  the  brain  in  front  of 
the  medulla  have  been  removed. 

The  violent  excitation  of  the  nerve-centres  soon  exhausts 
them,  and  all  the  more  readily  since  their  oxygen  supply 
(which  they  like  all  other  tissues  need  in  order  to  continue 
their  activity)  is  cut  off.  The  convulsions  therefore  gradu- 
ally cease,  and  the  animal  becomes  calm  again,  save  for  an 
occasional  act  of  breathing  when  the  oxygen  want  becomes 
so  great  as  to  lead  to  efficient  stimulation  even  of  the  dying 
respiratory  centre:  these  final  movements  are  inspirations 
and,  becoming  less  and  less  frequent,  at  last  cease,  and  the 
animal  appears  dead.  Its  heart,  however,  though  gorged 
with  extremely  dark  venous  blood  still  makes  some  slow 
feeble  pulsations.  So  long  as  it  beats  artificial  respiration  can 
restore  the  animal,  but  once  the  heart  has  finally  stopped 
restoration  is  impossible.  There  are  thus  three  distinguish- 
able stages  in  death  from  asphyxia.  (1)  The  stage  of 
dyspnoea.  (2)  The  stage  of  convulsions.  (3)  The  stage  of 
exhaustion;  the  convulsions  having  ceased  but  there  being 
from  time  to  time  an  inspiration.  The  end  of  the  third 
stage  occurs  in  a  mammal  about  five  minutes  after  the 
oxygen  supply  has  been  totally  cut  off.  If  the  asphyxia  be 
due  to  deficiency,  and  not  absolute  want  of  oxygen,  of  course 
all  the  stages  take  longer. 

Circulatory  Changes  in  Asphyxia.  During  death  by 
suffocation  characteristic  changes  occur  in  the  working  of 
the  heart  and  blood-vessels.  The  heart  at  first  beats  quicker, 
but  very  soon,  before  the  end  of  the  dyspneeic  stage,  more 


THE  RESPIRATORY  MECHANISM.  425 

slowly,  though,  at  first,  more  powerfully.  This  slowing  is 
due  to  the  fact  that  the  unusual  want  of  oxygen  leads  to 
stimulation  of  the  cardio-inhibitory  centre  in  the  medulla 
and  this,  through  the  pneumogastric  nerves,  slows  the 
heart's  beat.  Soon,  however,  the  want  of  oxygen  affects 
the  heart  itself  and  it  begins  to  beat  more  feebly,  and  also 
more  slowly,  from  exhaustion,  until  its  final  stoppage.  Dur- 
ing the  second  and  third  stages  the  heart  and  the  venae  cavse 
become  greatly  overfilled  with  blood,  because  the  violent 
muscular  contractions  facilitate  the  flow  of  blood  to  the 
heart,  while  its  beats  become  too  feeble  to  send  it  out  again. 
The  overfilling  is  most  marked  on  the  right  side  of  the  heart 
which  receives  the  venous  blood  from  the  Body  generally. 

During  the  first  and  second  stages  of  asphyxia  arterial 
pressure  rises  in  a  marked  degree.  This  is  due  to  excita- 
tion of  the  vaso-motor  centre  by  the  venous  blood,  and 
the  consequent  constriction  of  the  muscular  coats  of  the 
arteries  and  increase  of  the  peripheral  resistance.  In  the 
third  stage  the  blood-pressure  falls  very  rapidly,  because  the 
feebly  acting  heart  then  fails  to  keep  the  arteries  tense,  even 
although  their  diminished  calibre  greatly  slows  the  rate  at 
which  they  empty  themselves  into  the  capillaries. 

Another  medullary  centre  unduly  excited  during  asphyxia 
is  that  from  which  proceed  the  nerve-fibres  governing  those 
muscular  fibres  of  the  eye  which  enlarge  the  pupil.  During 
suffocation,  therefore,  the  pupils  become  widely  dilated. 
At  the  same  time  all  reflex  irritability  is  lost,  and  touching 
the  eyeball  causes  no  wink;  the  reflex  centres  all  over  the 
Body  being  rendered,  through  want  of  oxygen,  incapable  of 
activity.  The  same  is  true  of  the  higher  nerve-centres;  un- 
consciousness comes  on  during  the  convulsive  stage,  which, 
horrible  as  it  looks,  is  unattended  with  suffering. 

Modified  Respiratory  Movements.  Siglnng  is  a  deep 
long-drawn  inspiration  followed  by  a  shorter  but  correspond- 
ingly large  expiration.  Yawning  is  similar,  but  the  air  is 
mainly  taken  in  by  the  mouth  instead  of  the  nose,  and  the 
lower  jaw  is  drawn  down  in  a  characteristic  manner.  Hic- 
cough  depends  upon  a  sudden  contraction  of  the  diaphragm, 
while  the  aperture  of  the  larynx  closes;  the  entering  air, 
drawn  through  the  narrowing  opening,  causes  the  peculiar 
sound.  Coughing  consists  of  a  full  inspiration  followed  by  a 
violent  and  rapid  expiration,  during  the  first  part  of  which 


426  THE  HUMAN  BODY. 

the  laryngeal  opening  is  kept  closed ;  being  afterwards  sud- 
denly opened,  the  air  issues  forth  with  a  rush,  tending  to 
carry  out  with  it  anything  lodged  in  the  windpipe  or  larynx. 
Sneezing  is  much  like  coughing,  except  that,  while  in  a 
cough  the  isthmus  of  the  fauces  is  held  open  and  the  air 
mainly  passes  out  through  the  mouth,  in  sneezing  the  fauces 
are  closed  and  the  blast  is  driven  through  the  nostrils.  It  is 
commonly  excited  by  irritation  of  the  nasal  mucous  mem- 
brane, but  in  many  persons  a  sudden  bright  light  falling  into 
the  eye  will  produce  a  sneeze.  Laughing  consists  of  a  series 
of  short  expirations  following  a  single  inspiration;  the 
larynx  is  open  all  the  time,  and  the  vocal  cords  (CHap. 
XXXVII.)  are  set  in  vibration.  Crying  is,  physiologically, 
much  like  laughing  and,  as  we  all  know,  one  often  passes 
into  the  other.  The  accompanying  contractions  of  the  face 
muscles  giving  expression  to  the  countenance  are,  however, 
different  in  the  two. 

All  these  modified  respiratory  acts  are  essentially  reflex 
and  they  serve  to  show  to  what  a  great  extent  the  discharges 
of  the  respiratory  centre  can  be  modified  by  afferent  nerve 
impulses;  but,  with  the  exception  of  hiccough,  they  are  to  a 
certain  extent,  like  natural  breathing,  under  the  control  of 
the  will.  Most  of  them,  too,  can  be  imitated  more  or  less 
perfectly  by  voluntary  muscular  movements;  though  a  good 
stage  sneeze  or  cough  is  rare. 


CHAPTER  XXVIII. 

THE   KIDNEYS   AND   SKIN. 

General  Arrangement  of  the  Urinary  Organs.     These 
•consist   of   (1)    the   kidneys,  the  glands  which    secrete   the 
urine;  (2)  the  ureters  or  ducts  of  the  kidneys,  which  carry 
their  secretion  to    (3)    the  urinary  bladder,  a  reservoir  in 
which   it  accumulates  and  from  which  it  is  expelled  from 
time  to  time  through  (4)   an  exit  tube,  the  urethra.     The 
general  arrangement  of  these  parts,  as  seen  from  behind,  is 
represented   in  Fig.  132.     The  two  kidneys,  R,  lie  in   the 
dorsal  part  of  the  lumbar  region  of  the  abdominal  cavity, 
one  on  each  side  of  the  middle  line.     Each  is  a  solid  mass, 
with  a  convex  outer  and  a  concave   inner  border,  and    its 
upper   end    a   little  larger  than  the    lower.     From   the  ab- 
dominal aorta,  A,  a  renal  artery,  Ar,  enters  the  inner  border 
of  each  kidney,  to  break   up  within  it  into  finer  branches, 
ultimately  ending  in  capillaries.     The  blood  is  collected  from 
these  into  the  renal  veins.  Vr,  one  of  which  leaves  each  kid- 
ney and   opens  into  the  inferior  vena  cava,  Vc.     From  the 
concave  border  of  each  kidney  proceeds  also  the  ureter,  U,  a 
slender  tube  from  28  to    34  cm.    (11  to   13.5   inches)   long, 
opening  below  into  the  bladder,  Vu,  on  its  dorsal  aspect,  and 
near  its  lower  end.     From  the  bladder  proceeds  the  urethra, 
at    Ua.     The  channel  of  each   ureter   passes  very  obliquely 
through  the  wall  of  the  bladder  to  open  into  it;  accordingly 
if  the  pressure  inside  the  latter  organ  rises  above  that  of  the 
liquid  in  the  ureter,  the  walls  of   the  oblique  passage  are 
pressed   together   and   it   is   closed.      Usually   the   bladder, 
which  has  a  thick  coat  of  unstriped  muscular  tissue  lined  by 
a  mucous  membrane,  is  relaxed,  and  the  urine  flows  readily 
into  it  from  the  ureters.     While  urine  is  collecting,  the  be- 
ginning of  the  urethra  is  kept  closed,  in   part  at  least,  by 
bauds  of  clastic  tissue  around  it:  some  of  the  muscles  which 
surround  the  commencement  of  the  urethra  assist,  being  kept 
in  reflex  contraction  ;  it  is  found  that  in  a  dog  the  urinary 

427 


428 


111E  HUMAN  BODY. 


bladder  can  retain  liquid  under  considerably  higher  pressure 
when  the  spinal  cord  is  intact  than  after  destruction  of  its 


Fio.  132. — The  renal  organs,  viewed  f ram  behind.  R,  right  kidney;  A,  aorta:  Ar, 
right  renal  artery;  Vc,  inferior  vena  cava;  Vr,  right  renal  vein;  U,  right  ureter; 
Vu,  bladder;  Va,  commencement  of  urethra. 

lumbar  portion.  The  contraction  of  these  urethra-con- 
stricting muscles  can  be  reinforced  voluntarily.  When  some 
amount  of  urine  has  accumulated  in  the  bladder,  it  contracts 


THE  KIDNEYS  AND  SKIN.  429 

and  presses  on  its  contents;  the  ureters  being  closed  in  the 
way  above  indicated,  the  elastic  fibres  closing  the  urethral 
exit  are  overcome,  and  the  urethral  muscles  simultaneously 
relaxing,  the  liquid  is  forced  out. 

Naked.  Eye  Structure  of  the  Kidneys.  These  organs 
have  externally  a  red-brown  color,  which  can  be  seen  through 
the  transparent  capsule  of  peritoneum  which  envelops  them. 
When  a  section  is  carried  through  a  kidney  from  its  outer  to 
its  inner  border  (Fig.  133)  it  is  seen  that  a  deep  fissure,  the 
hilus,  leads  into  the  latter.  In  the  hilus  the  ureter  widens 
out  to  form  the  pelvis,  D,  which  breaks  up  again  into  a 
number  of  smaller  divisions,  the  cups  or  calices.  The  cut 
surface  of  the  kidney  proper  is  seen  to  consist  of  two  distinct 
parts;  an  outer  or  cortical  portion,  and  an  inner  or  medul- 
lary. The  medullary  portion  is  less  red  and  more  glistening 
to  the  eye,  is  finely  striated  in  a  radial  direction,  and  does  not 
consist  of  one  continuous  mass  but  of  a  number  of  conical 
portions,  the  pyramids  of  Malpiyhi,  2',  each  of  which  is 
separated  from  its  neighbors  by  an  inward  prolongation,  *,  of 
the  cortical  substance:  this,  however,  does  not  reach  to  the 
inner  end  of  the  pyramid,  which  projects,  as  the  papilla,  into 
a  calyx  of  the  ureter.  At  its  outer  end  each  pyramid  sepa- 
rates into  smaller  portions,  the  pyramids  of  Ferrein,  2", 
separated  by  thin  layers  of  cortex  and  gradually  spreading 
everywhere  into  the  latter.  The  cortical  substance  is  redder 
and  more  granular  looking  and  less  shiny  than  the  medullary, 
and  forms  everywhere  the  outer  layer  of  the  organ  next  its 
capsule,  besides  dipping  in  between  the  pyramids  in  the  way 
described. 

The  renal  artery  divides  in  the  hilus  into  branches  (5) 
which  run  into  the  kidney  between  the  pyramids,  giving  off 
;i  few  twigs  to  the  latter  and  ending  finally  in  a  much  richer 
vascular  network  in  the  cortex.  The  branches  of  the  renal 
vein  have  a  similar  course. 

The  Minute  Structure  of  the  Kidney.  The  kidneys 
are  compound  tubular  glands,  composed  essentially  of 
branched  microscopic  uriniferous  I ubules,  lined  by  epithe- 
lium. Each  tubule  commences  at  a  small  opening  on  a 
papilla  and  from  thence  lias  a  very  complex  course  to  its 
oilier  extremity:  usually  about  twenty  open,  side  by  side, 
on  one  papilla,  where  they  have  n  diameter  of  about  0.125 
mm.  (.,',„  inch).      Running  from  this  place  into  the  pyramid 


430 


Tin-:  it  u  max  noHY. 


each  tubule  divides  several  times.  At  first  the  branches  are 
smaller  than  the  main  tube;  hut  as  Boon  as  they  have  come 
down  to  abont  0.04  nun.  (,>,',„  inch)  this  diminution  in  size 
ceases,  and  division  continuing  while  the  tubules  retain  the 
same  diameter,  the  pyramid  thus  gets,  in  part,  its  conical  form. 
Ultimately  each  branch  runs  out  of  the  pyramid  somewhere, 
either  from  its  base  or  side,  into  the  cortex  and  there  dilates 


4^35>i 


Fig  133.— Section  through  the  right  Kidney  trom  its  outer  to  its  inner  border. 
i  cortex;  8,  medulla:  2',  pyramid  of  Malpighi;  -'"•  pyramid  of  Ken-em;  5,  small 
branches  of  the  renal  artery  entering  between  the  pyramids;  A,  a  branch  of  the 
renal  artery;  ]>.  the  pelvis  or  the  kidney;  U,  meter:  C,  a  calyx. 

and  is  twisted.  It  then  narrows  and  doubles  back  into  one  of 
the  pyramids  of  Ferrein  and  runs  as  a  straight  tube  towards 
the  papilla,  but  before  reaching  it  makes  a  loop  (loop  of  Henle), 
and  turns  back  again  as  a  straight  tube  towards  the  base  of 
the  pyramid,  where  it  once  more  enters  the  cortex,  dilates 
and  becomes  contorted,  and  then  ends  in  a  spherical  capsule, 


THE  KIDNEYS  AND  SKIN.  431 

containing  a  tuft  of  small  blood-vessels.  Or,  followed  the 
other  way,  each  tubule  commences  in  the  cortex  with  a 
globular  dilatation,  the  Malpighian  capsule.  From  this  it 
continues  as  a  convoluted  tubule  in  the  cortex;  this  passes 
into  a  pyramid  of  Ferrein,  becomes  straight,  and  runs  to  near 
a  pyramid  of  Malpighi  as  the  descending  limb  of  a  loop  of 
Henle.  Turning  at  the  loop,  it  continues  as  its  ascending 
limb,  and  this  passes  out  again  into  the  cortex  and  becomes 
the  convoluted  junctional  tubule,  which  passes  as  a  straight 
collecting  tubule  into  a  pyramid  of  Ferrein,  where  it  joins 
others  to  form  an  excretory  tubule;  the  excretory  tubules 
run  into  the  main  pyramid  and  unite  to  form  the  discharging 
tubules  which  open  on  the  papilla.  Throughout  its  course 
the  tubule  is  lined  by  a  single  layer  of  epithelium  cells  differ- 
ing in  character  in  its  different  sections:  they  are  flat  and 
clear  in  the  capsules,  and  very  granular  in  both  the  convo- 
luted parts,  where  their  appearance  suggests  that  they  are 
not  mere  lining  cells  but  cells  with  active  work  to  do;  they 
are  non-granular  and  flat  in  the  descending  limb  of  the  loop 
of  Henle,  clear  and  columnar  in  most  of  the  ascending,  and 
in  both  are  probably  only  protective;  in  the  collecting  and 
discharging  tubules  they  are  somewhat  cuboidal  in  form  and 
have  no  active  secretory  function.  All  the  tubes  are  bound 
together  by  a  sparse  amount  of  connective  tissue  and  by 
blood-vessels  to  form  the  gland.  The  lymph  spaces  are  large 
and  numerous,  especially  about  the  convoluted  portions  of  the 
tubules. 

The  Blood-flow  through  the  Kidney.  The  amount  of 
blood  brought  to  the  kidney  is  large  relatively  to  the  size  of 
the  organ  and  enters  under  a  very  high  pressure  almost  direct 
from  the  aorta,  and  leaves  under  a  very  low,  into  the  inferior 
cava  (Fig.  L32).  The  final  twigs  of  the  renal  artery  in  the 
cortex,  giving  off  a  few  branches  which  end  in  a  capillary 
network  around  the  convoluted  tubules  and  in  the  pyramids, 
are  continued  as  the  afferent  vessels  of  Malpighian  capsules, 
the  walls  of  which  arc  doubled  iii  before  them  (Fig.  134); 
there  each  breaks  up  into  a  little  knot  of  capillary  vessels 
called  the  glomerulus,  from  which  ultimately  an  efferent  vessel 
proceeds.  Where  the  wall  of  the  glomerulus,  w,  Pig.  134,  is 
doubled  in  before  the  blood-vessels,  its  lining  cells  continue 
covering,  c,  to  the  latter,  closely  adhering  to  the  vascular 
wall-.     A  space,  .1.  i-  left  between  the  epithelial  cells  of  the 


432 


THE  HUMAN  BODY. 


outside  of  the  capsule  and  those  involuted  on  the  vessels,  as 
there  would  be  in  the  interior  of  a  rubber  ball  one  side  of 
which  was  pushed  in  so  as  to  nearly  meet  the  other;  this 
cleft,  into  which  any  liquid  transuded  from  the  vessels  must 
enter,  opens  by  a  narrow  neck,  d,  into  the  commencement  of 
the  first  contorted  part  of  an  uriniferous  tubule.  The  effer- 
ent vein,  carrying  blood  away  from  the  glomerulus,  breaks 
up  into  a  close  capillary  network   around   the  neighboring 

tubules  of  the  cortex.  From  these 
capillaries  the  blood  is  collected 
into  the  renal  vein.  Most  of  the 
blood  flowing  through  the  kidney 
thus  goes  through  two  sets  of  capil- 
laries; one  found  in  the  capsules, 
and  the  second  formed  by  the 
breaking  up  of  their  efferent  veins. 
The  capillary  network  in  the  pyra- 
mids is  much  less  close  than  that 
in  the  cortex,  which  gives  reason  to 
suspect  that  most  of  the  secretory 
work  of  the  kidneys  is  done  in  the 
Fio.  134—  Diagram  showing  a  capsules    and   convoluted  tubules. 

kid  ne  v   glomerulus  and  the  com-   „.-.  •  -i    i     i  i        -in  i 

meneement  of  an  uriniferous  1  he  pyramidal  blood  flows  Ollly 
tubule,      a,   afferent    blood-vessel    ii  i  ■       t>  •■11  ,i 

pushing  in  the  wail,  to,  of  a  Maipi  through  one  set  of  capillaries,  there 
8XS^"SrBK*fi  beinS  no  glomeruli  in  the  kidney 

vein  e  issues:  c,  involuted  epithe-  rnedull'l 
Hum  covering  the  vascular  tuft; 

for  the  sake  of  distinctness  it  is        Trig     Renal     Secretion.      The 

represented  as  a  general  wrapping 

for  the  whole  tuft,  but  in  nature  amount  of  this  Carried  off  from  the 

it     forms     a     close      investment  .  . 

around    each   vessel    of    the    glo-  Body  m  24  llOUl'S  IS  SU  Dject  to  COU- 

merolus;  A.  space  in  capsule  into  .,        .  ,  ...         ,     .  .   ■,-, 

which     liquid   transuded    from  siderable  variation,  being  especially 

the    vessels     of     the     glomerulus     ■,.      ■     •   ■,       -,   i  ,-,   •  i    ~  i 

passes;  d.  neck  of  capsule  passing  diminished  by  anything  which  pro- 

into  commencement  of  first  con-  .  Tiprarn ration      iiul    inprpisprl 

voiuted  portion. //.  of  an  urinif-  motes   perspiration,   .iiiu  increased 

erous  tubule;  o,  granular  epithe-  k,t  prmrliHniis  ns  onld  to  flip  mir- 
lial    cells;     6,    basement      mem-    DJ    COIIUICIOUS,    as   LOIU    lO    ine    bur 

brane-  face,  which  diminish  the  skin  ex- 

cretion. Its  average  daily  quantity  varies  from  1200  to  1750 
cub.  cent.  (40  to  60  fluid  ounces).  The  urine  is  a  clear 
amber-colored  liquid,  of  a  slightly  acid  reaction;  its  specific 
gravity  is  about  1022,  being  higher  when  the  total  quantity 
excreted  is  small  than  when  it  is  greater,  since  the  amount  of 
solids  dissolved  in  it  remains  nearly  the  same  in  health;  the 
changes  in  its  bulk  being  dependent  mainly  on  changes  in 
the  amount  of  water  separated  from  the  blood  by  the  kidneys. 


THE  KIDNEYS  AND   SKIN. 


433 


Normal  urine  consists,  in  1000  parts,  of  about  960  water 
and  40  solids.  The  latter  are  mainly  crystalline  nitrogenous 
bodies  {urea  and  uric  acid),  but  small  quantities  of  pigments 
and  of  non-nitrogenous  organic  bodies  are  also  present,  and  a 
considerable  quantity  of  mineral  salts.  The  following  table 
gives  approximately,  in  the  first  column,  the  average  compo- 
sition of  the  urine  excreted  in  twenty-four  hours  expressed  in 
grams;  in  the  second  column  the  same  expressed  in  grains. 
The  third  column  gives  the  composition  of  1000  parts  of 
urine. 


Urine  in  24  hours. 

1500  grams. 

23,250  grains. 

In  1000  parts. 

Water 

1428.00 
72.00 

22,134.00 
1116.00 

952  00 

Solids  

48  00 

The  solids  consist  of— 

Urea 

33.00 
0.50 
0.40 
1.00 

10.00 
2.00 
3.00 
7.00 
0.75 
2.50 

11.00 
0.25 
0.20 

511.50 

7.75 

6.20 

15.50 

155  00 

31.00 

46.50 

108  50 

12.00 

38.75 

170.50 

3.80 

3.00 

22.00 

Uric  acid 

0.33 

Hippuric  acid 

0.27 

Kreatinin 

0.67 

6.66 

Sulphuric  acid 

1.33 

Phosphoric  acid 

2  Oo 

Chlorine 

4  67 

Ammonia 

0.50 

Potassium 

1.67 

Sodium 

7.33 

Calcium 

0.17 

0.13 

71.60 

1110.00 

47.73 

The  urine,  however,  even  in  health  is  subject  to  consid- 
erable variation  in  composition;  not  only  as  regards  the 
amount  of  water  in  it,  but  also  in  respect  to  its  solid  con- 
stituents; the  latter  are  especially  modified  by  the  quantity 
and  nature  of  the  food  taken. 

The  Crystalline  Nitrogenous  Constituents  of  the  Urine 
are  of  great  interest  as  they  represent  the  final  result  of  the 
breaking  down  in  the  Body  of  albuminous  and  gelatinaginous 
substances,  whether  due  to  tissue  waste  or  to  the  destruction 
in  the  bodily  liquids  of  proteids  and  albuminoids  existing  in 
them  in  solution.  Their  chemical  relationships  tend  to  cast 
some  light  on  the  structure  of  an  albumen  molecule  and  on 
the  metabolisms  it  undergoes  in  the  living  organism. 


434  THE  HUMAN  BODY. 

Urea  (NJI4CO)  is  the  chief  nitrogenous  waste  product 
of  the  human  Body  and  is  related  to  the  ammonia  group, 
being  readily  converted  into  ammonium  carbonate  by  hydra- 
tion, a  change  which  occurs  under  the  influence  of  some 
living  ferments  when  stale  urine  becomes  alkaline  and  ac- 
quires its  well-known  offensive  ammoniacal  odor — 


N9H4CO  +  2HaO  =  (NH4)aC01 


On  another  side  urea  is  allied  to  the  cyanogen  group  of  sub- 
stances, being  isomeric  with  ammonium  cyanate,  which  is 
converted  into  it  by  simple  heating. 

Uric  acid  (C6H4N403)  is  present  in  but  small  quantity  in 
normal  human  urine,  but  is  the  chief  nitrogenous  excretion 
of  birds  and  reptiles.  Its  molecular  structure  is  more  com- 
plex than  that  of  urea,  and  when  it  is  decomposed  by  various 
methods  urea  is  very  frequently  one  of  the  products.  It  is  a 
less  complete  product  of  proteid  degradation  than  urea. 
Some  of  its  decompositions  indicate  relations  to  oxalic  acid 
and  to  amido-acetic  acid  (glycin),  and  through  this  latter  to 
the  ammonias  and  the  fatty  acids  series.  In  human  urine 
uric  acid  exists  chiefly  in  the  form  of  salts  of  potassium  and 
sodium;  these  are  less  soluble  in  cold  than  in  warm  water, 
and  are  sometimes  deposited  as  a  flocculent  precipitate  when 
originally  clear  urine  is  left  to  cool.  The  precipitate  dis- 
appears on  reheating  the  liquid. 

Hippuric  acid  (C7H602)  is  scanty  in  normal  human  urine 
but  abundant  in  the  urine  of  herbivora.  Chemically  it  is 
related  to  the  aromatic  series,  being  formed  when  benzoic 
acid  and  glyciu  are  made  to  unite  with  dehydration;  and  it 
is  broken  up  into  those  substances  when  boiled  with  mineral 
acids.  Certain  aromatic  bodies  allied  to  benzoic  acid  are 
found  in  hay  and  similar  foods  and  account  for  the  large 
amount  of  hippurates  in  herbivorous  urine.  But  proteids 
when  broken  up  by  putrefaction  also  yield  bodies  of  the  ben- 
zoic group,  and  the  hippuric  acid  of  human  urine  probably 
has  its  origin  in  the  liberation  of  benzoic  residues  in  metabolic 
activities  of  some  of  the  living  cells  of  the  Body;  these 
residues  being  then  combined  with  glycin  to  form  hippuric 
acid.  That  glycin  is  formed  in  the  Body  is  shown  by  the 
fact  that  benzoic  acid  given  in  food  reappears  in  human  urine 
as  hippuric  acid,  having  been  somewhere  united  to  a  glycin 
residue. 


THE  KIDNEYS  AND  SKIN.  435 

Kreatinin  (C4H,NsO)  is  closely  allied  tokreatin  (C4H9N302), 
of  which  it  is  a  simple  dehydration  product.  Kreatin  is  a 
normal  constituent  of  muscle  (0.2-0.3$),  being,  indeed,  most 
conveniently  prepared  from  Liebig's  extract;  it  is  also  known 
that  kreatin  introduced  into  the  Body  is  converted  into 
kreatiuin;  for  if  given  in  the  food  it  causes  an  equivalent 
increase  of  the  kreatinin  excreted  in  the  urine.  Kreatin 
formed  in  the  muscles  has  accordingly  been  supposed  to  be  a 
source  of  the  kreatinin  of  the  urine,  but  this  does  not  appear 
to  be  the  case,  as  all  kreatinin  disappears  from  the  urine 
during  starvation.  The  kreatinin  of  normal  urine  probably 
has  its  source  in  the  kreatin  of  flesh  eaten  as  food. 

Tlte  Urinary  Pigments  are  still  very  imperfectly  known, 
but  appear  in  part  to  be  derived  from  uro-bilin,  which,  as  we 
have  seen  (Chap.  XXIV"),  is  itself  probably  a  derivative  of 
haemoglobin. 

Of  the  inorganic  salts  sodium  chloride  is  by  far  the  most 
abundant,  but  the  phosphates  deserve  notice  because  the 
acidity  of  normal  fresh  urine  is  dependent  on  the  presence  of 
acid  sodium  phosphate. 

In  various  diseases  abnormal  substances  are  found  in  the 
urine:  the  more  important  are  albumens  in  albuminuria  ox 
Bright's  disease;  grape  sugar  or  glucose  in  diabetes;  bile 
salts  ;  bile  pigments. 

The  Secretory  Actions  of  Different  Parts  of  an  Urinif- 
erous  Tubule. — The  microscopic  structure  of  the  kidneys  is 
such  as  to  suggest  that  in  those  organs  we  have  to  do  with 
two  essentially  distinct  secretory  apparatuses :  one  represented 
by  the  glomeruli,  with  their  capillaries  separated  only  by  a 
single  layer  of  flat  epithelial  cells  from  the  cavity  of  the- 
capsule  and  especially  adapted  for  filtration  and  dialysis;  the 
other  represented  by  the  contorted  portions  of  the  tubules, 
witb  their  large  granular  cells,  which  clearly  have  some  more 
active  part  to  play  than  that  of  a  mere  passive  transudation 
membrane.  And  Ave  find  in  the  urine  substances  which  like 
the  water  and  mineral  salts  may  easily  be  accounted  for  by 
mere  physical  processes,  and  others,  urea  especially,  which  are 
preseni  in  such  proportion  as  must  be  due  to  some  active 
physiological  work  of  the  kidney,  whether  a  merely  selective 
activity  of  its  (ills  or  a  constructive  one.  More  direct  evi- 
dence  docs,  in  fact,  justify  us  in  saying  that,  in  general  the 
glomeruli  are  transudation  organs,  the  contorted  portions  of 


436  THE  HUMAN  BODY. 

the  tubuli  secretory  organs,  while  the  loops  of  Henle  and  the 
collecting  and  discharging  tubules  arc  merely  passive  channels 
for  the  gathering  and  transmission  of  liquid.  Even  in  the 
glomeruli,  however,  the  renal  cells  provide  something  more 
than  a  merely  passive  physical  membrane  for  dialysis  and 
liltration:  to  a  certain  extent  they  control  the  passage  of 
substances  through  them;  while  they  are  in  health  no  serum 
albumen  or  paraglobulin  passes  through  them  into  the  urine, 
though  egg  albumen  injected  into  the  blood  of  a  living 
mammal  does.  But  when  they  are  altered  in  disease  or  even 
by  a  temporary  stoppage  of  their  blood-supply,  then  they 
allow  the  normal  blood  proteids  to  transude:  if  the  blood- 
supply  of  a  kidney  be  cut  off  for  some  minutes  by  clamping 
the  renal  artery,  then  the  urine  secreted  for  some  time  after 
the  clamp  is  removed  is  albuminous. 

The  structure  of  the  glomerular  epithelium  and  its  rela- 
tion to  the  blood-vessels  are  such  as  to  make  it  almost  certain 
that  when  albumen  appears  in  the  urine  it  enters  through 
them  and  not  through  other  parts  of  the  tubule;  but  in  some 
amphibia  we  get  direct  evidence  of  the  entry  of  substances  other 
than  salts  and  water  into  the  renal  secretion  by  the  path  of 
the  Malpighian  capsules.  In  amphibia  the  blood  carried  to 
the  kidney,  like  that  supplying  the  mammalian  liver,  has  two 
sources,  one  venous  and  one  arterial  ;  the  arterial  supply 
comes  from  the  renal  arteries,  the  venous  from  the  veins  of 
the  leg  by  the  reniportal  vein.  Both  bloods  leave  the  organ 
by  the  renal  veins,  but  their  distribution  in  it  is  in  great  part 
distinct;  the  arteries  supply  the  glomeruli,  the  reniportal 
vein  the  tubules  of  the  cortex,  though  mixed  there  with  blood 
from  the  efferent  vessels  of  the  glomeruli.  In  some  small 
amphibia  it  is,  in  fact,  possible  to  observe  the  circulation  in 
the  living  kidney  and  to  see  that  all  blood-flow  in  the  glomer- 
uli ceases  when  the  renal  arteries  are  tied,  though  it  con- 
tinues elsewhere  throughout  the  organ.  When  sugar  or 
peptone  is  injected  into  the  blood  of  such  an  animal  those 
substances  appear  in  the  urine;  but  if  the  renal  arteries  be  pre- 
viously tied  they  do  not.  It  is  true  that  under  those  circum- 
stances all  secretion  of  urine  usually  ceases,  but  it  may  be 
excited  by  administering  certain  drugs,  and  then  is  found  to 
be  free  from  sugar  and  peptone.  Grape-sugar  when  present 
in  the  blood  of  mammals  beyond  a  certain  small  percentage 
passes  out  in  the  urine;  and   the  same  is  true  of  peptone: 


THE  KIDNEYS  AND  SKIN.  437 

indeed,  the  absence  of  peptone  (or  of  all  but  the  merest  traces 
of  it)  from  healthy  human  urine  is  one  of  the  main  reasons 
for  believing  that  peptone  absorbed  from  the  alimentary  canal 
is  converted  at  once  by  the  lymphoid  tissues  of  the  mucous 
membrane  into  the  proper  proteids  of  the  blood  plasma. 
When  sugar  appears  in  the  urine  either  in  disease  or,  as  some- 
times happens  temporarily,  in  health,  after  a  meal  rich  in 
starchy  substances  we  have  from  the  results  of  experiment 
on  amphibia  reason  to  believe  that  its  path  of  excretion  is 
through  the  glomeruli.  In  hemoglobinuria,  a  condition  in 
which  haemoglobin  is  found  in  solution  in  urine  (not  in  blood- 
corpuscles,  for  in  that  case  it  may  have  come  from  ruptured 
vessels  anywhere  in  the  renal  apparatus),  the  haemoglobin  also 
passes  out  through  the  Malpighian  bodies:  for  when  some 
laky  blood  (Chap.  IV)  is  injected  into  the  vessels  of  an  animal 
and  the  secretion  of  urine  at  the  same  time  made  slow,  col- 
lections of  haemoglobin  may  be  found  in  the  cavities  of  the 
capsules.  While,  however,  we  have  evidence  that  the  epithe- 
lium of  the  capsule  has  a  certain  selective  power  and  is  the 
special  seat  of  transmission  of  particular,  especially  abnormal, 
urinary  constituents,  yet  on  the  whole  the  glomeruli  provide 
a  merely  physical  apparatus.  Through  them  most  of  the 
bulk  of  the  urine  passes  out,  and,  flushing  the  more  active 
portions  of  the  tubules  on  its  course  to  the  pelvis  of  the  kid- 
ney, picks  up  from  them  the  more  specific  urinary  con- 
stituents. 

Urea  is  the  most  important  and  most  abundant  of  the 
characteristic  ingredients  of  urine,  and  it  has  a  very  marked 
influence  on  kidney  activity,  the  injection  of  some  of  it  into 
blood  causing  a  greatly  increased  secretion  of  urine,  in  which 
the  injected  urea,  is  quickly  passed  out.  Judging  from  ex- 
periments on  amphibia,  urea  is  not  excreted  or  at  any  rate  not 
chiefly  excreted  by  the  glomeruli.  On  tying  the  renal  arte- 
ries of  one  of  these  animals  urinary  secretion  ceases,  there 
being  then  no  blood-pressure  in  the  glomeruli  to  cause  the 
transudation  of  liquid;  but  if  some  urea  be  now  injected 
into  the  blood  tlie  ephithelial  cells  of  other  parts  of  the 
tubules  are  stimulated  to  secrete,  and  urine  rich  in  urea  is 
formed;  but  in  these  circumstances  it  cannot  come  from  the 
lialpighian  bodies.  It,  would  seem  then  that  urea  is  a  special 
stimulant  to  .some  cells  of  the  tubules,  and  that  an  excess 
of  it  in  the  blood  can  stir  them  up  to  its  elimination  along 


438  THE  HUMAN  BODY. 

with  some  water,  quite  independently  of  any  formation  of 
transudation  urine.  In  mammalia  we  cannot  Beparate  the 
glomerular  secretion  from  the  tubular  as  in  amphibia;  and 
the  diuresis  which  administration  of  urea  causes  in  them  is 
in  pari  due  to  increased  glomerular  activity,  as  urea  dilate-; 
the  kidney  vessels  and  causes  more  blood  flow  through  the 
glomeruli,  which  causes  the  transudation  of  more  water 
through  them;  but  the  simultaneous  increase  of  urea  is 
almost  certainly  due  to  special  activity  of  the  other  parts  of 
the  tubules. 

The  proteids  and  albuminoids  of  food  may  while  within 
the  organism  have  been  built  up  into  tissue  or  may  have 
remained  in  solution  in  the  liquids;  but  in  either  case  they 
are  sooner  or  later  broken  up  and  oxidized,  the  main  final 
products  being  carbon  dioxide,  water,  and  urea.  But  this 
breaking  down  may  and  does  occur  in  many  stages  and  by 
different  modes  in  the  various  tissues;  and  there  is  no  doubt 
that  most  of  the  intermediate  processes  in  the  chemical 
degradation  of  albuminous  compounds  take  place  outside 
the  kidneys.  It  was,  however,  at  one  time  believed  that  the 
urea  itself  was  a  kidney  product  :  that  the  penultimate  ni- 
trogenous products  of  proteid  degradation  were  brought  to 
the  kidneys,  and  that  there  the  final  formation  of  urea  took 
place.  But  if  this  were  so  there  could  not  be  less  urea  in  the 
blood  leaving  the  kidneys  by  their  veins  than  in  that  brought 
to  them  by  the  renal  arteries;  yet  such  is  the  case.  And 
further,  if  urea  be  made  in  the  kidneys  it  ought  not  to 
accumulate  in  the  blood  of  animals  from  whom  both  kidneys 
have  been  removed,  as  it  is  now  known  to  do,  though  not 
the  immediate  cause  of  the  symptoms  of  so-called  urmmic 
poisoning  seen  in  persons  with  extensive  kidney  disease. 
So  far,  then,  as  urea  is  concerned  the  cells  of  the  kidney 
tubules  are  not  its  producers;  they  have  a  special  affinity  for 
it  and  pick  it  up  from  the  lymph  of  the  kidney,  which  in 
turn  gets  it  from  the  blood.  The  cells  then  pass  it  on  with 
some  water,  and  no  doubt  other  things,  into  the  tubules 
which  they  line.  That  it  is  the  epithelial  cells  of  the 
contorted  portions  of  the  tubules  which  especially  exer- 
cise this  selective  power  is,  so  far  as  urea  is  concerned, 
a  presumption  based  on  their  histological  characters,  but 
there  is  evidence  that  these  cells  have  a  special  selective 
power  for  some  other  substances  circulating  in  the  blood. 


THE  KIDNEYS  AND  SKIN.  439 

A  bine  substance  known  as  sodium  sulphindogate  after  in- 
jection, in  solution,  into  a  vein  of  an  animal  is  excreted  in 
the  bile  and  urine.  If  the  animal  be  killed  during  the 
excretion  no  traces  of  this  body  can  under  normal  circum- 
stances be  detected  in  any  special  part  of  the  kidney;  it  is 
in  fact  washed  away  by  the  urine  as  fast  as  the  cells  pick  it 
up  and  pass  it  into  the  tubuli.  But  if  the  blood-pressure  of 
the  animal  be  made  so  low  (as  by  cutting  the  main  vaso- 
constrictor nerves)  as  to  bring  the  renal  secretion  to  a  stand, 
and  the  animal  be  killed  some  time  after  injection  of  the 
indigotate,  the  glomeruli  and  most  of  the  tubules  are  found 
free  of  the  blue,  which  lies  only  in  the  contorted  portions, 
just  where  the  cells  which  gathered  it  from  the  circulating 
liquid  had  passed  it  out. 

Though  the  renal  epithelium  does  not  make  urea  it 
has  constructive  powers  as  regards  some  other  urinary  constit- 
uents. As  already  stated,  benzoic  acid  taken  with  the  food 
leaves  the  Body  as  hippuric  acid,  having  been  combined 
with  glycin.  If  blood  containing  benzoic  acid  and  glycin  be 
artificially  circulated  through  a  perfectly  fresh  still  living 
kidney,  the  renal  vein  blood  will  contain  hippuric  acid. 
Even  if  no  glycin  be  provided  in  the  blood  injected  through 
the  renal  artery  the  returning  blood  will  still  yield  hippuric 
acid.  So  living  kidney  cells  caii  not  only  perform  the 
synthesis,  with  dehydration,  necessary  to  form  hippuric  acid, 
but  can  also  form  and  supply  the  required  glycin.  The 
process  is  closely  dependent  on  the  vitality  of  the  cells;  the 
experiment  fails  if  the  organ  be  not  perfectly  fresh  and  unin- 
jured, and  if  the  blood  supplied  be  not  properly  arterialized. 

The  Influence  of  Renal  Blood-flow  on  the  Amount  of 
Urine  Secreted.  From  the  structure  of  the  glomeruli  and 
the  fact  that  most  of  the  water  of  the  urine  is  derived 
from  them  it  is  a  priori  probable  that  anything  tending  to 
increase  the  pressure  of  blood  in  them  will  increase  the  bulk 
of  urine  secreted,  and  anything  diminishing  that  pressure 
lo'Tnase  the  urine.  This  is  confirmed  by  experiment.  The 
kidney  is  supplied  with  both  vaso-constrictor  and  vaso-dilator 
nerves  which  reach  it  mainly  through  the  solar  plexus, 
though  both  seta  come  ultimately  from  the  spinal  cord. 
When  the  Bpinal  cord  is  cut  in  the  neck  region  of  a  dog  the 
kidm  ■  i-  ae  well  as  those  of  the  rest  of  its  body  dilate 

and    blood-pressure   everywhere   is   very    low.     Under   these 


440  THE  HUMAN  BODY. 

circumstances  the  secretion  of  urine  is  suppressed.  If  the 
lower  end  of  the  cut  cord  be  stimulated  the  vessels  all  over 
the  body  of  the  animal  contract,  and  blood-pressure  every- 
where becomes  very  high.  But  the  kidney  vessels  being 
constricted  with  the  rest  allow  very  little  blood  to  enter 
the  glomeruli  in  spite  of  the  high  aortic  pressure,  and  little 
or  no  urine  is  secreted.  If,  however,  the  vasoconstrictor 
nerves  of  the  kidney  be  cut  before  the  stimulation  of  the 
cord,  we  get  a  dilatation  of  the  kidney  vessels  with  a  con- 
striction of  vessels  elsewhere,  and  abundant  blood  flows 
through  the  glomeruli  under  high  pressure  :  the  whole 
kidney  swells  and  abundant  urine  is  formed.  When  the 
skin  vessels  contract  on  exposure  to  cold,  more  blood  flows 
through  internal  organs,  the  kidneys  included,  and  the  blood- 
pressure  in  these  is  if  anything  increased,  the  expansion  of 
internal  arteries  not  at  the  most  more  than  counterbalancing 
the  constriction  of  the  cutaneous.  Hence  the  greater  secre- 
tion of  urine  in  cold  weather.  Injection  of  a  little  water 
into  a  vein  of  an  animal  causes  a  very  transient  constriction 
of  the  kidney  vessels  followed  by  a  dilatation;  and  general 
blood-pressure  not  being  at  the  same  time  lowered,  pressure 
in  the  renal  glomeruli  is  high  and  the  secretion  of  urine 
increased.  Urea  introduced  into  the  blood  acts  in  a  similar 
way,  but  more  markedly ,  so  that  this  substance  causes 
diuresis  not  merely,  as  we  have  seen,  by  stimulating  the  cells 
of  the  tubuli,  but  also  by  exciting  the  vaso-dilator  nerves  of 
the  glomerular  arteries.  Solution  of  sodium  acetate  or  even 
of  common  salt  injected  into  the  veins  causes  very  marked 
local  vascular  dilatation  in  the  kidney,  and  hence  great  Mow 
through  the  organ  under  high  pressure  and  a  marked  in- 
crease in  the  quantity  of  urine  excreted.  Even  if  the  nerves 
going  to  the  kidney  be  first  cut,  the  above  results  follow, 
these  salts  appearing  to  act  directly  on  a  local  renal  vaso- 
dilator mechanism.  They  may  of  course  also,  like  urea, 
directly  stimulate  the  cells  of  the  contorted  tubules,  but  this 
is  not  proved.  The  increased  secretion  of  urine  after  drink- 
ing much  water  is  probably  produced  by  the  dilution  of  the 
blood  by  the  liquid  absorbed  by  the  alimentary  canal,  essen- 
tially in  the  same  manner  as  the  extra  secretion  caused  by 
direct  injection  of  water  into  the  blood-vessels.  That  the 
central  nervous  system  may  influence  the  renal  secretion  is 
well    known,  fear,   for   example,    stimulating    it.     Probably 


THE  KIDNEYS  AND  SKIN  441 

such  influence  is  mainly  due  to  vaso-motor  changes — either 
paralvsis  of  the  renal  vaso-constrictor  nerves  or  stimulation 
of  the  vaso-dilator.  Such  changes  would  account  for  the 
phenomenon,  and  there  is  no  evidence  of  the  existence  of 
true  secretory  nerves  acting  directly  on  the  cells  of  the 
organ  as  certain  fibres  of  the  chorda  tympani  (Chap.  XIX) 
do  on  the  cells  of  the  submaxillary  gland. 

The  Skm,  which  covers  the  whole  exterior  of  the  Body, 
consists  everywhere  of  two  distinct  layers  ;  an  outer,  the  cuti- 
cle or  epidermis,  and  a  deeper,  the  dermis,  cutis  vera,  or 
corium.  A  blister  is  due  to  the  accumulation  of  liquid  be- 
tween these  two  layers.  The  hairs  and  nails  are  excessively 
developed  parts  of  the  epidermis. 

The  Epidermis,  Fig.  135,  consists  of  cells,  arranged  in 
many  layers,  and  united  by  a  small  amount  of  cementing 
substance.  The  deepest  layer,  d,  is  composed  of  elongated 
or  columnar  cells,  set  on  with  their  long  axes  perpendicular 
to  the  corium  beneath.  To  it  succeed  several  layers  of  round- 
ish cells,  b,  the  deepest  of  which,  prickle  cells,  are  covered  by 
minute  processes  (not  indicated  in  the  figure)  which  do  not 
interlock  but  join  end  to  end  so  as  to  leave  narrow  spaces 
between  the  cells  ;  in  more  external  layers  the  cells  become 
more  and  more  flattened  in  a  plane  parallel  to  the  surface. 
The  outermost  epidermic  stratum  is  composed  of  many  layers 
of  extremely  flattened  cells  from  which  the  nuclei  (conspicu- 
ous in  the  deeper  layers)  have  disappeared.  These  super- 
ficial cells  are  dead  and  are  constantly  being  shed  from  the 
surface  of  the  Body,  while  their  place  is  taken  by  new  cells, 
formed  in  the  deeper  layers,  and  pushed  up  to  the  surface 
and  flattened  in  their  progress.  The  change  in  the  form  of 
the  cells  as  they  travel  outwards  is  accompanied  by  chemical 
changes,  and  they  finally  constitute  a  semitransparent  dry 
horny  stratum,  a,  distinct  from  the  deeper,  more  opaque  and 
softer  Malpighinn  or  mucous  layer,  b  and  d,  of  the  epider- 
mic The  cells  of  this  latter,  in  spite  of  their  name,  are  not 
muceginous;  they  are  soluble  in  acetic  acid;  those  of  the 
homy  stratum  not. 

The  rolls  of  material  which  are  peeled  off  the  skin  in  the 
" shampooing "  of  the  Turkish  bath,  or  by  rubbing  with  a 
rough  towel  after  an  ordinary  warm  bath,  are  the  dead  outer 
scales  of  the  horny  stratum  of  the  epidermis. 


442 


THE  HUMAN  BODY. 


In  dark  races  the  color  of  the  skin  depends  mainly  on 
minute  pigment  granules  lying  in  the  cells  of  the  deeper  part 
of  i  lie  Malpighian  layer. 

No  blood  or  lymphatic  vessels  enter  the  epidermis,  which 
is  entirely  nourished  by  mutters  derived   from  the  subjacent 


i  x—  a 


Ipfi: 


i 


<■■• -a 


Fig.  135. — A  section  through  the  epidermis,  somewhat  diagrammatic,  highly 
magnified.  Below  is  seen  a  papilla  of  the  dermis,  with  its  artery,  /,  and  veins,  gg ; 
a,  the  horny  layer  of  the  epidermis  ;  b,  the  retr  mucosum  or  Malpighian  layer;  d, 
the  layer  of  columnar  epidermic  cells  in  immediate  contact  with  the  dermis  ;'  It,  the 
duct  of  a  sweat-gland. 

corium.     Fine  nerve-fibres  run  into  it  and  end  there  among 
the  cells. 

The  Corium,  Cutis  Vera,  or  True  Skin,  Fig.  136,  consists 
fundamentally  of  a  close  feltwork  of  elastic  and  white  fibrous 
tissue,  which,  becoming  wider  meshed  below,  passes  gradually 
into  the  subcutaneous  areolar  (issue  (Chap.  VI II)  which 
attaches  the  skin  loosely  to  parts  beneath.    In  tanning  it  is  the 


THE  KIDNEYS  AND  SKIN. 


443 


cutis  vera  which  is  turned  into  leather,  its  white  fibrous  tissue 
forming  an  insoluble  and  tough  compound  with  the  tannin 
of  the  oak-bark  employed.  Wherever  there  are  hairs,  bun- 
dles of  plain  muscular  tissue  are  found  in  the  corium  ;  it 
contains  also  a  close  capillary  network  and  numerous  lym- 
phatics and  nerves.  In  shaving,  so  long  as  the  razor  keeps 
in  the  epidermis  there  is  no  bleediug;  but  a  deeper  cut  shows 
at  once  the  vascularity  of  the  true  skin. 

The   outer  surface  of   the  corium   is  almost  everywhere 
raised  into   minute   elevations,  called  the  papillae,  on  which 


m 


d 


/--! 


Fig.   i36.— A  section   through    the   skin   and   subcutaneous  areolar  tissue,      h, 

bornv  stratum,  and  m, deeper  more  opaque  layer  of  the  epidermis;  d,  demur. 

_'  below  into  wr,  loose  areolar  tissue,  with  fat,  f,  in  its  meshes  :  above,  dermic 

papillae  are  seen,  projecting  into  ilie  epidermis  which  is  moulded  on  them,    a, 

opening  of  a  sweat  gland  ;  '//.  the  gland  itw»lf. 

the  epidermis  is  moulded,  so  thai  its  deep  side  presents  pits 
corresponding  to  the  projections  of  the  dermis.  In  Fig.  135 
i-:  shown  a  papilla  of  the  corium  containing  ;i  knot  of  blood- 
jupplied  by  the  small  artery,/,  and  having  the  blood 
carried  <>i\  from  them  by  the  two  little  veins.////.  Other 
papilla?  contain  no  capillary  loops  bul  special  organs  connected 
with  nerve-fibres,  and  supposed  to  be  concerned  in  thesen.se 


444  THE  III  MAX  BODY. 

of  touch  (Chap.  X  X  X  V).  On  t  lie  palmar  surface  of  the  hand 
the  dermic  papilla'  are  especially  well  developed  (as  they  are 

in  most  parts  where  tin:  sense  of  touch  is  acute)  and  are  fre- 
quently compound,  or  branched  at  the  tip.  On  the  front  of 
the  hand,  they  are  arranged  in  rows;  the  epidermis  fills  up  the 
hollows  between  the  papilla-  of  the  same  row,  but  dips  down 
between  adjacent  rows,  and  thus  are  produced  the  finer  ridges 
seen  on  the  palms.  In  many  places  the  corium  is  also  fur-: 
rowed,  as  opposite  the  finger-joints  and  on  the  palm.  Else- 
where such  furrows  are  less  marked,  but  they  exist  over  the 
whole  skin.  The  epidermis  closely  follows  all  the  hollows, 
and  thus  they  are  made  visible  from  the  surface.  The 
wrinkles  of  old  persons  are  due  to  the  absorption  of  subcu- 
taneous fat  and  of  other  soft  parts  beneath  the  skin,  which, 
not  shrinking  itself  at  the  same  rate,  is  thrown  into  folds. 

Hairs.  Each  hair  is  a  long  filament  of  epidermis  devel- 
oped on  the  top  of  a  special  dermic  papilla,  seated  at  the 
bottom  of  a  depression  reaching  down  from  the  skin  into  the 
tissue  beneath,  and  called  the  hair-follicle.  The  portion  of 
a  hair  buried  in  the  skin  is  called  its  root ;  this  is  succeeded 
by  a  stem  which,  in  an  uncut  hair,  tapers  off  to  a  point.  The 
stem  is  covered  by  a  single  layer  of  overlapping  scales  form- 
ing the  hair-cuticle;  the  projecting  edges  of  these  scales  are 
directed  towards  the  top  of  the  hair.  Beneath  the  hair-cuti- 
cle comes  the  cortex,  made  up  of  greatly  elongated  cells 
united  to  form  fibres;  and  in  the  centre  of  the  shaft  there 
is  found,  in  many  hairs,  a  medulla,  made  up  of  more  or  less 
rounded  cells.  The  color  of  hair  is  mainly  dependent  upon 
pigment  granules  lying  between  the  fibres  of  the  cortex. 
Ail  hairs  contain  some  air  cavities,  especially  in  the  medulla. 
They  are  very  abundant  in  white  hairs  and  cause  the  white- 
ness by  reflecting  all  the  incident  light,  just  as  a  liquid  beaten 
into  fine  foam  looks  white  because  of  the  light  reflected  from 
the  walls  of  all  the  little  air  cavities  in  it.  In  dark  hairs  the 
air  cavities  are  few. 

The  hair-follicle  (Fig.  137)  is  a  narrow  pit  of  the  dermis, 
projecting  down  into  the  subcutaneous  areolar  tissue,  and 
lined  by  an  involution  of  the  epidermis.  At  the  bottom  of 
the  follicle  is  a  papilla,  and  the  epidermis,  turning  up  over 
this,  becomes  continuous  with  the  hair.  On  the  papilla  epi- 
dermic cells  multiply  rapidly  so  long  as  the  hair  is  growing, 
and  the  whole  hair  is  there  made  up  of  roundish  cells.     As 


THE  KIDNEYS  AND  SKIN. 


445 


these  get  pushed  up  by  fresh  ones  formed  beneath  them,  the 
outermost  layer  become  fiat-  ~  q 

tened  and  form  the  hair 
cuticle;  several  succeeding 
layers  elongate  and  form 
the  cortex;  while,  in  hairs 
with  a  medulla,  the  middle 
cells  retain  pretty  much 
their  original  form  and  size. 
Pulled  apart  by  the  elongat- 
ing cortical  cells,  these  cen- 
tral   ones    then     form    the 

medulla  with  its  air  Cavities.         Fig.  |137.— Parts  of  two  hairs  imbedded 

.,.,       .  ,    ,  „    . ,  in  their  follicles,    o,  the  skin,  which  is  seen 

1  he  innermost    layer    Of    the  todlp  down  and  line  the  follicle;  b,  thesub- 

•  j  ....  ,i       p   it    i  cutaneous  tissue;  c,  the  musclesof  the  hair- 

epidermiS  lining  the  follicle,  follicle,    which  by  their   contraction    can 

has  its   Cells  projecting,  with  erect  the  hair;  o,  sebaceous  gland. 

overlapping  edges  turned  downwards.  Accordingly  these  inter- 
lock with  the  upward  directed  edges  of  the  cells  of  the  hair- 
cuticle;  consequently  when  a  hair  is  pulled  out  the  epidermic 
lining  of  the  follicle  is  usually  brought  with  it.  So  long  as  the- 
dermic  papilla  is  left  intact  a  new  hair  will  be  formed,  but  not 
otherwise.  Slender  bundles  of  unstriped  muscle  (c,  Fig.  137) 
run  from  the  dermis  to  the  side  of  the  hair-follicles.  The  latter 
are  in  most  regions  obliquely  implanted  in  the  skin  so  that 
the  hairs  lie  down  on  the  surface  of  the  Body,  and  the  mus- 
cles are  so  fixed  that  when  they  shorten,  they  erect  the  hair 
and  cause  it  to  bristle,  as  may  be  seen  in  an  angry  cat,  or 
sometimes  in  a  greatly  terrified  man.  Opening  into  each  hair- 
follicle  are  usually  a  couple  of  sebaceous  or  oil  glands.  Hairs  are 
found  all  over  the  skin  except  on  the  palms  of  the  hands  and 
the  soles  of  the  feet;  the  back  of  the  last  phalanx  of  the  fingers 
and  toes,  the  upper  eyelids,  and  one  or  two  other  regions. 

Nails.  Each  nail  is  a  part  of  the  epidermis,  with  its 
horny  stratum  greatly  developed.  The  back  part  of  the  nail 
tit-  behind  into  a  furrow  of  the  dermis  and  is  called  its  root. 
The  visible  part  consists  of  a  body,  fixed  to  the  dermis  be- 
neath (which  forms  the  bed  of  tin;  nail),  and  of  a  free  ethic 
Near  the  rool  ie  a  little  area  whiter  than  the  rest  of  the  nail 
and  called  the  lunula.  The  whiteness  is  due  in  part  to  the 
nail  being  really  more  opaque  there  and  partly  to  the  fact 
that  its  bed,  which  seen  through  the  nail  causes  its  pink 
color,  is  in  this  region  less  vascular. 


446 


THE  HUMAN  BOD  Y. 


The  portion  of  the  corium  on  which  the  nail  is  formed 
is  called  its  matrix.  Posteriorly  this  forms  a  furrow  lodging 
the-  root,  and  it  is  by  new  cells  added  on  there  that  the  nail 
grows  in  length.  The  part  of  the  matrix  lying  beneath  the 
body  of  the  nail,  and  called  its  bed,  is  highly  vascular  and 
raised  up  into  papillae  which,  except  in  the  region  of  the 
lunula,  arc  arranged  in  longitudinal  rows,  slightly  diverging 
as  they  run  towards  the  tip  of  the  finger  or  toe.  It  is  by 
new  cells  formed  on  its  bed  and  added  to  its  under  surface 
that  the  nail  grows  in  thickness,  as  it  is  pushed  forward  by 
the  new  growth  in  length  at  its  root.  The  free  end  of  a 
nail  is  therefore  its  thickest  part.  If  a  nail  is  "  cast "  in 
consequence  of  an  injury,  or  torn  off,  a  new  one  is  produced, 
provided  the  matrix  is  left. 

The  Glands  of  the  Skin  are  of  two  kinds,  the  sudo- 
riparous or  sweat  glands,  and  the  sebaceous  or  oil  gland*. 
The  former  belong  to  the  tubular,  the  latter  to  the  race- 
mose type.  The  sweat-glands,  Fig.  138,  lie  in  the  subcu- 
taneous tissue,  where  they  form  little  globular  masses  com- 
posed of  a  coiled  tube.  From  the  coil  a  duct  (sometimes 
double)  leads  to  the  surface,  being  usually 
spirally  twisted  as  it  passes  through  the  epi- 
dermis. The  secreting  part  of  the  gland 
consists  of  a  connective-tissue  tube,  continu- 
ous along  the  duct  with  the  dermis;  within 
this  is  a  basement  membrane;  and  the  final 
secretory  lining  consists  of  several  layers  of 
gland-cells.  A  close  capillary  network  inter- 
twines with  the  coils  of  the  gland.  Sweat- 
glands  are  found  on  all  regions  of  the  skin, 
but  more  closely  set  in  some  places,  as  the 
palms  of  the  hands  and  on  the  brow,  than 
elsewdiere:  there  are  altogether  about  two 
and  a  half  millions  of  them  opening  on  the 
surface  of  the  Body. 

The  sebaceous  glands  nearly  always  open 
into  hair-follicles,  and  are  tound  wherever 
there  are  hairs.  Each  consists  of  a  duct 
opening  near  the  mouth  of  a  hair-follicle 
and  branching  at  its  other  end:  the  final 
branches  lead  into  globular  secreting  saccules, 
-which,  like  the  ducts,  are  lined  with  epithelium.     In  the 


Fig.  188.— A  sweat 
gland.  (/.  horny 
layer  of  cuticle;  c, 
Malpighian  layer;  6, 
dermis.  The  coils 
of  the  gland  proper, 
imbedded  in  the  sub- 
cutaneous fat,  are 
seen  below  the  der- 
mis. 


THE  KIDNEYS  AND  SKIN.  447 

saccules  the  substance  of  the  cells  becomes  charged  with  oil- 
drops,  the  protojjlasm  disappearing;  and  finally  the  whole 
cell  falls  to  pieces,  its  detritus  constituting  the  secretion. 
New  cells  are,  meanwhile,  formed  to  take  the  place  of  those 
destroyed.  Usually  two  glands  are  connected  with  each  hair- 
follicle,  but  there  may  be  three  or  only  one.  A  pair  of  seba- 
ceous glands  are  represented  on  the  sides  of  each  of  the  hair- 
follicles  in  Fig.  137. 

The  Skin  Secretions.  The  skin  besides  forming  a  pro- 
tective covering  and  serving  as  a  sense-organ  (Chap.  XXXV) 
also  plays  an  important  part  in  regulating  the  temperature  of 
the  Body,  and,  as  an  excretory  organ,  in  carrying  off  certain 
waste  products. 

The  sweat  poured  out  by  the  sudoriparous  glands  is  a 
transparent  colorless  liquid,  with  a  peculiar  odor,  varying  in 
different  races  and,  in  the  same  individual,  in  different  regions 
of  the  Body.  Its  quantity  in  twenty-four  hours  is  subject  to 
great  variations,  but  usually  lies  between  700  and  2000  grams 
(10,850  and  31,000  grains).  The  amount  is  influenced  mainly 
by  the  surrounding  temperature,  being  greater  when  this  is 
high;  but  it  is  also  increased  by  other  things  tending  to 
raise  the  temperature  of  the  Body,  as  muscular  exercise. 
The  sweat  may  or  may  not  evaporate  as  fast  as  it  is  secreted; 
in  the  former  case  it  is  known  as  insensible,  in  the  latter  as 
sensible  perspiration.  By  far  the  most  passes  off  in  the  in- 
sensible form,  drops  of  sweat  only  accumulating  when  the 
secretion  is  very  profuse,  or  the  surrounding  atmosphere  so 
humid  that  it  does  not  readily  take  up  more  moisture.  The 
perspiration  is  acid,  and  in  1000  parts  contains  990  of  water 
to  10  of  solids.  Among  the  latter  are  found  urea  (1.5  in 
1000),  fatty  acids,  sodium  chloride,  and  other  salts.  In  dis- 
eased conditions  of  the  kidneys  the  urea  may  be  greatly 
increased,  the  skin  supplementing  to  a  certain  extent  defi- 
ciencies of  those  organs. 

The  Nervous  and  Circulatory  Factors  in  the  Sweat 
Secretion.  It  used  to  be  believed  that  an  increased  flow  of 
blood  through  the  skin  would  suffice  of  itself  to  cause  in- 
creased perspiration;  but  against  this  view  are  the  facts  that, 
in  terror  f,,r  example,  there  may  be  profuse  sweating  with  a 
cold  pallid  skin;  and  that  in  many  febrile  states  the  skin  may 
be  li"'  and  its  vessels  full  of  blood,  and  yet  there  may  be  no 
sweating. 


448  THE  HUMAN  B(J1>  7. 

Direct  experimenl  Bhows  thai  the  secretory  activity  of 
the  sweat --lands  is  under  immediate  control  of  nerve-fibres, 
and  is  only  indirectly  dependent  <>n  the  blood-supply  in  their 
neighborhood.  Stimulating  the  sciatic  nerve  of  the  freshly 
amputated  leg  of  a  cat  will  cause  the  balls  of  its  feel  to 
sweat,  although  there  is  no  Mood  flowing  through  the  Limb. 
On  the  other  hand,  if  the  sciatic  nerve  be  cut  so  as  to  para* 
lyze  it,  in  a  living  animal,  the  skin  arteries  dilate  and  the 
foot  gets  more  blood  and  becomes  wanner;  but  it  does  not 
sweat.  The  sweat-fibres  originate  in  certain  sweat-centres  iu 
the  spinal  cord,  which  may  either  be  directly  excited  by 
blood  of  a  higher  temperature  than  usual  flowing  through 
them  or,  reflexly,  by  warmth  acting  on  the  exterior  of  the 
Body  and  stimulating  the  sensory  nerves  there.  Both  of 
these  agencies  commonly  also  excite  the  vaso-dilator  nerves 
of  the  sweating  part,  and  so  the  increased  blood-supply  goes 
along  with  the  secretion;  but  the  two  phenomena  are  funda- 
mentally independent. 

The  Sebaceous  Secretion.  This  is  oily,  semifluid,  and 
of  a  special  odor.  It  contains  about  50  per  cent  of  fats  (olein 
and  palmatin).  It  lubricates  the  hairs  and  usually  renders 
them  glossy,  even  in  persons  who  use  none  of  the  various 
compounds  sold  as  "  hair-oil.''  No  doubt,  too,  it  gets  spread 
more  or  less  over  the  skin  and  makes  the  cuticle  less  permea- 
ble by  water.  Water  poured  on  a  healthy  skin  does  not  wet 
it  readily  but  runs  off  it,  as  "  off  a  duck's  back  "  though  to  a 
less  marked  degree. 

Hygiene  of  the  Skin.  The  sebaceous  secretion,  and  the 
solid  residue  left  by  evaporating  sweat,  constantly  form  a 
solid  film  over  the  skin,  which  must  tend  to  choke  the 
mouths  of  the  sweat-glands  (the  so-called  "pores"  of  the 
skin)  and  impede  their  activity.  Hence  the  value  to  health 
of  keeping  the  skin  clean:  a  daily  bath  should  be  taken  by 
every  one.  "Women  cannot  well  wash  their  hair  daily  as  it 
takes  so  long  to  dry,  but  a  man  should  immerse  his  head 
when  he  takes  his  bath.  As  a  general  rule,  soap  should  only 
be  used  occasionally;  it  is  quite  unnecessary  for  cleanliness, 
except  on  exposed  parts  of  the  Body,  if  frequent  bathing  be  a 
habit  and  the  skin  be  well  rubbed  afterwards  until  dry. 
Soap  nearly  always  contains  an  excess  of  alkali  which  in  itself 
injures  some  skins,  and,  besides,  is  apt  to  combine  chemically 
with  the  sebaceous  secretion  and  carry  it  too  freely  away. 


THE  KIDNEYS  AND  SKIN.  449 

Persons  whose  skin  will  not  stand  soap  can  find  a  good  sub- 
stitute, for  washing  the  hands  and  face,  in  a  little  corn  meal. 
Xo  doubt  many  folk  go  about  in  very  good  health  with  very 
little  washing;  contact  with  the  clothes  and  other  external 
objects  keeps  its  excretions  from  accumulating  on  the  skin 
to  any  very  great  extent.  But  apart  from  the  duty  of  per- 
sonal cleanliness  imposed  on  man  as  a  social  animal  in  daily 
intercourse  with  others,  the  mere  fact  that  the  healthy  Body- 
can  manage  to  get  along  under  unfavorable  conditions  is  no 
reason  for  exposing  it  to  them.  A  clogged  skin  throws  more 
work  than  their  fair  share  on  the  lungs  and  kidneys,  and  the 
evil  consequences  may  be  experienced  any  day  when  some- 
thing else  puts  another  extra  strain  on  them. 

Animals,  a  considerable  portion  of  whose  skin  has  been 
varnished,  die  within  a  few  hours.  This  used  to  be  thought 
due  to  poisoning  by  retained  ingredients  of  the  sweat.  But 
the  main  cause  of  death  seems  to  be  an  excessive  radiation 
of  heat  from  the  surface  of  the  body,  dependent  mainly  on 
dilatation  of  the  cutaneous  vessels  caused  by  the  varnish, 
though  possibly  the  retention  of  some  poisonous  substance 
usually  excreted  by  the  skin  may  have  an  influence.  The 
bodily  temperature  falls  in  consequence  of  the  great  loss  of 
heat  until  it  reaches  the  fatal  point,  about  20°  C.  (68°  F.)  for 
rabbits.  If  the  animal  be  packed  in  raw  cotton  or  kept  in 
an  atmosphere  at  a  temperature  of  30°  C.  (86°  F.)  it  does  not 
die  as  a  consequence  of  the  varnishing,  or  at  least  not  nearly 
so  soon  as  it  would  otherwise  die. 

Bathing.  The  general  subject  of  bathing  may  be  consid- 
ered here.  One  object  of  it  is  that  above  mentioned — to 
cleanse  the  skin;  but  it  is  also  useful  to  strengthen  and  in- 
vigorate the  whole  frame.  For  strong  healthy  persons  a  cold 
bath  is  the  best,  except  in  extremely  severe  weather,  when  the 
temperature  of  the  water  should  be  raised  to  15°*C.  (about 
60  K.i.  at  which  it  still  feels  quite  cold  to  the  surface.  The 
first  effect  of  a  cold  bath  is  to  contract  all  the  skin-vessels 
and  make  the  surface  pallid.  This  is  soon  followed  by  a 
reaction,  in  which  the  skin  becomes  red  and  congested,  and  a 
glow  of  warmth  is  felt  in  it.  The  proper  time  to  come  out  is 
while  this  reaction  lasts,  and  after  emersion  it  should  be  pro- 
moted by  a  good  nib.  If  the  stay  in  the  cold  water  be  too 
prolonged  the  state  of  reaction  passes  off,  the  skin  becomes 
cold     and    pale    and    the  person    feels   chilly,   uncomforta- 


•450  THE  HUMAN  BODY. 

ble,  and  depressed  all  day.  Then  bathing  is  injurious  instead 
of  beneficial;  it  lowers  instead  of  stimulating  the  activities 
of  the  Body.  How  long  a  stay  in  the  cold  water  may  be 
made  with  benefit  depends  greatly  on  the  individual :  a  vigor- 
ous man  ca)i  bear  and  set  up  a  healthy  reaction  after  much 
longer  immersion  than  a  feeble  one;  moreover,  being  used  to 
cold  bathing  renders  a  longer  stay  safe,  and,  of  course,  the 
temperature  of  the  water  has  a  great  influence:  water  called 
"cold"  may  vary  within  very  wide  limits  of  temperature,  as 
indicated  by  the  thermometer;  and  the  colder  it  is  the  shorter 
is  the  time  which  it  is  wise  to  remain  in  it.  Persons  who  in 
the  comparatively  warm  water  of  Narragansett  during  the 
summer  months  stay  with  benefit  and  pleasure  in  the  sea, 
have  to  content  themselves  with  a  single  plunge  on  parts  of 
the  coast  where  the  water  is  colder.  The  nature  of  the  water 
has  some  influence;  the  salts  contained  in  sea-water  stimu- 
late the  skin-nerves  and  promote  the  afterglow.  Many  per- 
sons who  cannot  stand  a  simple  cold  fresh-water  bath  take 
one  with  benefit  when  some  salines  are  previously  dissolved 
in  the  water.  The  best  for  this  purpose  are  probably  those 
sold  in  the  shops  under  the  name  of  "sea-salts." 

It  is  perfectly  safe  to  bathe  when  warm,  provided  the  skin 
is  not  perspiring  profusely,  the  notion  commonly  prevalent  to 
the  contrary  notwithstanding.  On  the  other  hand,  no  one 
should  enter  a  cold  bath  when  feeling  chilly,  or  in  a  depressed 
vital  condition.  It  is  not  wise  to  take  a  bath  immediately 
after  a  meal,  since  the  afterglow  tends  to  draw  away  too 
much  blood  from  the  digestive  organs,  which  are  then  ac- 
tively at  work.  The  best  time  for  a  long  bath  is  about  three 
hours  after  breakfast;  but  for  an  ordinary  daily  dip,  lasting 
but  a  short  time,  there  is  no  better  period  than  on  rising  and 
while  still  warm  from  bed. 

The  shower-bath  abstracts  less  heat  from  the  skin  than  an 
ordinary  cold  bath  and,  at  the  same  time,  gives  it  a  greater 
stimulus:  hence  it  has  certain  advantages. 

Persons  in  feeble  health  may  diminish  the  shock  to  the 
system  by  raising  the  temperature  of  the  water  they  bathe  in 
up  to  any  point  at  which  it  still  feels  cool  to  the  skin.  Bath- 
ing in  water  which  feels  hot  is  not  advisable:  it  tends  gen- 
erally to  diminish  the  vital  activity  of  the  Body.  Hence  warm 
baths  should  only  be  taken  occasionally  and  for  special  pur- 
poses, other  than  mere  luxury. 


CHAPTEE  XXIX. 
NUTRITION. 

The  Problems  of  Animal  Nutrition.  We  have  iu  pre- 
ceding chapters  traced  certain  materials,  consisting  of  foods 
more  or  less  changed  by  digestion,  into  the  Body  from  the 
alimentary  canal,  and  oxygen  into  it  from  the  lungs.  We 
have  also  detected  the  elements  thus  taken  into  the  Body  in 
their  passage  out  of  it  again  by  lungs,  kidneys,  and  skin;  and 
found  that  for  the  most  part  their  chemical  state  was  differ- 
ent from  that  in  which  they  entered;  the  difference  being 
expressible  in  general  terms  by  saying  that  more  oxidized 
forms  of  matter  leave  the  Body  than  go  into  it.  We  have  now 
to  consider  what  happens  to  each  food  during  the  journey 
through  the  Body :  is  it  changed  at  all  ?  is  it  oxidized  ?  if  so 
where  ?  what  products  does  its  oxidation  give  rise  to  ?  Is 
the  oxidation  direct  and  complete  at  once,  or  does  it  occur  in 
successive  steps  ?  Has  the  food  been  used  first  to  make  part 
of  a  living  tissue  and  is  that  then  oxidized;  or  has  it  been 
oxidized  without  forming  part  of  a  living  tissue  ?  if  so, 
where?  in  the  blood  stream,  or  outside  of  it?  Finally,  if 
the  chemical  changes  undergone  in  the  Body  are  such  as 
to  liberate  energy,  how  has  this  energy  been  utilized  ?  to 
maintain  the  temperature  of  the  Body  or  to  give  rise  to  mus- 
cular work,  or  for  other  purposes  ?  This  is  a  long  string  of 
questions,  the  answers  to  many  of  which  Physiology  has  still 
to  seek. 

The  Seat  of  the  Oxidations  of  the  Body.  According 
to  elder  views  oxidation  mainly  took  place  in  the  blood  while 
flowing  through  the  lungs.  Those  organs  were  considered  a 
BOrl  of  furnace  in  which  heat  was  liberated  by  blood  oxidation, 
;md  then  distributed  by  the  circulation.  But  if  this  were  so 
the  lunge  onghl  to  be  the  hottest  parts  of  the  Body,  and  the 
blood  leaving  them  by  the  pulmonary  veins  much  hotter  than 
that  brought  to  them  by  the  pulmonary  artery  after  it  had 
been  cooled  by  warming  all  the  tissues;  and  neither  of  these 

451 


452  THE  HUMAN  BODY. 

things  is  true.  A  small  amount  of  heat  is  liberated  when 
haemoglobin  combines  with  oxygen  in  the  pulmonary  capil- 
laries, but  the  affinities  thus  satisfied  are  so  feeble  that  the 
energy  liberated  is  trivial  in  amount  when  compared  with 
t  hat  set  free  when  this  oxygen  subsequently  forms  stabler  com. 
pounds  elsewhere.  There  is  good  reason  to  believe  that 
hardly  any  of  this  latter  class  of  oxidations  occurs  in  the 
living  circulating  blood  at  all;  its  cells  do,  no  doubt,  use  up 
some  oxygen  and  set  free  some  carbon  dioxide;  but  not 
enough  to  be  detected  by  ordinary  methods  of  analysis.  The 
percentage  of  oxygen  liberated  in  a  vacuum  by  two  specimens 
of  the  blood  of  an  animal,  taken  one  from  an  artery  near  the 
heart,  and  the  other  from  a  distant  one,  are  practically  the 
same;  showing  that  during  the  time  occupied  in  flowing  two 
or  three  feet  through  an  artery  the  blood  uses  up  no  appreci- 
able quantity  of  its  own  oxygen;  while  in  its  brief  capillary 
transit  it  almost  suddenly  loses  so  much  oxygen  as  to  become 
venous.  The  difference  is  explained  by  the  fact  that  the 
blood  gives  off  oxygen  gas  through  the  thin  capillary  walls 
to  the  surrounding  tissues;  and  in  the  latter  the  oxidation 
takes  place.  As  we  have  already  seen,  a  freshly  excised 
muscle  deprived  of  blood  can  still  be  made  to  contract ;  and  for 
some  considerable  time  if  it  be  the  muscle  of  a  cold-blooded 
animal.  During  its  contraction  it  evolves  large  amounts 
of  carbon  dioxide,  although  the  resting  fresh  muscle  contains 
hardly  any  of  that  gas.  Here  we  have  direct  evidence  of 
oxidation  taking  place  in  a  living  tissue  and  in  connection 
with  its  functional  activity;  and  what  is  true  of  a  muscle 
is  probably  true  of  all  tissues:  the  oxidations  which  supply 
them  with  energy  take  place  within  the  living  cells  themselves. 
The  statement  frequently  made  that  the  oxygen  in  the  cir- 
culating blood  exists  as  ozone,  rests  on  no  sufficient  basis; 
decomposing  haemoglobin  does  ozonize  some  oxygen  when 
exposed  to  the  air,  but  there  is  no  ozone  in  fresh  blood.  Ex- 
periments made  by  adding  various  combustible  substances,  as 
sugar,  to  newly  drawn  blood,  also  fail  to  prove  the  occurrence 
of  any  oxidation  of  such  bodies  in  that  liquid. 

Tissue-Building  and  Energy-Yielding  Foods.  The 
Human  Body,  like  that  of  other  animals,  is,  on  the  whole, 
chemically  destructive;  it  takes  in  highly  complex  substances 
as  food,  and  eliminates  their  elements  in  much  simpler 
compounds,  which   can  again  be  built   up  to  their  original 


NUTRITION.  453 

condition  by  plants.  Nevertheless  the  Body  has  certain  con- 
structive powers:  it,  at  least,  builds  up  protoplasm  from 
proteids  and  other  substances  received  from  the  exterior; 
and  there  is  reason  to  believe  it  does  a  good  deal  more  of  the 
same  kind  of  work,  though  never  an  amount  equalling  its 
chemical  destructions.  Given  one  single  proteid  in  its  food, 
say  egg  albumen,  the  Body  can  do  very  well;  making  serum 
albumen  and  paraglobulin  out  of  it  for  the  blood,  myosinogen 
for  the  muscles,  and  so  on :  in  such  cases  the  original  proteid 
must  have  been  taken  more  or  less  to  pieces,  remodelled,  and 
built  up  again  by  the  living  tissues;  and  it  is  extremely 
doubtful  if  anything  different  occurs  in  other  cases,  when 
the  proteid  eaten  happens  to  be  one  found  in  the  Body.  In 
fact,  during  digestion  the  proteids  are  broken  down  some- 
what and  turned  into  peptones;  in  this  state  they  are  absorbed 
and  must  somewhere  again  be  built  up  into  the  proteids  of 
the  tissues. 

The  constructive  powers  of  the  Body  used  to  be  rather 
too  much  ignored.  Foods  were  divided  into  assimilable  and 
combustible,  the  former  serving  directly  to  renew  the  organs 
or  tissues  as  they  were  used  up,  or  to  supply  materials  for 
growth;  these  were  mainly  proteids  and  fats;  no  special 
chemical  syntbesis  was  thus  supposed  to  take  place,  the  living 
cells  being  nourished  by  the  reception  from  outside  of  mole- 
cules similar  to  those  they  had  lost.  Fat-cells,  it  was  sup- 
posed, grew  by  picking  up  fatty  molecules  like  their  contents, 
received  from  the  food;  and  albumen-rich  tissues  by  the  re- 
ception of  ready-made  proteid  molecules,  needing  no  further 
manufacture  in  the  cell.  The  combustible  foods,  on  the  other 
hand,  were  the  carbohydrates  and  some  fat:  the  carbohy- 
drates, according  to  the  hypothesis,  were  incapable  of  being 
made  into  parts  of  a  living  tissue,  and  were  merely  oxidized 
in  order  to  maintain  the  bodily  warmth.  It  having  been 
proved,  however,  that  more  fat  might  accumulate  in  the  body 
of  an  animal  than  was  taken  in  its  food,  this  excess  was  ac- 
counted for  by  supposing  it  was  due  to  excess  of  com- 
bustible Foods,  converted  into  fats  and  stored  away  as  oil- 
droplete  in  various  cells;  but  not  actually  built  up  into  true 
living  adipose  tissue.  Liebig,  somewhat  similarly,  classed  all 
food-  into  plastic,  concerned  in  making  new  tissue,  mid 
respiratory,  directly  oxidized  before  they  ever  constituted 
part  of  a  tissue.     The  plastic  foods  were  the  proteids,  but 


454  THE  III  MAN  BODY. 

these  also  indirectly  gave  rise  to  the  energy  expended  in 
muscular  work,  and  to  some  heat:  the  proteid  muscular  fibre 
being  broken  first  into  a  highly  nitrogenous  part  (urea,  or 
some  body  well  on  the  road  to  become  urea)  and  a  non-nitro- 
genized  richly  hydrocarbonous  part  ;  and  this  latter  was  then, 
oxidized  and  gave  rise  to  heat.  Several  facts  may  be  urged 
against  this  view:  (1)  Men  in  tropical  climates  live  mainly 
on  non-proteid  foods,  yet  their  chief  needs  are  not  heat  pro- 
duction, but  tissue  formation  and  muscular  work:  according 
to  Liebig's  view  their  diet  should  be  mainly  nitrogenous. 
(•.')  Carnivorous  animals  live  on  a  diet  very  rich  in  proteids, 
nevertheless  develop  plenty  of  animal  heat,  and  that  without 
doing  the  excessive  muscular  work  which,  on  Liebig's  theory, 
must  first  be  gone  through  in  order  to  break  up  the  proteids,. 
with  the  production  of  a  non-azotized  part  which  could  then 
be  oxidized  for  heat-production.  (3)  Great  muscular  work 
can  be  done  on  a  diet  poor  in  proteids;  beasts  of  burden  are 
for  the  most  part  herbivorous.  (4)  Further,  we  know  exactly 
how  much  energy  can  be  liberated  by  the  oxidation  of  pro- 
teids to  that  stage  which  occurs  in  the  Body;  and  it  is  pos- 
sible to  estimate  with  considerable  accuracy  the  amount  of 
urea  and  uric  acid  excreted  in  a  given  time;  from  their  sum 
the  amount  of  proteid  oxidized  and  the  amount  of  energy 
liberated  in  that  oxidation  can  be  calculated;  if  this  be  done 
it  is  found  that,  nearly  always,  the  muscular  work  done  dur- 
ing the  same  period  represents  far  more  energy  expended 
than  could  be  yielded  by  the  proteids  broken  down. 

The  Source  of  the  Energy  Expended  in  Muscular  Work. 
This  important  question,  which  was  postponed  in  the  chap- 
ters dealing  with  the  physiology  of  the  muscular  tissues, 
needs  now  consideration.  It  may  be  put  thus  :  Does  a 
•muscle-fibre  work  by  the  oxidation  of  its  proteids,  i.e.  by 
breaking  them  down  into  compounds  which  are  then  re- 
moved from  it  and  conveyed  out  of  the  Body  ?  or  does  it 
work  by  the  energy  liberated  by  the  oxidation  of  carbon  and 
hydrogen  compounds  only?  The  problem  maybe  attacked 
in  two  ways:  first,  by  examining  the  excretions  of  a  man,  or 
other  animal,  during  work  and  rest;  second,  by  examining 
directly  the  chemical  changes  produced  in  a  muscle  when  it 
contracts.  Both  methods  point  to  the  same  conclusion,  viz., 
that  proteid  oxidation  is  not  the  source  of  the  mechanical 
energy  expended  by  the  Body. 


NUTRITION.  455 

One  gram  (15.5  grains)  of  pure  albumen  when  completely 
burnt  liberates,  as  heat,  an  amount  of  energy  equal  to  2117 
kilogrammeters  (15,270  foot-pounds).  But  in  the  Body  pro- 
teids  are  not  fully  oxidized  ;  part  of  their  carbon  is,  to  form 
carbon  dioxide,  and  part  of  the  hydrogen,  to  form  water; 
but  some  carbon  and  hydrogen  pass  out,  combined  with  ni- 
trogen and  oxygen,  in  the  incompletely  oxidized  state  of  urea. 
Therefore  all  of  the  energy  theoretically  obtainable  is  not  de- 
rived from  proteids  in  the  Body:  from  the  above  full  amount 
for  each  gram  of  proteid  we  must  take  the  quantity  carried 
oft  in  the  urea,  which  will  be  the  amount  liberated  when  that 
urea  is  completely  oxidized.  Each  gram  (15.5  grains)  of 
proteid  oxidized  in  the  Body  gives  \  of  a  gram  (5.14  grains) 
of  urea ;  since  one  gram  of  urea  liberates,  on  oxidation, 
energy  amounting  to  934  kilogrammeters  (G740  foot-pounds), 
each  gram  of  proteid  oxidized,  so  far  as  is  possible  in  the 
Body,  will  yield  during  the  process  2117  —  ^f  *  =  1805.7  kilo- 
grammeters (13,037  foot-pounds)  of  energy.  Knowing  that 
urea  carries  off  practically  all  the  nitrogen  of  proteids  broken 
up  in  the  Body,  and  contains  46.6  per  cent  of  nitrogen,  while 
proteids  contain  16  per  cent,  it  is  easy  to  find  that  each  gram 
of  urea  represents  the  decomposition  of  about  2.80  grams  of 
proteid  and,  therefore,  the  liberation  of  5060.00  kilogram- 
meters (36,533.0  foot-pounds)  of  energy.  If,  therefore,  we 
know  how  much  urea  a  man  excretes  during  a  given  time, 
and  how  much  mechanical  work  he  does  during  the  same 
time,  we  can  readily  discover  if  the  latter  could  possibly  have 
been  done  by  the  energy  set  free  by  proteid  decomposition. 
Let  us  take  a  special  case.  Fick  and  Wislecenus,  two  Ger- 
man observers,  climbed  the  Faulhorn  mountain,  which  is 
1956  meters  (about  6415  feet)  high.  Fick  weighed  66  kilo- 
grams and,  therefore,  in  lifting  his  Body  alone,  did  during 
the  ascent  129,096  kilogrammeters  (932,073  foot-pounds)  of 
work.  Wislecenus,  who  weighed  76  kilograms,  did  similarly 
148,656  kilogrammeters  (1,073,296  foot-pounds)  of  work. 
But  during  the  ascent,  and  for  five  hours  afterwards,  Fick 
secreted  urine  containing  urea  answering  only  to  37.17  grams 
of  proteid,  and  Wislecenus  urea  answering  to  37  grams. 
Since  each  gram  of  proteid  broken  up  in  the  Body  liberates 
J 805.7  kilogrammeters  (13,0:57  foot-pounds)  of  energy,  the 
amount  that  Fick  could  possibly  have  obtained  from  such  a 
source    is    1805.7x37.17  =  67,117  kilogrammeters   (484,584 


456  THE  HUMAN  BODY. 

foot-pounds),  and  Wislecenus  L805.7  X  37  =  66,810  kilo- 
grammeters.  If  to  the  muscular  work  done  in  actually  rais- 
ing their  bodies,  we  add  that  done  simultaneously  by  the 
heart  and  the  respiratory  muscles,  and  in  such  movements 
of  the  limbs  as  were  not  actually  concerned  in  lifting  the 
weight,  we  should  have,  at  least,  to  double  the  above  total 
muscular  work  done  ;  and  the  amount  of  energy  liberated 
meanwhile  by  proteid  oxidation,  becomes  utterly  inadequate 
for  its  execution.  It  is  thus  clear  that  muscular  work  is  not 
wholly  done  at  the  expense  of  the  oxidation  of  muscle  pro- 
teid; and  it  is  very  probable  that  none  is  so  done  under  ordi- 
nary circumstances,  for  the  urea  excretion  during  rest  is 
about  as  great  as  that  during  work,  if  the  diet  remain  the 
same:  if  the  work  be  very  severe,  as  in  long-distance  walking- 
matches,  the  urea  quantity  is  sometimes  temporarily  raised, 
but  this  increase,  which  no  doubt  represents  an  abnormal 
wear  and  tear  of  muscle-fibre,  is  probably  independent  of  the 
liberation  of  energy  in  the  form  in  which  a  muscle  can  use  it, 
more  likely  taking  the  form  of  heat  ;  and  is,  moreover,  com- 
pensated for  afterwards  by  a  diminished  urea  excretion. 
Thus,  hourly,  before  the  ascent  Fick  and  Wislecenus  each 
excreted  on  the  average  about  4  grams  (62  grains)  of  urea  ; 
during  the  ascent  between  7  and  8  grams  (108-124  grains); 
but  during  the  subsequent  16  hours,  when  any  urea  formed 
in  the  work  would  certainly  have  reached  the  urine,  only  an 
average  of  about  3  grams  (46.5  grains)  per  hour. 

It  may  still  be  objected,  however,  that  a  good  deal  of  the 
muscle  work  may  be  done  by  the  energy  of  oxidized  muscle 
proteid ;  that  the  amount  of  this  oxidation  occurring  in  a 
muscle  during  rest  or  ordinary  work  is  pretty  constant  and 
simply  takes  different  forms  in  the  two  cases,  much  as  a 
steam-engine  with  its  furnace  in  full  blast  will  burn  as  much 
coal  when  resting  as  when  working,  but  in  the  former  case 
lose  all  the  generated  euergy  in  the  form  of  heat,  and  in  the 
latter  partly  as  mechanical  work.  Thus  the  small ness  of  in- 
crease in  urea  excretion  as  a  consequence  of  muscular  activity 
could  be  explained,  while  still  a  good  deal  of  utilizable  energy 
might  come  from  proteid  degradation.  But  if  this  were  so, 
then  the  working  Body  should  eliminate  no  more  carbon 
dioxide  than  the  resting;  the  amount  of  chemical  changes  in 
its  muscles  being  by  hypothesis  the  same,  the  carbon  dioxide 
eliminated  should  not  be  increased.     Experiment,  however, 


NUTRITION.  457 

shows  that  it  is,  and  that  to  a  very  large  extent,  even  when 
the  work  done  is  quite  moderate  and  falls  within  the  limits 
which  could  be  performed  by  the  normal  proteid  degradation 
of  the  Body.  Quite  easy  muscular  work  doubles  the  carbon 
dioxide  excreted  in  twenty-four  hours,  and  in  a  short  period 
of  very  hard  work  it  may  rise  to  five  times  the  amount  elimi- 
nated during  rest.  Since  the  urea  is  not  increased,  or  but 
slightly  increased,  at  the  same  time,  this  carbon  dioxide  can- 
not be  due  to  increased  proteid  metamorphosis;  and  it  there- 
fore indicates  that  a  muscle  works  by  the  oxidation  of  car- 
bonaceous non-nitrogenous  compounds.  Since  all  the  carbon 
compounds  oxidized  in  the  Body  contain  hydrogen  this 
element  is  also  no  doubt  oxidized  during  muscular  work;  but 
the  estimation  of  the  amount  so  used  is  difficult  and  has 
not  been  satisfactorily  made,  because  the  Body  contains  so 
much  water  ready  formed  that  a  large  quantity  is  always 
ready  for  increased  evaporation  from  the  lungs  and  skin, 
whenever  the  respirations  are  quickened,  as  they  are  by 
exercise.  It,  thus,  is  very  difficult  to  say  how  much  of  the 
extra  water  eliminated  from  the  Body  during  work  is  due 
merely  to  this  cause  and  how  much  to  increased  hydrogen 
oxidation. 

The  conclusion  we  are  led  to  is,  then,  that  a  muscle 
works  by  the  oxidation  mainly,  if  not  entirely,  of  carbon  and 
hydrogen;  and  that  the  proteid  constituents  of  the  living 
muscle  substance  are  essentially  the  machinery  determining 
in  what  way  the  energy  shall  be  spent:  they  may  and  do 
suffer  some  wear  and  tear,  but  this  bears  no  direct  proportion 
to  the  work  done;  as  a  steam-engine  may  rust,  so  muscle 
proteid  may  and  does  oxidize,  but  not  to  supply  the  organ 
with  energy  for  use.  This  conclusion,  arrived  at  by  a  study 
of  t  lie  excretions  of  the  whole  Body,  is  confirmed  by  the  re- 
sults obtained  by  the  chemical  study  of  a  single  muscle. 
A  fresh  frog's  muscle  (which  agrees  in  all  essential  points 
with  a  man's)  contains  practically  no  carbon  dioxide,  yet 
made  to  work  in  a,  vacuum  gives  off  that  gas,  and  more  the 
more  it  works.  Some  carbon  dioxide  is  therefore  formed  in 
the  working  muscle.  If  the  muscle,  after  contracting  as  long 
a-  it  can  I"'  made  to  do  so,  be  thrown  into  death  rigor  it 
gives  off  more  carbon  dioxide;  and  if  taken  perfectly  fresh 
and  sent,  into  rigor  mortis  without  contracting,  it  gives  off 
carbon   dioxide  also,  in   amount  equal  to  the  sum  of  that 


4."i8  THE  HUMAN  BODY. 

which  it  would  have  given  off  in  two  stages,  if  first  worked 
and  then  sent  into  rigor.  The  muscle  must,  therefore,  con- 
tain a  certain  store  of  a  carbon-dioxide-yielding  body,  and 
the  decomposition  of  this  is  associated  with  the  occurrence 
both  of  muscular  activity  and  death  stiffening.  Similar 
things  are  true  of  the  acid  simultaneously  developed;  the 
muscle  when  it  works  produces  some  sarcolactic  acid,  and 
when  thrown  into  rigor  mortis  still  more.  No  increase  of 
urea  or  kreatin  or  any  similar  product  of  nitrogenous  de- 
composition is  found  in  a  worked  muscle  when  compared 
with  a  rested  one,  but  the  total  carbohydrates  are  rather  less 
in  the  former.  These  facts  make  it  clear  that  muscular  work 
is  not  done  at  the  expense  of  proteid  oxidation;  and  we  have 
already  seen  (Chap.  XXVI)  that  the  oxygen  a  muscle  uses  in 
contracting  is  not  taken  up  by  it  at  the  time  it  is  used,  since 
a  muscle  containing  no  oxygen  will  still  contract  in  a  vacuum 
and  form  carbon  dioxide.  It  is  probable  that  the  chemical 
phenomena  occurring  in  contraction  and  rigor  are  essentially 
the  same;  the  death  stiffening  results  when  they  occur  to  an 
extreme  degree.  Provisionally  one  may  explain  the  facts  as 
follows:  A  muscle  in  the  Body  takes  up  from  the  blood, 
oxygen,  proteids,  and  non-nitrogenous  (carbohydrate  or  fatty) 
substances.  These  it  builds  up  into  a  highly  complex  and 
very  unstable  compound,  comparable,  for  example,  to  nitro- 
glycerine. When  the  muscle  is  stimulated  this  falls  down 
into  simpler  substances  in  which  stronger  affinities  are  satis- 
fied; among  these  are  carbon  dioxide  and  sarcolactic  acid  and 
a  proteid  (myosinogen).  The  energy  liberated  is  thus  in- 
dependent of  any  simultaneous  taking  up  of  oxygen;  the 
amount  possible  depends  only  on  how  much  of  the  decom- 
posable body  existed  in  the  muscle.  Under  natural  condi- 
tions the  carbon  dioxide  is  carried  off  in  the  blood  and  per- 
haps the  sarcolactic  acid  also,  the  latter  to  be  elsewhere 
oxidized  further  to  form  water  and  more  carbon  dioxide. 
The  myosinogen  remains  in  the  muscle-fibre  and  is  combined 
with  more  oxygen, and  with  compounds  of  carbon  and  hydrogen 
taken  from  the  blood,  and  built  up  into  the  unstable  energy- 
yielding  body  again ;  no  increased  quantity  of  nitrogenous 
material,  under  ordinary  circumstances,  leaves  the  working 
muscle.  If,  however,  the  blood-supply  be  deficient,  myosin 
forms  from  myosinogen  and  clots  (Chap.  IX)  before  this 
restitution   takes  place,  and    cannot  be  directly  rebuilt  into 


NUTRITION.  459 

living  muscle  material;  in  excessive  work  the  same  thing 
partially  occurs,  decomposition  occurring  faster  than  recom- 
position;  clotted  myosin  is  then  broken  up  into  simpler 
bodies  as  kreatin,  and  these  are  somewhere  turned  into  urea 
and  excreted.  In  rigor  mortis  all  the  myosinogen  passes  into 
clotted  myosin  and  causes  the  rigidity.  A  working  muscle 
takes  up  more  oxygen  from  the  blood  than  a  resting  one,  as 
is  shown  by  a  comparison  of  the  gases  of  the  venous  blood 
of  the  two;  this  oxygen  assumption  is  not  necessarily  pro- 
portionate to  the  carbon-dioxide  elimination  at  the  same 
time;  for  the  latter  depends  on  the  breaking  down  of  a 
material  already  accumulated  in  the  muscle  during  rest,  and 
this  breaking  down  may  occur  faster  than  the  reconstruction. 
We  are  thus  enabled,  also,  to  understand  how,  during  exercise, 
the  carbon  dioxide  evolved  from  the  lungs  may  contain  more 
oxygen  than  that  taken  up  at  the  same  time;  for  it  is  largely 
oxygen  previously  stored  during  rest  which  then  appears  in 
the  carbon  dioxide  of  the  expired  air.  The  kreatin  which 
can  always  be  found  even  in  muscles  suddenly  killed  after 
long  rest,  represents  the  breaking  down  of  proteid  in  the 
chemical  processes  of  the  living  fibres,  in  their  vital  meta- 
bolisms, which  are  not  necessarily  similar  to  the  special 
chemical  changes  associated  with  a  contraction. 

Are  any  Foods  Respiratory  in  Liebig's  Sense  of  the 
Term  ?  We  find,  then,  that  Liebig's  classification  of  foods 
cannot  be  accepted  in  an  absolute  sense.  There  is  no  doubt 
that  the  substance  broken  down  in  muscular  contraction  is 
proper  living  muscular  tissue;  and  if  this  (its  proteid  con- 
stituent being  retained)  be  reconstructed  from  foods  con- 
taining no  nitrogen  (whether  carbohydrates  or  fats)  then 
the  term  plastic  or  tissue-forming  cannot  be  restricted  to  the 
proteids  of  the  diet.  We  must  rather  conclude  that  any 
alimentary  principle  containing  carboii  may  be  used  to  re- 
place the  oxidized  carbon,  and  any  containing  hydrogen  to 
replace  the  oxidized  hydrogen,  of  a  tissue;  and  so  even  non- 
proteid  foods  may  be  plastic.  A  certain  proportion  of  the 
foods  digested  may  perhaps  be  oxidized  to  yield  energy. 
before  they  ever  form  part  of  a  tissue;  and  so  correspond 
pretty  much  to  Liebig's  respiratory  foods;  but  no  hard  and 
fast  lino  can  be  drawn,  milking  all  proteid  foods  plastic  and 
all  oxidizable  non-proteid  foods  respiratory. 

Luxu8  Consumption.     Not  only,  as  above   pointed  out, 


460  THE  UVMAN  BODY. 

may  non-nitrogenous  foods  be  plastic  but,  on  the  other  hand, 
it  is  certain  tli.it  if  any  foods  are  oxidized  at  once  before 
being  organized  into  a  tissue,  proteids  are  under  certain 
circumstances;  namely,  when  they  are  contained  in  excess  in 
a  diet  If  an  animal  be  starved  it  is  found  that  its  non- 
nitrogenous  tissues  go  first;  an  insufficiently  fed  animal  loses 
its  fat  first,  and  if  it  ultimately  dies  of  starvation  is  found  to 
have  lost  97  per  cent  of  its  adipose  tissue  and  onlyaboul  30 
per  cent  of  its  proteid-rich  muscular  tissue,  and  almost  none 
of  its  brain  and  spinal  cord;  all  of  course  reckoned  by  their 
dry  weight.  It  is  thus  clear  that  the  proteids  of  the  tissues 
resist  oxidation  much  better  than  fat  does.  But,  on  the 
other  hand,  if  a  well-fed  animal  be  given  a  very  rich  proteid 
diet  all  the  nitrogen  of  its  food  reappears  in  its  urine,  and 
that  when  it  is  laying  up  fat;  so  that  then  we  get  a  state  of 
things  in  which  proteids  are  broken  up  more  easily  than 
fats.  This  indicates  that  proteid  in  the  Body  may  exist 
under  two  conditions  ;  one,  when  it  forms  part  of  a  living 
tissue  and  is  protected  to  a  great  extent  from  oxidation, 
and  another,  in  which  it  is  oxidized  with  readiness  and  is 
presumably  in  a  different  condition  from  the  first,  being  not 
yet  built  up  into  part  of  a  living  cell.  The  use  of  proteids 
for  direct  oxidation  is  known  as  luxus  consumption  ;  how 
far  it  occurs  under  ordinary  circumstances  will  be  considered 
presently.  The  main  point  now  to  be  borne  in  mind  is  that 
while  all  organic  non-nitrogenous  foods  cannot  be  called 
respiratory,  neither  can  proteids  under  all  circumstances  be 
called  plastic,  in  Liebig's  sense. 

The  Antecedents  of  Urea.  In  the  long-run  the  pro- 
genitors of  the  urea  excreted  from  the  Body  are  the  proteids 
taken  in  the  food;  but  it  remains  still  to  be  considered  what 
intermediate  steps  these  take  before  the  excretion  of  their 
nitrogen  in  the  urine. 

In  seeking  antecedents  of  urea  one  naturally  turns  first 
to  the  muscles,  which  form  by  far  the  largest  mass  of  pro- 
teid tissues  in  the  Body.  Analysis  shows  that  they  always 
yield  kreatin,the  quantity  of  this  in  muscles  being  practically 
unaffected  by  work,  and  from  0.2  to  0.3  per  cent  of  the  dry 
weight  of  the  muscle.  Since  it  is  readily  soluble  and  dialyz- 
able,  and  therefore  fitted  to  pass  rapidly  out  of  the  muscles 
into  the  blood  stream,  it  is  a  fair  conclusion  that  a  good  deal 
of  it  is  formed  in  the  muscles  daily  and  carried  off  from  them. 


NUTRITION.  461 

Kreatin,  too,  exists  in  the  brain,  and  probably  there  and  else- 
where in  the  nervous  system  is  produced  by  chemical  degra- 
dation of  protoplasm ;  the  spleen  also  contains  a  good  deal 
of  kreatin,  and  so  do  many  glands.  This  substance  would 
therefore  seem  to  be  constantly  produced  in  considerable 
quantities  by  the  protoplasmic  tissues  generally;  and  since 
it  belongs  to  a  group  of  nitrogenous  compounds  which  the 
Body  is  unable  to  utilize  for  reconstruction  into  proteids,  it 
must  be  carried  off  somehow.  The  urine,  however,  contains 
no  kreatin  and  but  little  of  its  immediate  derivative,  krea- 
tinin,  and  what  kreatinin  it  does  contain  depends  mainly  on 
the  feeding,  since  its  amount  varies  with  the  diet  and  it 
disappears  during  starvation.  Kreatin  can  readily  be  chem- 
ically broken  up  with  hydration,  yielding  urea  and  sarkosin; 
and  sarkosin  in  turn  can  be  decomposed  so  as  to  yield  its 
nitrogen  in  the  form  of  urea.  Hence  there  are  no  great 
chemical  difficulties  in  regarding  kreatin  as  the  main  im- 
mediate source  of  the  urea  of  normal  urine.  There  are  some 
reasons  for  thinking  that  kreatin  is  not  the  form  of  the 
actual  nitrogen  waste  in  living  muscle  but  a  post-mortem 
product  from  that  waste;  but  that  is  not  of  importance  in 
the  present  connection.  Whatever  the  original  form  of  the- 
waste  substance  be,  if  it  be  not  kreatin  it  is  certainly  very 
easily  converted  into  it.  The  formation  of  the  final  product, 
urea,  does  not  occur  in  the  muscles.  They  never  contain 
urea;  and  very  little  of  it,  if  any,  can  be  extracted  from  the 
brain. 

Where  the  kreatin  is  finally  changed  into  urea  is  doubt- 
ful. We  have  seen  (Chap.  XX  VIII)  that  it  is  not  formed  in 
the  kidneys  but  merely  separated  by  them  from  the  blood. 
A  good  deal  of  urea  is  found  in  the  liver,  which  suggests  some 
part  played  by  that  organ  in  urea  formation.  Further,  in 
certain  cases  of  hepatic  disease  (;icute  yellow  atrophy)  in 
winch  the  liver  cells  are  profoundly  changed,  the  area  of  the 
urine  is  greatly  diminished  and  a  quite  different  substance, 
leucin,  takes  its  place;  and  this  favors  the  view  that  the  liver 
has  much  to  do  with  the  final  elaboration  of  urea.  It  may 
also  be  noted  in  this  connection  that, quite  apart  from  kreatin 
at  a  source  of  area,  then-  may  be  aunt  her  in  leucin,  for  leucin 

is  very  widely  distributed  through  the  Body,  and  when  proteids 
are   decomposed  by  various   chemical    methods    leucin    is  very 

constant  among  the  products.    It  is  therefore  a  possible  form 


462  77/ A'  III' MAS   BODY. 

for  the  primary  nitrogen  waste  of  many  tissues.  Chemically 
leucin  is  an  ammonium  derivative,  being  the  amide  of  caproic 
(a  fatty)  acid. 

\\  bile  the  urea  resulting  from  further  changes  in  the 
kreatin,  leucin,  or  similar  .substances  formed  in  the  tissues,  is 
a  measure  of  the  wear  and  tear  of  their  protoplasm,  pari  of 
the  urea  excreted  has  probably  a  different  source;  being  due 
to  the  oxidation  of  proteids  as  energy  liberators  or  respira- 
tory foods,  before  they  have  ever  formed  a  tissue.  When 
abundant  proteid  food  is  taken  the  urea  excretion  is  largely 
increased  and  that  very  rapidly,  within  a  couple  of  hours  for 
example,  and  before  we  can  well  suppose  the  proteids  eaten 
to  have  been  built  up  into  tissues,  and  these  in  turn  broken 
down;  in  fact  there  need  be,  and  usually  is,  under  such  cir- 
cumstances no  sign  of  any  special  activity  of  any  group  of 
tissues,  such  as  one  would  expect  to  see  if  the  urea  always 
came  from  the  breaking  down  of  formed  histological  ele- 
ments. This  urea  is  thus  indicative  of  a  utilization  of  pro- 
teids for  other  than  plastic  purposes;  and  the  same  fact  is 
indicated  by  the  storage  of  carbon  and  elimination  of  all 
the  nitrogen  of  the  food  when  a  diet  very  rich  in  proteid 
alimentary  principles  is  taken.  This  luxus  consumption  may 
be  compared  to  the  paying  out  of  gold  by  a  merchant  instead 
of  greenbacks  when  he  has  an  abundance  of  both.  Only  the 
o-old  can  be  used  for  certain  purposes,  as  settling  foreign 
debts,  but  any  quantity  above  that  needed  for  such  a  purpose 
is  harder  to  store  than  the  paper  money,  and  not  so  con- 
venient to  handle;  so  it  is  paid  out  in  preference  to  the 
paper  money,  which  is  really  somewhat  less  valuable,  as 
available  at  par  only  for  the  settlement  of  domestic  debts. 
Similarly,  only  proteids  can  be  used  for  certain  final  stages  of 
tissue  building,  but  an  excess  of  them  is  more  difficult  to 
store  than  fats  or  carbohydrates,  and  so  is  eliminated  in  pref- 
erence to  them. 

In  artificial  pancreatic  digestions,  when  long  carried  on, 
two  bodies,  called  leucin  and  tyrosin,  are  produced  from 
proteids.  It  is  found  that  when  leucin  is  given  to  an  ani- 
mal in  its  food,  it  reappears  in  the  urine  as  urea;  so  the  Body 
can  turn  leucin  into  that  substance.  Hence  a  possible  source 
of  some  of  the  luxvs-consumption  urea  is  leucin  produced  dur- 
ing intestinal  digestion;  and  this  is  very  likely  turned  into 
urea  in  the  liver.     Mammalia  rapidly  die  when   the  liver  is 


NUTRITIOX.  463 

removed,  but  some  birds  survive  for  a  time.  In  them  it  lias 
been  found  that  the  uric  acid  (which  in  avian  urine  has  the 
predominance  which  urea  takes  in  mammalian)  excreted  is 
diminished  after  extirpation  of  the  liver;  and  also  that  leucin 
which  when  given  to  the  normal  bird  reappears  in  the  urine 
as  uric  acid,  in  the  bird  from  which  the  liver  has  been  removed 
is  excreted  unaltered. 

Circulating  and  Fixed  Proteid.  When  an  animal  is 
fed  on  food  deficient  in  proteids,  or  containing  none  of  them 
at  all,  its  urea  excretion  falls  very  rapidly  during  the  first  day 
or  two,  but  then  much  more  slowly  until  death:  there  is  thus 
indicated  a  double  source  of  urea, apart  resulting  from  tissue 
wear  and  tear,  and  always  present;  and  a  part  resulting  from 
the  breaking  down  of  proteids  not  built  up  into  tissue,  and 
ceasing  when  the  amount  of  this  proteid  in  the  Body  (in  the 
blood  for  example)  falls  below  a  certain  limit  as  a  result  of 
the  starvation.  As  the  nitrogen-starved  Body  wastes,  its 
bulk  of  proteid  tissues  is  slowly  reduced  and  the  urea  result- 
ing from  their  degradation  diminishes  also.  How  well  pro- 
teid built  up  into  a  tissue  resists  removal  is  shown  by  the 
tacts  already  mentioned  as  to  the  relative  losses  of  the  pro- 
teid-rich  and  proteid-poor  tissues  during  starvation. 

On  the  other  hand,  if  an  animal  be  taken  while  starving 
and  losing  weight  and  have  a  small  amount  of  flesh  given  it, 
it  will  continue  to  lose  weight,  and  more  urea  than  before 
will  appear  in  the  urine;  increased  proteid  diet  increases  the 
proteid  metamorphosis,  and  the  animal  still  loses,  though 
]<•—  rapidly  than  it  did.  A  little  more  proteid  still  increases 
proteid  metamorphosis  in  its  body  and  its  urea  elimination, 
and  so  on  for  some  time;  but  each  increment  of  proteid  in 
the  food  increases  the  nitrogenous  metamorphosis  in  propor- 
tion to  itself  somewhat  less  than  the  preceding  one  did,  until, 
finally,  a  point  is  reached  at  which  the  nitrogen  egesta  and 
ingesta  balance:  in  a  dog  this  occurs  when  the  animal  gets 
daily  J(|  Ltfi  weight  of  lean  meat,  along  with  the  necessary 
water.  More  flesh  if  then  given  is  at  first  stored  up  and  the 
animal  increases  in  weight;  but  very  soon  the  greater  wear 
a  lid  tear  of  the  larger  mass  of  tissues  shows  itself  as  increased 
una  excretion,  anil  again  the  egesta  and  ingesta  balance,  ami 

the  animal   comes   to   a   new  weight  equilibrium  at,  the  higher 

level.  More  meat  now  causes  a  repetition  of  the  phenomenon: 
at  ftrsl  increase  of  tissue,  and   nitrogen  storage;  and  then  a 


4»'>4  TEE  HUMAN  BODY. 

cessation  of  the  gain  in  weight,  and  an  excretion  in  twenty- 
four  hours  of  all  the  nitrogen  taken.  And  so  on,  until  the 
animal  refuses  to  cat  a  larger  quantity. 

These  facts  seem,  very  clearly,  to  show  thai  proteids  can- 
not be  built  up  quickly  into  tissues.  .Meat  given  to  the 
starving  animal  lias  its  proteids,  at  first,  \\>rd  up  mainly  in 
I uzns  consumption — while  a  little  is  stored  as  tissue,  though 
at  first  not  enough  to  counterbalance  the  daily  tissue  waste. 
When  a  good  deal  more  proteid  is  given  than  answers  to  the 
nitrogen  excretion  during  starvation,  the  animal  builds  up 
as  much  into  living  tissue  as  it  breaks  down  in  the  vital 
processes  of  these,  the  rest  going  in  luxus  consumption  J  it 
thus  neither  gains  nor  loses.  Still  more  proteid  if  now  given 
does  not  all  appear  in  the  urine  at  once;  some  is  u>vA  to 
build  up  new  tissue,  but  only  slowly;  the]),  after  some  days, 
the  increased  metabolism  of  the  increased  mass  of  living 
tissues  balances  the  excess  of  nitrogen  in  the  diet,  and  equi- 
librium is  again  attained.  But,  all  through,  it  seems  clear 
that  the  tissue  formation  is  slow  and  gradual;  and  so  it  be- 
comes additionally  probable  that  the  increased  urea  excretion 
soon  after  a  meal  is  not  due  to  rapidly  increased  tissue  forma- 
tion and  degradation,  but  to  a  more  direct  proteid  destruction. 
The  more  stable  proteid,  that  which  breaks  down  slowly  in 
starvation  and  is  rebuilt  slowly  when  food  is  given,  has  been 
distinguished  as  fixed  or  tissue  albumen  from  the  less  stable 
portion,  which  from  the  belief  that  it  mainly  exists  in  the 
liquids  of  the  Body  has  been  named  circulating  albumen 
Feeding  experiments  further  show  the  important  fact  that 
the  gelatinous  or  albuminoid  foods  cannot  be  converted 
into  fixed  proteid;  for  its  formation  true  albumens  are 
required.  The  tissues  of  an  animal  deprived  of  all  proteid 
food-s!  nil's  waste,  no  matter  how  much  albuminoids  be  given  : 
but  given  some  of  the  latter  the  Body  can  build  tissues  and 
maintain  their  integrity  with  less  true  proteid  than  would 
otherwise  be  necessary,  so  the  gelatin-yielding  foods  are  by 
no  means  without  nutritive  value. 

The  Storage  Tissues.  Every  healthy  cell  of  the  Body 
eou tains  at  any  moment  some  little  excess  of  material  laid 
by  in  itself,  above  what  is  required  for  its  immediate  neces- 
sities. The  capacity  of  contracting,  and  the  concomitant 
evolution  of  carbon  dioxide,  exhibited  by  an  excised  muscle 
in   a  vacuum,   seem   to   show    that   even    oxygen,  of    which 


NUTRITION.  465 

warm-blooded  animals  have  but  a  small  reserve,  may  "be 
stored  up  in  the  living  tissues  in  such  forms  that  they  can 
utilize  it,  even  when  the  air-pump  fails  to  extract  any  from 
them.  But  in  addition  to  the  supplies  for  immediate  spend- 
ing, contained  in  all  the  cells,  we  find  special  food  reserves 
in  the  Body,  on  which  any  of  the  tissues  can  call  at  need. 
These,  especially  the  oxygen  and  proteid  reserves,  are  found 
for  most  part  in  the  blood.  Special  oxygen  storage  is,  however, 
rendered  unnecessary  by  the  fact  that  the  Body  can,  except 
under  very  unusual  circumstances,  get  more  from  the  air  at 
any  time,  so  the  quantity  of  this  substance  laid  by  is  only 
small;  hence  death  from  asphyxia  follows  very  Vapidly  when 
the  air-passages  are  stopped;  while,  on  account  of  the  re- 
serves laid  up,  death  from  other  forms  of  starvation  is  a 
much  slower  occurrence.  Proteids,  also,  we  have  learnt  from 
the  study  of  muscle,  are  probably  but  little  concerned  in 
energy-production  in  the  tissues.  Speaking  broadly,  the 
work  of  the  Body  is  carried  on  by  the  oxidation  of  carbon 
and  hydrogen,  and  we  find  in  the  Body,  in  correspondence 
with  this  fact,  two  great  storehouses  of  fatty  and  carbo- 
hydrate foods,  which  serve  to  supply  the  materials  for  the 
performance  of  work  and  the  maintenance  of  the  bodily 
temperature  in  the  intervals  between  meals,  and  during 
longer  periods  of  starvation.  One  such  store,  that  of  car- 
bohydrate material,  is  found  in  the  liver-cells;  the  other, 
or  fatty  reserve,  is  laid  by  in  the  adipose  tissue  and  to  a  cer- 
tain extent  in  oil  droplets  found  in  other  cells,  and  sometimes 
in  blood  and  lymph.  That  such  substances  are  true  reserves, 
nol  for  any  special  local  purpose  but  for  the  use  of  the  Body 
generally,  is  shown  by  the  way  they  disappear  in  starvation; 
tin'  liver  reserve  in  a  few  days,  and  the  fat  somewhat  later 
and  more  slowly,  but  very  Largely  before  any  of  the  other 
tissues  has  been  seriously  affected.  By  using  these  accumu- 
lated matters  the  Body  can  work  and  keep  warm  during 
ral  days  of  more  or  less  deficient  feeding;  and  the  fatter 
an  animal  is  at  the  beginning  of  a.  starvation  period  flic 
Longer  will  it  live:  which  would  not,  be  the  case  could  not  its 
fat  he  utilized  by  the  working  tissues.  Hibernating  animals 
prove  tlf  same  thing;  bears,  before  their  winter  sle  p,  are 
very  fat,  and  at  the  end  of  it  commonly  very  thin;  while 
their  muscular  and  nervous  systems  are  not  noticeably 
diminished    in    mass.     During  tin;  whole   winter,  then,  the 


460  TEE   II r  MAN  BODY. 

energy  needed  to  keep  the  heart  and  respiratory  muscles  at 
work,  and  to  maintain  the  temperature  of  the  body,  must 
have   been  obtained   from   the   oxidation  of  the  fat   reserve 

with  which  the  animal  started. 

Glycogen.  The  size  of  the  liver  was  long  a  Btumbling- 
block  to  physiologists:  it  was  difficult  to  understand  why  so 
large  an  organ  should  be  developed  for  the  mere  secretion  of 
some  bile,  a  not  very  important  digestive  liquid.  Bui  even 
centuries  ago  some  glimmering  of  the  truth  was  guessed, 
and  the  liver  was  believed  to  be  concerned  in  the  elaboration 
of  nutritive  Mood,  which  was  distinguished  from  the  blood, 
charged  witfi  vital  spirits,  which  came  from  the  lungs  and 
the  left  side  of  the  heart.  Harvey's  discovery  of  the  real 
course  of  the  circulation,  and  Lavoisier's  interpretation  of 
the  meaning  of  respiration,  upset  these  crude  doctrines;  and 
for  long  the  germ  of  truth  which  they  contained  was  lost  to 
view  in  the  glare  of  the  new  light.  We  have  now  learned, 
on  a  new  basis  of  actual  experiment,  that  the  liver  is  very 
largely  concerned  in  the  nutritive  processes  of  the  Body:  its 
relation  to  proteid  metabolism  and  urea  formation  has 
already  been  considered,  and  we  have  now  to  study  its 
activity  in  regard  to  the  formation,  and  storage,  and  trans- 
mission of  a  carbohydrate  substance,  glycogen. 

If  a  liver  be  cut  up  two  or  three  hours  after  removal  from 
the  body  of  a  healthy  well-fed  animal,  and  thoroughly  ex- 
tracted with  water,  it  will  yield  much  grape-sugar.  If,  on  I  he 
other  hand,  a  perfectly  fresh  liver  be  heated  rapidly  to  the  tem- 
perature of  boiling  water,  and  be  then  pounded  up  and  ex- 
tracted, it  will  yield  a  milky  solution,  containing  little  grape- 
sugar,  but  much  glycogen;  a  substance  which  chemically 
has  the  same  empirical  formula  as  starch  (C6H10O6),  and  in 
other  ways  is  closely  allied  to  that  body.  The  salivary  and 
pancreatic  secretions  rapidly  convert  it  into  the  sugar  maltose, 
as  they  do  starch.  The  transformation  of  glycogen  into  glucose 
(grape  sugar)  which  occurs  in  the  liver  after  death  and  prob- 
ably also  during  life  is  then  quite  different  from  that  brought 
about  by  the  digestive  enzymes;  and  in  fact  no  enzyme  has 
been  extracted  from  fresh  liver.  The  change  is  apparently 
not  a  fermentative  one,  but  one  dependent  on  some  vital 
metabolic  activity  of  the  liver-cells,  which  activity  is  greatly 
accelerated  during  their  period  of  dying:  hence  the  need  of 
killing  them  rapidly  by  boiling,  if  any  considerable  amount  of 


NUTRITION.  467 

glycogen  is  to  be  obtained  from  the  organ.  Pure  glycogen 
is  a  white  amorphous  inodorous  powder,  readily  soluble  in 
water,  forming  an  opalescent  milky  solution;  insoluble  in 
alcohol,  and  giving  with  iodine  a  red  coloration  which  dis- 
appears on  heating  and  reappears  on  cooling  again. 

About  four  per  cent  of  glycogen  can  be  obtained  from 
the  liver  of  a  well-nourished  animal  (dog  or  rabbit).  This 
for  the  human  liver,  which  weighs  about  1500  grams  (53 
oz  ),  would  give  about  GO  grams  (2.1  oz.)  of  glycogen  at  any 
one  moment,  The  quantity  actually  formed  daily  is,  how- 
ever, much  in  excess  of  that,  since  glycogen  is  constantly 
being  removed  from  the  liver  and  carried  elsewhere,  while  a 
fresh  supply  is  formed  in  the  organ.  Its  quantity  is  subject, 
also,  to  considerable  fluctuations;  being  greatest  about  two 
hours  after  a  good  meal,  and  falling  from  that  time  until  the 
next  digestion  period  commences,  when  it  begins  to  rise 
until  it  again  attains  its  maximum.  If  a  warm-blooded 
animal  be  starved  glycogen  disappears  from  its  liver  in  the 
course  of  four  or  five  days.  We  are,  thus,  led  to  believe  that 
glycogen  is  being  constantly  used  up.  and  that  its  mainte- 
nance  in  normal  quantity  depends  on  food  supply. 

The  accumulation  and  disappearance  of  glycogen  can  be 
demonstrated  histologically.     The  liver  is  essentially  a  com- 
pound tubular  -land,  but  its  structure  is  obscured  by  the  fact 
thai   the   hepatic  cells   are  very  large  in   proportion  to  the 
tubules  which  they  surround,  so  that  these  are  reduced  to 
nv  re  ductules,  formed  by  the  apposition  of  grooves  on  the 
adjacent  sides  of  two  cells;  and  by  the  fact  that  cells  and 
ductules  form  an  irregular  network  interlaced  with  the  capil- 
laries of  the  lobule  (Chap.  XXII),  which  capillaries  are  far 
larger  than  the  interlobular  bile-ducts.     When  properly  pre- 
pared  hepatic    cells,  taken  from  a  healthy   well-fed  animal, 
are  examined,  the  side  of  the  cell  nearest  the  bile-ductule  is 
seen  to  be  granular,  and  it  also  picks  up  readily  most  of  the 
ordinary  protoplasmic  stains.     The  rest  of  the  cell  contains 
few  granules  and  does  not  stain  with  carmine,  but  it  does 
stain" red  with  iodine.     It  is  in  fact  mainly  filled  with  glyco- 
gen, and  if  this  be  dissolved  out  by  digestion  with  saliva  there 
,.    left  a  Loose  protoplasmic   network.     If  sections  from  the 
liver   of    a  starved   animal   be   compared   with   those   from  a 
well-fed,  ih''  liver-celle  are  seen  to  be  considerably  smaller,  to 
gianular  throughout,  and  to  stain  everywhere  with  carmine 


468  THE  HUMAN  BOUT. 

;uid  not  at  all  with  iodine:  they  contain  no  glycogen  and  may 
be  compared  with  the  cells  of  the  pancreas  in  a  late  stage  of 
digestion  (Chap.  X  IX). 

In  the  liver  we  have  \>>  deal  with  cells  of  I  wofold  function; 
ihe  granular  portion  of  each  especially  concerned  with  bile 
secretion,  and  the  larger  [tort ion  of  the  cell  with  the  making 
of  glycogen.  In  a  salivary  gland  we  have  cells  whose  sole 
apparent  function  is  the  formation  of  secretion  to  be  poured 
into  the  gland  duets;  in  the  thyroid  and  suprarenal  bodies 
we  find  cells  forming  special  materials  which  are  passed  into 
blood  or  lymph.  The  hepatic  cells  do  both,  and  it  should  be 
borne  in  mind  that  possibly  all  gland-cells  do.  In  fact  it  has 
already  been  pointed  out  that  the  pancreas  has  still  an- 
other function  than  the  formation  of  pancreatic  juice.  As 
regards  the  liver-cells,  we  naturally  ask  whether  the  two 
processes,  bile-making  and  glycogen-making,  are  distinct  and 
independent  activities,  or  whether  bile  and  glycogen  are 
simultaneous  products  of  a  single  metabolic  activity,  as  soap 
and  glycerine  are  of  the  chemical  process  of  soap-making: 
but  to  this  question  it  is  not  possible  yet  to  give  a  satisfactory 
answer. 

The  Source  and  Destination  of  Liver  Glycogen.  All 
foods  are  not  equally  efficacious  in  keeping  up  the  stock  of 
glycogen  in  the  liver;  fats  by  themselves  are  useless;  proteids 
by  themselves  give  a  little;  by  far  the  most  is  formed  on 
a  diet  rich  in  starch  and  sugar  ;  so  it  would  seem  that  glyco- 
gen is  mainly  formed  from  carbohydrate  materials  absorbed 
from  the  alimentary  canal  and  curried  to  the  hepatic  cells  by 
the  portal  vein.  The  chief  of  these  materials  is  probably 
glucose,  since,  although  saliva  and  the  amylolytic  ferment  of 
the  pancreas  convert  starch  info  maltose  (C^H^O,,  +  H„0), 
of  the  cane-sugar  group,  the  intestinal  secretion  rapidly  con- 
verts this  into  grape-sugar  or  glucose.  Thi-  is  taken  up  by 
the  liver- cells,  modified  by  them  and  stored  as  glycogen:  and 
by  their  further  activity  from  time  to  time  reconverted  into 
o-iucose  and  passed  into  the  blood  according  to  the  needs  of 
the  Body  in  general.  The  cells  then  do  distinctly  chemical 
work  on  the  carbohydrate  material:  possibly,  indeed  even 
probably,  they  build  that  supplied  into  their  own  living  sub- 
stance and  then  by  partial  breaking  down  of  this,  deposit  some 
of  it  for  a  time  as  glycogen:  and  by  further  living  activity 
turn  this  into  glucose  and  send  it  on  to  the  blood,  when  the 


NUTRITION.  469 

sugar  in  that  liquid  falls  below  a  certain  percentage.  That 
the  chief  part  of  the  glycogen  found  in  the  normal  liver  has 
its  ultimate  source  in  carbohydrate  foods  is  shown  by  several 
facts.  (1)  Sugar  if  it  exist  in  the  blood  in  above  a  certain 
small  percentage,  passes  out  by  the  kidneys  and  appears  in  the 
urine,  constituting  the  characteristic  symptom  of  the  disease 
called  diabetes.  In  health,  however,  even  after  a  meal  very 
rich  in  carbohydrates,  sugar  rarely  appears  in  the  urine,  and 
then  but  temporarily;  so  that  the  large  quantity  of  it  absorbed 
from  the  alimentary  canal  within  a  brief  time  under  such  cir- 
cumstances, must  be  stopped  somewhere  before  it  reaches  the 
general  blood-current.  (2)  Glucose  injected  into  one  of  the 
general  veins  of  an  animal,  if  in  any  quantity,  soon  appears 
in  the  urine;  but  the  same  amount  injected  into  the  portal 
vein,  or  one  of  its  radicles,  causes  no  diabetes,  but  an  accumu- 
lation of  glycogen  in  the  liver.  We  may  therefore  conclude 
that  the  sugar  absorbed  from  the  alimentary  canal  is  taken 
by  the  portal  vein  to  the  liver,  and  there  converted  into 
glycogen  and  stayed  for  a  time;  and  later  slowly  passed  on 
into  the  hepatic  veins  during  the  intervals  between  meals. 
Thus  in  spite  of  the  intervals  which  elapse  between  meals  the 
carbohydrate  content  of  the  blood  is  kept  pretty  constant: 
during  digestion  it  is  not  suffered  to  rise  very  high,  nor  dur- 
ing ordinary  periods  of  fasting  to  fall  very  much  below  the 
average. 

In  what  form  glycogen  leaves  the  liver  is  not  certain  ;  it 
might  be  dissolved  out  and  carried  off  as  such,  or  previously 
turned  again  into  glucose  and  sent  on  in  that  form;  since  the 
fresh  liver-cells  are  capable  of  changing  glycogen  into  glucose 
the  latter  view  is  the  more  probable.  Analyses  of  portal  and 
hepatic  bloods,  made  with  the  view  of  determining  whether 
more  sugar  was  carried  out  of  the  liver  during  fasting  than 
into  it.  are  conflicting;  and  considering  the  great  amount  of 
blood  which  flows  through  the  liver  in  twenty-four  hours,  a 
verv  slight  increase  of  sugar  (falling  within  the  limit.}  of 
error  of  the  difficult  quantitative  determination  of  that  sub- 
stance in  the  blood)  in  the  hepatic  vein  would  represent  a 
Urge  total  amount  during  the  whole  day.  The  main  fact, 
however,  remains,  that  somehow  this  carbohydrate  reserve  in 
the  liver  is  steadily  carried  off  to  he  used  elsewhere:  and 
animal  glycogen  thus  answers  pretty  much  to  vegetable  starch, 
which,  made  in  the  green  leaves,  is  dissolved  and  carried  away 


470  Til E  HUMAN   liODY. 

by  the  sap  currents  to  distant  and  not  green  parts  (as  the 
grains  of  corn  or  tubers  of  a  potato,  which  cannot  make  starch 
for  themselves)  and  in  them  is  again  laid  down  in  the  form  of 
solid  starch  grains,  which  are  subsequently  dissolved  and  used 
for  the  growth  of  the  germinating  seed  or  potato.  Reasons 
have  been  given  in  an  early  part  of  this  chapter  for  believing 
that  the  carbohydrate  leaving  the  liver  is  not  oxidized  in  the 
blood,  but  only  after  it  has  passed  out  of  that  into  the  organ- 
ized tissue.  Among  these  the  muscles  at  least  seem  to  get 
some,  since  a  fresh  muscle  always  contains  glycogen,  and  even 
to  retain  it  in  normal  amount  after  an  animal  has  been  starved 
for  some  time;  the  muscle-fibres  then,  so  to  speak,  drawing  on 
the  balance  with  their  banker  (the  liver)  so  long  as  there  is 
any.  When  a  muscle  contracts,  this  glycogen  disappears  and 
some  glucose  appears,  but  not  an  amount  equivalent  to  the 
glycogen  used  up;  so  that  the  working  muscle,  it  is  probable, 
uses  this  substance,  among  others,  for  its  repair  after  each 
contraction. 

How  it  is  that  the  glycogen,  which  is  so  rapidly  converted 
into  grape-sugar  by  the  dying  liver,  escapes  such  rapid  con- 
version during  life  has  not  been  satisfactorily  answered.  It 
may  be  that  the  metabolisms  of  the  dying  hepatic  cell  include 
processes  which  are  an  exaggeration  of  those  occurring  dur- 
ing normal  life;  in  some  such  way  as  the  production  of  myo- 
sin in  dying  muscle  is  apparently  an  exaggeration  of  chemical 
changes  occurring  in  normal  contracting  muscle:  or  the  gly- 
cogen in  the  living  cell  may  not  exist  free,  but  combined 
with  other  portions  of  the  cell  substance  so  as  to  be  pro- 
tected ;  while,  after  death,  post-mortem  changes  may  rapidly 
liberate  it  in  a  condition  to  be  acted  upon. 

Diabetes.  The  study  of  this  disease  throws  some  light 
upon  the  history  of  glycogen.  Two  distinct  varieties  of  it 
are  known;  one  in  which  sugar  appeal's  in  the  urine  only 
when  the  patient  takes  carbohydrate  foods;  the  other  in 
which  it  is  still  excreted  when  he  takes  no  such  foods,  and 
must,  therefore  form  sugar  in  his  Body  from  substances  not  at 
all  chemically  allied  to  it.  The  more  probable  source  of  the 
sugar  in  the  latter  case  is  proteids;  since  some  glycogen  is 
found  in  the  livers  of  animals  fed  on  proteids  only,  while  fats 
by  themselves  give  none  of  it.  It  seems  that  the  proteid 
molecule,  in  some  complex  way,  is  split  up  in  the  liver  into  a 
highly  nitrogenized  part  (urea  or  an  antecedent  of  urea)  and 


NUTKITION.  471 

a  uonazotized  part,  glycogen.  On  this  view  the  more  severe 
form  of  diabetes  would  be  due  to  an  increased  activity  of  a 
normal  proteid-decomposing  function  of  the  hepatic  cells;  and 
sometimes  the  urea  and  sugar  in  the  urine  of  diabetics  rise 
and  fall  together,  thus  seeming  to  indicate  a  community  of 
origin.  Diabetes  dependent  on  carbohydrate  food  might  be 
produced  in  several  ways.  The  liver-cells  might  cease  to 
stop  the  sugar  and,  letting  it  all  pass  on  into  the  general  cir- 
culation, suffer  it  to  rise  to  such  a  percentage  in  the  blood 
after  a  meal,  that  it  attained  the  proportion  in  which  the 
kidneys  pass  it  out;  or  the  tissues  might  cease  to  use  their 
natural  amount  of  sugar,  and  this,  sent  on  steadily  out  of  the 
liver,  at  last  rise  in  the  blood  to  the  point  of  excretion.  Or 
the  liver  might  transform  (into  glucose)  and  pass  on  its  gly- 
cogen faster  than  the  other  tissues  used  it,  and  so  diabetes 
might  arise;  but  this  would  only  be  temporary,  lasting  until 
the  liver  stock  was  used  up  by  the  rapid  conversion.  Arti- 
ficially we  can,  in  fact,  produce  diabetes  in  several  of  these 
ways;  curari  poisoning,  for  example,  paralyzing  the  motor 
nerves,  makes  the  skeletal  muscles  lie  completely  at  rest,  and 
so  diminishes  the  glycogen  consumption  of  the  Body  and  pro- 
duces diabetes.  Carbon-monoxide  poisoning  produces  dia- 
betes also,  presumably  by  checking  bodily  oxidation.  Fi- 
nally, pricking  a  certain  spot  in  the  medulla  oblongata  causes 
a  temporary  diabetes.  This  might  conceivably  be  due  to  the 
fact  that  the  operation  injures  that  part  of  the  vasomotor 
centre  which  controls  the  muscular  coat  of  the  hepatic  artery, 
and  this  artery,  then  dilating,  carries  so  much  blood  through 
the  liver  that  an  excess  of  glycogen  is  carried  off  by  the 
hepatic  veins;  and  in  favor  of  this  opinion  is  the  fact  that  if 
the  splanchnic  nerves  be  cut  the  whole  arteries  of  the  ab- 
dominal viscera  dilate  no  diabetes  follows.  This  has  been 
explained  as  din;  to  the  fact  that  so  many  vessels  are  dilated 
that  a  great  part  of  the  blood  of  the  Body  accumulates  in 
them,  and  there  is  in  consequence  no  noticeably  increased 
flow  through  the  liver.  Others,  however,  maintain  that  the 
"piqdre "  diabetes  (as  that  due  to  pricking  the  medulla  is 
called)  h  due  to  irritation  of  trophic  nerve-fibres  originating 
there,  and  governing  the  rate  at  which  the  liver-cells  produce 
glycogen  or  convert  it  into  glucose.  This  latter  view, 
though  perhaps  the  less  commonly  accepted,  is  probably  the 
more  correct.     The    hepatic   cells   do  not  merely  hold  back 


472  THE  HUMAN  BODY. 

glucose  carried  through  the  liver  so  that  it  is  there  to  be 
washed  out  by  a  greater  blood-flow,  bul  they  feed  on  sugar 
and  protcids  and  make  glycogen;  and  this  is  later  converted 
into  glucose  and  carried  off.  Glycogen,  except,  for  its  dis- 
charge into  the  blood  instead  of  a  gland  duct,  would  then  be 
comparable  to  the  materials  stored  in  the  cells  of  the  salivary 
and  some  other  glands  (Chap.  XIX);  and  the  transforma- 
tion of  such  hodies  into  the  specific  element  of  a  secretion 
we  have  already  seen  to  he  directly  under  the  control  of  the 
nervous  system,  and  almost  entirely  or  quite  independent  of 
the  simultaneous  blood-flow. 

The  History  of  Fats.  While  glycogen  forms  a  reserve 
store  of  material  which  is  subject  to  rapid  alterations,  deter- 
mined by  meal-times,  the  fats  are  much  more  stable;  their 
periods  of  fluctuation  are  regulated  by  days,  weeks,  or  months 
of  good  or  had  nutrition,  and  during  starvation  they  are  not 
so  readily,  or  at  least  so  rapidly,  called  upon  as  the  hepatic 
glycogen.  If  we  carry  on  the  simile  by  which  we  compared 
the  reserve  in  each  cell  to  pocket-money,  the  glycogen 
would  answer  somewhat  to  a  balance  on  the  right  side 
with  a  man's  banker;  while  the  fat  would  represent  assets  or 
securities  not  so  rapidly  realizable;  as  capital  in  business,  or 
the  cargoes  afloat  in  the  argosies  of  Antonio,  the  ''Merchant 
of  Venice."  Fat,  in  fact,  is  slowly  laid  clown  in  fat-cells  and 
surrounded  in  these  by  a  cell-wall,  and,  being  itself  insoluhle 
in  blood  plasma  or  lymph,  it  must  undergo  chemical  changes, 
which  no  doubt  require  some  time,  before  it  can  be  taken 
into  the  blood  and  carried  off  to  other  parts. 

When  adipose  tissue  is  developing  it  is  seen  that  undif- 
ferentiated cells  in  the  connective  tissues  (especially  areolar) 
show  minute  oil-drops  in  their  protoplasm;  these  increase 
in  size  and  ultimately  fuse  together  and  form  one  larger 
oil-droplet,  while  most  of  the  original  protoplasm  disappears. 

The  oily  matter  would  thus  seem  due  to  a  chemical  meta- 
morphosis of  the  cell  protoplasm,  during  which  it  gives  rise 
to  a  non-azotized  fatty  residue  which  remains  behind,  and  a 
highly  nitrogenous  part  which  is  carried  off.  In  many  parts 
of  the  Body  protoplasmic  masses  are  subject  to  a  similar  but 
less  complete  metamorphosis;  fatty  degeneration  of  the  heart, 
for  example,  is  a  more  or  less  extensive  replacement  of  the 
proper  substance  of  its  muscular  fibres  by  fat-droplets;  and 
the  cream  of  milk  and  the  oily  matter  of  the  sebaceous  secre 


NUTRITION. 


473 


tion  are  due  to  a  similar  fatty  degeneration  in  gland-cells. 
Moreover,  careful  feeding  experiments  undoubtedly  show  that 
fat  can  come  from  proteids;  when  an  animal  is  very  richly 
supplied  with  these  all  the  nitrogen  taken  in  them  reappears 
in  its  excretions,  but  all  the  carbon  does  not;  it  is  in  part 
stored  in  the  Body:  and,  since  such  feeding  produces  but 
little  glycogen,  this  carbon  can  only  be  stored  as  fat, 

While  there  is,  then,  no  doubt  that  some  fat  may  have  a 
proteid  origin,  it  is  not  certain  that  all  has  such.  During 
digestion  a  great  deal  of  fat  is  ordinarily  absorbed,  in  a 
chemically  unchanged  state,  from  the  alimentary  canal;  it  is 
merely  emulsified  and  carried  off  in  minute  drops  by  the  chyle 
to  be  poured  into  the  blood;  and  this  fat  might  conceiveably 
be  directly  deposited,  as  such,  in  adipose  tissue.  There  are, 
however,  good  reasons  for  supposing  that  all  the  fat  in  the 
Body  is  manufactured.  The  fat  of  a  man,  of  a  dog,  and  of  a 
cat  varies  in  the  proportions  of  palmatin,  stearin,  margarin, 
and  olein  in  it;  and  varies  in  just  the  same  way  if  all  be  fed 
on  the  same  kind  of  food,  which  could  not  be  the  case  if  the 
fat  eaten  were  simply  deposited  unchanged.  Moreover,  if 
aii  animal  be  fed  on  a  diet  containing  one  kind  of  fat  only, 
Bay  olein,  but  a  very  slightly  increased  percentage  of  that 
particular  fatty  substance  is  found  in  its  adipose  tissue, 
which  goes  to  show  that  if  fats  come  from  fats  eaten,  these 
latter  are  first  pulled  to  bits  by  the  living  cells  and  built  up 
again  into  the  forms  normal  to  the  animal;  so  that,  even  with 
fatty  food,  the  fats  stored  up  seem  to  be  in  most  part  manu- 
factured in  the  Body. 

In  still  another  way  it  is  proved  that  fats  can  be  con- 
structed in  the  Body.  In  animals  fed  for  slaughter,  the  total 
fat  stored  up  in  them  during  the  process  is  greatly  in  excess 
of  that  taken  with  their  food  during  the  same  time.  For 
example,  a  fattening  pig  may  store  up  nearly  five  hundred 
parts  of  bit  Cor  every  hundred  in  its  food,  and  this  fat  must  be 
made  from  proteids  or  carbohydrates.  Whether  it  can  come 
from  the  latter  is  still  perhaps  an  open  question;  for,  while 
all  fattening  foods  are  rich  in  starch  or  similar  bodies,  there 
are  considerable  chemical  difficulties  in  supposing  an  origin 

of  fats  from  BUCh;  and  it  is  on  the  whole  more  probable  that 
they  Bimply  ftd  by  sparing  from  use  fats  simultaneously 
formed  or  stored  in  the  body,  and  which  would  have  other- 
wi-e    bee,,    called    upon.       They    make    glycogen,   and    this 


474  THE  HUMAN  BODY. 

shelters  the  fats.  Liebig,  indeed,  in  a  very  celebrated  dis- 
cussion, maintained  that  fats  were  formed  from  carbohydrates 
He  showed  that  a  cow  gave  out  more  butter  in  its  milk  than 
it  received  fats  in  its  food;  and  Iluber,  the  blind  naturalist, 
showed  that  bees  still  made  wax  (a  fatty  body)  for  a  time 
when  fed  on  pure  sugar;  ami  indefinitely  when  fed  on  honey. 
Consequently,  for  a  long  time,  an  origin  of  fats  from  carbo- 
hydrates was  supposed  to  be  proved;  but  their  possible  origin 
from  proteids  (a  possibility  now  shown  to  be  a  certainty)  was 
neglected,  and  the  validity  of  the  above  proofs  of  their  carbo- 
hydrate origin  is  thus  upset.  The  cow  may  have  made  its 
butter  from  proteids;  the  bees,  fed  on  sugar,  their  wax  for  a 
time  from  proteids  already  in  their  bodies;  and,  indefinitely, 
when  fed  on  honey,  from  the  proteids  in  that  substance. 
Moreover,  animals  (ducks)  fed  on  abundant  rice,  which  con- 
tains much  carbohydrate  but  very  little  proteid  or  fat,  remain 
lean;  while  if  some  fat  be  added  they  lay  up  fat. 

Persons  who  fatten  cattle  for  the  butcher  find  that  the 
foods  useful  for  the  purpose  all  contain  proteids,  carbohy- 
drates, and  fats,  and  that  rapid  fattening  is  only  obtained 
with  foods  containing  a  good  deal  of  fat;  as  oilcake,  milk, 
or  Indian  corn.  Taking  all  the  facts  into  account  we  shall 
probably  not  be  wrong  in  concluding  that  nearly  all  the 
bodily  fat  is  manufactured  either  from  fats  or  proteids; 
from  fats  easier  than  from  anything  else,  but  when  much 
proteid  is  eaten  some  is  made  from  it  also.  Carbohydrates 
alone  do  not  fatten;  the  animal  body  cannot  make  its  pal- 
matin,  etc.,  out  of  them.  Nevertheless  they  are,  indirectly, 
important  fattening  foods  when  given  with  others,  since, 
being  oxidized  instead  of  it,  they  protect  the  fat  formed. 

Dietetics.  That  "one  man's  meat  may  be  another  man's 
poison"  is  a  familiar  saying,  and  one  that,  no  doubt,  ex 
presses  a  certain  amount  of  truth ;  but  the  difference  probably 
depends  on  the  varying  digestive  powers  of  individuals 
rather  than  on  peculiarities  in  their  laws  of  cell  nutrition: 
we  all  need  about  the  same  amount  of  proteids,  fats,  and 
carbohydrates  for  each  kilogram  of  body  weight;  but  all  of 
us  cannot  digest  the  same  varieties  of  them  equally  well:  it 
is  also  a  matter  of  common  experience  that  some  foods  have 
peculiar,  almost  poisonous,  effects  on  certain  persons.  Some 
people  are  made  ill  by  mutton,  which  the  majority  digest 
better  than  beef. 


NUTRITION.  475 

The  proper  diet  must  necessarily  vary,  at  least  as  to 
amount,  with  the  work  done;  whether  it  should  vary  in  kind 
with  the  nature  of  the  work  is  not  so  certain.  Provided  a 
man  gets  enough  proteids  to  balance  those  lost  in  the  wear 
and  tear  of  his  tissues,  it  jn'obably  matters  little  whether  he 
gets  for  oxidation  and  the  liberation  of  energy  either  fats  or 
carbohydrates,  or  even  excess  of  proteids  themselves;  any  one 
of  the  three  will  allow  him  to  work  either  his  brain  or  his 
muscles,  aud  to  maintain  his  temperature.  Proteids,  how- 
ever, are  wasteful  foods  for  mere  energy-yielding  purposes: 
in  the  first  place,  they  are  more  costly  than  the  others; 
secondly,  they  are  incompletely  oxidized  in  the  Body;  and, 
thirdly,  it  is  probably  more  laborious  to  the  system  to  get  rid 
of  urea  than  of  the  carbon  dioxide  and  water,  which  alone 
are  yielded  by  the  oxidation  of  fats  and  carbohydrates.  Be- 
tween fats  and  carbohydrates  similar  considerations  lead  to 
a  use  of  the  latter  when  practicable:  starch  is  more  easily 
utilized  in  the  Body  than  fats,  as  shown  by  the  manner  in 
which  it  protects  the  latter  from  oxidation;  and  a  given 
weight  of  starch  fully  oxidized  in  the  Body  will  liberate 
about  one  half  as  much  energy  as  the  same  amount  of  butter, 
while  it  costs  considerably  less  than  half  the  money.  Also, 
starch  is  more  easily  digested  than  fats  by  most  persons: 
children  especially  are  apt  to  be  fond  of  starchy  or  saccharine 
foods  and  to  loathe  fats;  and  the  appetite  in  cuch  cases  is 
a  good  guide.  As  a  rule  the  people  of  the  United  States 
differ  very  markedly  from  the  English  in  their  love  of  sweet 
foods  of  all  kinds;  whether  this  is  correlated  with  their  char- 
acteristic activity,  calling  for  some  food  that  can  be  rapidly 
used,  is  an  interesting  question. 

It  is  certain  that  no  general  rules  for  the  best  dietary 
for  all  persons  can  be  formulated,  but  on  broad  principles 
the  best  diet  is  that  which  contains  just  the  amount  of  pro- 
teid  necessary  for  tissue  repair,  and  so  much  carbohydrates 
as  can  be  well  digested;  the  balance  needed,  if  any,  being 
made  up  by  fats  and  gelatinoids.  Such  a  food  would  be 
the  cheapest;  that  is,  the  supplying  of  it  would  call  for  less 
of  the  time  and  energy  of  the  nation  using  it,  and  leave  more 
work  to  spare  for  other  pursuits  than  food  production — for 
all  the  arts  which  make  life  agreeable  and  worth  living,  and 
which  elevate  civilized  man  above  the  merely  material  life  of 
the  savage  whose  time  is  devoted  to  catching  and   eating. 


176  77//-;  HUMAN  BODY 

We  have  high  authority  for  saying  thai  man  does  not  live  by 
bread  alone;  in  other  words,  his  highesl  development  is 
impossible  when  he  is  totally  absorbed  in  "  keeping  body  and 
sonl  together/' and  the  more  labor  that  can  be  spared  from 
getting  enough  food  the  better  chance  has  he,  if  he  use  his 
leisure  rightly,  of  becoming  a  more  worthy  man.  While 
there  is,  thus,  a  theoretically  best  diet,  it  is  nevertheless 
impossible  to  say  what  that  is  lor  each  individual;  but  what 
the  general  experience  is  may  he  approximately  gathered  by 
taking  an  average  of  the  dietaries  of  a  number  of  public 
institutions  in  which  the  health  of  many  people  is  main- 
tained as  economically  as  possible.  Such  an  examination 
made  by  Moleschott  gives  us  as  its  result  a  diet  containing 
daily — 

Proteids 30  grams  or       465  grains 

Fats 84       "      or    1,300       " 

Carbohydrates 404       "      or    6, '.262 

Salts..'. 30       "      or       465       " 

Water 2800       "      or  43,400 

People  in  easy  circumstances  take  as  a  rule  more  proteids 
and  fats  and  less  amyloids;  and  this  selection,  when  a  choice 
is  possible,  probably  indicates  that  such  a  diet  is  the  better 
one:  the  proteids  in  the  above  table  seem  especially  deficient. 
Experimenting  on  himself  the  physiologist  Ranke  found  that 
when  he  was  in  good  health,  neither  gaining  nor  losing 
weight,  and  excreting  daily  as  much  nitrogen  as  he  took  in 
food,  he  maintained  this  condition  of  equilibrium  on  a  diet 
containing 

Proteids 100  grams  (    1550  grains) 

Fats 100      "      (    1550      "     ) 

Carbohydrates 240      "      (    3720      "      ) 

Salts 25      "      (      437      "      ) 

Water 2600      "      (40,400      "     ) 

Other  experimenters  have  since  arrived  at  very  similar  re- 
sults; and  such  a  diet  is  probably  about  the  normal  for  per- 
sons of  our  race  living  in  a  temperate  climate. 


CHAPTER  XXX. 

THE  PRODUCTION  AND  REGULATION  OF  THE  HEAT  OF 
THE  BODY. 

Cold-  and  Warm-blooded  Animals.  All  animals,  so 
long  as  they  are  alive,  are  the  seat  of  chemical  changes  by 
which  heat  is  liberated;  hence  all  tend  to  be  somewhat 
warmer  than  their  ordinary  surroundings,  though  the  differ- 
ence may  not  be  noticeable  unless  the  heat  production  is 
considerable.  A  frog  or  a  fish  is  a  little  hotter  than  the  air 
or  water  m  which  it  lives,  but  not  much;  the  little  heat  that 
it  produces  is  lost,  by  radiation  or  conduction,  almost  at  once. 
Hence  such  animals  have  no  proper  temperature  of  their  own; 
on  a  warm  day  they  are  warm,  on  a  cold  day  cold,  and  are 
accordingly  known  as  chimgeable-temperatured  (poikilo-ther- 
mous)  or,  in  ordinary  language,  "cold-blooded"  animals. 
Man  and  other  mammals,  as  well  as  birds,  on  the  contrary, 
are  the  seat  of  very  active  chemical  changes  by  which  much 
heat  is  produced,  and  so  maintain  a  tolerably  uniform  tem- 
perature of  their  own,  much  as  a  fire  does  whether  it  be  burn- 
ing in  a  warm  or  a  cold  room ;  the  heat  production  during 
any  given  time  balancing  the  loss,  a  normal  body  temperature 
is  maintained,  and  usually  one  considerably  higher  than  that 
of  the  medium  in  which  they  live;  such  animals  are  com- 
monly named  "  warm-blooded."  This  name,  however,  does 
not  properly  express  the  facts;  a  lizard  basking  in  the  sun 
on  a  warm  summer's  day  may  be  quite  as  hot  as  a  man  usu- 
ally is;  but  on  the  cold  day  the  lizard  becomes  cold,  while 
the  average  temperature  of  the  healthy  Human  Body  is, 
within  a  degree,  the  same  in  winter  or  summer;  within  the 
arctic  circle  or  on  the  equator.  Hence  it  is  better  to  call 
such  animals  "  homotherm  cms"  or  of  uniform   temperature. 

Moderate  warmth  accelerates  protoplasmic  activity;  com- 
pare ;t  frog  dormant  in  the  winter  with  the  same  animal  ac- 
tive in  the  warm  months:  what  is  true  of  the  whole  frog  is 
true  of  each  of  its  living  cells.     Its  muscles  contract  more 

477 


478  TEE  HUMAN  BODY. 

rapidly  when  warmed,  and  the  white  corpuscles  of  its  blood 
when  heated  up  to  the  temperature  of  the  Human  Body  are 
seen  (with  the  microscope)  to  exhibit  much  more  active  amoe- 
boid movements  than  they  do  at  the  temperature  of  frog's 
blood.  In  summer  a  frog  or  other  cold-blooded  animal  uses 
much  more  oxygen  and  evolves  much  more  carbon  dioxide 
than  in  winter,  as  shown  not  only  by  direct  measurements  of 
its  gaseous  exchanges,  but  by  the  fact  that  in  winter  a  frog 
can  live  a  long  time  after  its  lungs  have  been  removed  (being 
able  to  breathe  sufficiently  through  its  moist  skin),  while  in 
warm  weather  it  dies  of  asphyxia  very  soon  after  the  same 
loss.  The  wanner  weather  puts  its  tissues  in  a  more  active 
state;  and  so  the  amount  of  work  the  animal  does,  and  there- 
fore the  amount  of  oxygen  it  needs,  depend  to  a  great  extent 
upon  the  temperature  of  the  medium  in  which  it  is  living. 
With  the  warm-blooded  animal  the  reverse  is  the  case.  Within 
very  wide  limits  of  exposure  to  heat  or  cold  it  maintains  its 
temperature  at  that  at  which  its  tissues  live  best;  accordingly 
in  cold  weather  it  uses  more  oxygen  and  sets  free  more  carbon 
dioxide  because  it  needs  a  more  active  internal  combustion  to 
compensate  for  its  greater  loss  of  heat  to  the  exterior.  And 
it  does  not  become  warmer  in  warm  weather,  partly  because 
its  oxidations  are  less  than  in  cold  (other  things  being  equal), 
and  partly  because  of  physiological  arrangements  by  which  it 
loses  heat  faster  from  its  body.  In  fact  the  living  tissues  of 
a  man  may  be  compared  to  hothouse  plants,  living  in  an  arti- 
ficially maintained  temperature;  but  they  differ  from  the 
plants  in  the  fact  that  they  themselves  are  the  seats  of  the 
combustions  by  which  the  temperature  is  kept  up.  Since, 
within  wide  limits,  the  Human  Body  retains  the  same  temper- 
ature no  matter  whether  it  be  in  cold  or  warm  surroundings, 
it  is  clear  that  it  must  possess  an  accurate  arrangement  for 
heat  regulation;  either  by  controlling  the  production  of  heat 
in  it,  or  the  loss  of  heat  from  it,  or  both. 

The  Temperature  of  the  Body.  The  parts  of  the  Body 
are  all  either  in  contact  with  one  another  directly  or,  if  not, 
at  least  indirectly  through  the  blood,  which,  flowing  from 
part  to  part,  carries  heat  from  warmer  to  colder  regions. 
Thus,  although  at  one  time  one  group  of  muscles  may  espe- 
cially work,  liberating  heat,  and  at  other  times  another,  or 
the  muscles  may  be  at  rest  and  the  glands  the  seat  of  active 
oxidation,  the  temperature  of  the  whole  Bodv  is  kept  pretty 
much  the  same.     The  skin,  however,  which  is  in  direct  con- 


THE  HEAT  OF  THE  BODY.  479 

tact  with  external  bodies,  usually  colder  than  itself,  is  cooler 
than  the  internal  organs;  its  temperature  in  health  is  from 
30°  to  37°  C.  (9G.8-9S.50  F.),  being  warmer  in  more  protected 
parts,  as  the  hollow  of  the  armpit.  In  internal  organs,  as 
the  liver  and  brain,  the  temperature  is  higher;  about  43°  C. 
(107°  F.)  in  health.  In  the  lungs  there  is  a  certain  quantity 
of  heat  liberated  when  oxygen  combines  with  haemoglobin,  but 
this  is  more  than  counterbalanced  by  loss  of  the  heat  carried 
out  by  the  expired  air  and  that  used  up  in  evaporating  the 
water  carried  out  in  the  breath,  so  the  blood  returned  to  the 
heart  by  the  pulmonary  veins  is  slightly  colder  than  that 
carried  from  the  right  side  of  the  heart  to  the  lungs. 

The  Sources  of  Animal  Heat.  Apart  from  heat  received 
from  its  surroundings  in  hot  food  and  drink  the  sources  of 
heat  in  the  Body  are  twofold — direct  and  indirect.  Heat  is 
directly  produced  wherever  oxidation  is  taking  place;  and, 
since  almost  invariably  the  chemically  degrading  or  katabolic 
processes  going  on  in  a  living  organ  exceed  the  anabolic,  the 
living  tissues  at  rest  produce  heat  as  one  result  of  the  chemical 
changes  supplying  them  with  energy  for  the  maintenance  of 
their  vitality:  and  whenever  an  organ  is  active  and  its  chemi- 
cal metamorphoses  are  increased  it  becomes  hotter:  a  secret- 
ing gland  or  a  contracting  muscle  is  warmer  than  a  resting 
one,  and  the  venous  blood  leaving  noticeably  warmer  than 
the  arterial  supplied  to  it.  Indirectly,  heat  is  developed 
within  the  Body  by  the  transformation  of  other  forms  of  en* 
orgy:  mainly  mechanical  work,  but  also  of  electricity.  All 
movements  of  parts  of  the  Body  which  do  not  move  it  in 
space  or  move  external  objects,  are  transformed  into  heat 
within  it:  and  the  energy  they  represent  is  lost  in  that  form. 
Every  cardiac  contraction  sets  the  blood  in  movement,  and 
this  motion  is  for  the  most  part  turned  into  heat  within  the 
Body  by  friction  within  the  blood-vessels.  The  same  trans- 
formation of  energy  occurs  with  respect  to  the  movements  of 
the  alimentary  canal,  except  in  so  far  as  they  expel  matters 
from  the  Body;  and  every  muscle  in  contracting  has  part  of 
the  mechanical  energy  expended  by  it  turned  into  heat  by 
friction  against  neighboring  parts.  Similarly  the  movements 
ot  cilia  and  of  amoeboid  colls  are  for  the  most  part  converted 
in  tin-  Body  into  heat.  The  muscles  and  nerves  are  also  the 
of  manifestations  of  electricity,  which,  though  small  in 
amount,  for  the  most  part  do  not  leave  the  Body  in  that  form 
but  are  first  converted  into  heat. 


480  THE  HUMAN  BODY. 

The  Energy  Lost  by  the  Body  in  Twenty-four  Houry 

Practically  speaking,  the  Body  only  loses  energy  in  two 
forms;  as  heat  and  mechanical  work:  by  applying  conduct- 
ors to  different  parts  of  its  surface  small  amounts  of  elec- 
tricity can  be  carried  off,  but  the  amount  is  quite  trivia!  in 
comparison  with  the  total  daily  energy  expenditure.  During 
complete  rest,  that  is,  when  no  more  work  is  done  than  that 
necessary  for  the  maintenance  of  life,  nearly  all  the  loss  takes 
the  form  of  heat.  The  absolute  amount  of  this  will  vary 
with  the  surrounding  temperature  and  other  conditions,  but 
mi  an  average  a  man  loses,  during  a  day  of  rest,  2700  calories; 
that  is  enough  to  raise  2700  kilograms  (5940  lbs.)  of  water 
from  0°  to  1"  C.  (from  32°  to  33. 8°  F.);  otherwise  expressed, 
this  amount  of  heat  would  boil  27  kilos  (59.4  lbs.)  of  ice-cold 
water.  This  does  not  quite  represent  all  the  energy  lost  by 
the  Body  in  that  time:  since  a  small  jiroportion  is  lost  as 
mechanical  work  in  moving  the  clothes  and  air  by  the  respir- 
atory movements,  and  even  by  the  beat  of  the  heart,  which  at 
each  systole  pushes  out  the  chest-wall  a  little  and  moves  the 
things  in  contact  with  it.  The  working  Body  liberates  and 
loses  much  more  energy;  part  as  mechanical  work  done  on 
external  objects,  part  as  increased  heat  radiated  or  conducted 
from  the  surface,  or  carried  off  by  the  expired  air  in  the 
quickened  respirations.  Every  one  knows  that  he  feels 
warmer  when  he  takes  exercise,  and  this  is  due  to  the  greater 
amount  of  blood  then  carried  to  the  skin  and  raising  for  the 
time  its  temperature.  The  general  temperature  of  the  Body 
as  measured  in  the  mouth  is  not  at  all  or  only  very  slightly 
raised,  however,  as  the  greater  loss  of  heat  from  the  skin  keeps 
the  average  temperature  of  the  blood  at  its  normal  level.  This 
greater  loss  corresponding  to  the  greater  production  has  been 
measured  on  persons  enclosed  in  specially  constructed  calori- 
meters; and  though  there  are  considerable  difficulties  in  the 
May  of  getting  quite  accurate  results,  the  measurements  show 
that  the  heat  produced  and  lost  in  a  day  of  moderate  work  is 
about  one  third  greater  than  that  in  a  day  of  rest.  The  fol- 
lowing table  gives  more  definite  numbers: 

Day  of  Rest.  Day  of  Work. 


Rest  16  hrs.     Sleep  8  hrs.  Rest  8  hrs.    Work  8  hrs.  Sleep  8  hrs. 

Ur^o6u%T\  ^0A_  J*0__         _123_5.^_  J16Q.6 m_ 

=van  a  I  10.885  Fall. -lb.  \  »~oi  q  /  14.S88  Fah.-lb.  \ 

-w*  \     heat-units.     /  *<*»•»  ^     heat-units.     J 


THE  HEAT  OF  THE  BODY.  481 

The  mechanical  work  done  on  the  working  day  presented 
in  addition  an  expenditure  of  energy  of  213,344  kilogram- 
meters,  which  is  equal  to  502  calories.  Of  the  excess  heat  in 
the  working  day,  part  is  directly  produced  by  the  increased 
chemical  changes  in  the  quicker  working  heart  and  respira- 
tory muscles,  and  the  other  muscles  set  at  work;  while  part 
is  indirectly  due  to  heat  arising  from  increased  friction  in  the 
blood-vessels  as  the  blood  is  driven  faster  around  them,  and 
to  friction  of  the  various  muscles  used.  The  average  cardiac 
work  in  twenty-four  hours  is  about  60,000  kilogrammeters; 
that  of  the  respiratory  muscles  about  14,000;  and  since  nearly 
all  of  both  is  turned  finally  into  heat  within  the  Bod}-,  we 
have  74,000  kilogrammeters  of  energy  answering  to  about  174 
calories  (6786  Fah.-lb.  units)  indirectly  produced  in  the  rest- 
ing Body  daily  from  these  sources. 

Of  100  parts  of  heat  lost  from  the  resting  Body,  about 
74.7  are  carried  off  in  radiation  or  conduction  from  the  skin. 
14. ."J  are  carried  off  in  evaporation  from  the  skin. 

5.4   "         «  «  «  «        «    lungs. 

3.6    "         "  "      expired  air. 

1.8   "         "  "      the  excretions. 

In  a  day  of  average  work,  of  every  100  parts  of  energy  lost 
in  any  form  from  the  Body — 

1-2  go  as  heat  in  the  excreta. 

3-4  in  heating  the  expired  air. 
20-30  in  evaporating  water  from  the  lungs  and  skin. 
60-75  in  heat  radiated  or  conducted  from  the  surfaces  and  in 
external  mechanical  work. 
It  is  obvious,  however,  that  such  numbers  are  only  rough 
approximations  and  must  vary  greatly  with  the  temperature 
and  moisture  of  the  surrounding  air,  the  rate  of  respiration, 
and  other  circumstances. 

The  Superiority  of  the  Body  as  a  Working  Machine. 
Du liner  eight  hours  of  work  we  find  (see  table)  the  Body 
loses  2169.6  calories  of  energy  as  heat,  and  can  do  simul- 
taneously work  equivalent  to  502  calories.  So  of  all  the 
energy  lost  from  if  in  that  time  about  4  may  take  the  form  of 
mechanical  work;  this  is  a  very  large  proportion  of  the  total 
energy  expended,  being  a  much  higher  percentage  than  that 
given  by  ordinary  machines.  The  best  steam-engines  can 
utilize  as  mechanical  work  only  about  I  of  the  total  energy 
liberated  in  them  and  lost  from  them  in  a  given   time,  the 


482  TUB  HUMAN  BODY. 

remainder  is  transmitted  directly  as  heat  to  the  exterior,  and 
is  lost  to  the  engine  for  all  useful  purposes. 

The  Maintenance  of  an  Average  Temperature.  This  is 
necessary  for  the  continuance  of  the  life  of  a  warm-blooded 
animal;  should  the  temperature  rise  above  certain  limits 
chemical  changes,  incompatible  with  life,  occur  in  the  tissues 
for  example  at  about  4!>'J  0.  (120°  F.)  the  muscles  begin  to 
become  rigid.  On  the  other  hand,  death  ensues  if  the  Body 
be  cooled  down  to  about  19°  0.  (06°  F.).  Hence  the  need 
of  means  for  getting  rid  of  excess  heat,  and  of  protection 
from  excessive  cooling.  Either  end  may  be  gained  in  two 
ways:  by  altering  the  rate  at  which  heat  is  lost  or  that  at 
which  it  is  produced.  As  regards  heat-loss,  by  far  the  most 
important  regulating  organ  is  the  skin:  under  ordinary  cir- 
cumstances nearly  90  per  cent  of  the  total  heat  given  off  from 
the  Body  in  24  hours  goes  by  the  skin  (73  by  radiation  and 
conduction,  14.5  by  evaporation).  This  loss  may  be  con- 
trolled— 

1.  By  clothing  ;  we  naturally  wear  more  in  cold  and  less 
in  warm  weather;  the  effect  of  clothes  being,  of  course,  not 
to  warm  the  Body  but  to  diminish  the  rate  at  which  the  heat 
produced  in  it  is  lost. 

2.  Increased  temperature  of  the  surrounding  medium  in- 
creases the  activity  of  the  heart  and  lungs.  A  hastened  cir- 
culation by  itself  does  not,  as  already  pointed  out  (Chap. 
XXVI),  increase  the  general  tissue  activity  of  the  Body,  or 
the  oxidations  occurring  in  it,  and  so,  apart  from  the  harder- 
working  heart  itself,  does  not  influence  the  amount  of  heat 
liberated  in  the  Body  during  a  given  time:  but  the  more  rapid 
blood-flow  through  the  skin  carries  more  of  that  fluid  through 
this  cool  surface  in  each  minute  and  in  that  way  increases 
the  loss  of  heat.  The  quickened  respirations,  too,  increase  the 
evaporation  of  water  from  the  lungs  and,  thus,  the  loss  of  heat. 

3.  Warmth,  mainly  through  reflex  vaso-moter  actions  leads 
to  dilatation  of  the  skin-vessels  and  cold  to  contraction.  In 
a  warm  room  the  vessels  on  the  surface  dilate  as  shown  by  its 
redness,  while  in  a  cold  atmosphere  they  contract  and  the 
skin  becomes  pale.  But  the  more  blood  that  flows  through 
the  skin  the  greater  will  be  the  heat  lost  from  the  surface— 
and  vice  versa. 

4.  Heat  induces  sweating  and  cold  checks  it;  the  heat 
appears  to  act,  partly,  reflexly  through   afferent   cutaneous 


THE  HEAT  OF  THE  BODY.  483 

nerve  fibres  exciting  the  sweat-centres  from  which  the 
secretory  nerves  for  the  sudoriparous  glands  arise  and,  partly, 
directly  on  those  centres,  as  they  are  thrown  into  activity,  at 
least  in  health,  as  soon  as  the  temperature  of  the  blood  flow- 
ing through  the  spinal  cord  is  raised.  In  fever  of  course  we 
may  have  a  high  temperature  with  a  dry  non-sweating  skin. 
The  more  there  is  sweat  poured  out,  the  more  heat  is  used 
up  in  evaporating  it  and  the  more  the  Body  is  cooled. 

5.  Our  sensations  induce  us  to  add  to  or  diminish  the 
heat  in  the  Body  according  to  circumstances;  as  by  cold  or 
warm  baths,  and  iced  or  hot  drinks. 

As  regards  temperature-regulation  by  modifying  the  rate 
of  heart  production  in  the  Body,  the  following  points  may  be 
noted ;  on  the  whole,  such  regulation  is  far  less  important 
than  that  brought  about  by  changes  in  the  rate  of  loss,  since 
the  necessary  vital  work  of  the  Body  always  necessitates  the 
continuance  of  oxidative  processes  which  liberate  a  tolerably 
large  quantity  of  heat.  The  Body  cannot  therefore  be  cooled 
by  diminishing  such  oxidations;  nor,  on  the  other  hand,  can 
it  be  safely  warmed  by  largely  increasing  them.  Still,  within 
certain  limits,  the  heat  production  may  be  controlled  in 
several  ways  : 

1.  Cold  increases  hunger;  and  increased  ingestion  of 
food  increases  bodily  oxidation,  as  shown  by  the  greater 
amount  of  carbon  dioxide  excreted  in  the  hours  succeeding 
a  meal.  This  increase  is  probably  due  to  the  activity  into 
which  the  digestive  organs  and  such  metabolic  organs  as  the 
liver  are  thrown ;  hepatic-vein  blood  is  about  one  degree  cen- 
tigrade (nearly  two  degrees  Fahrenheit)  warmer  than  portal- 
vein  blood,  and  during  digestion  much  more  blood  flows 
through  the  liver. 

2.  Cold  inclines  us  to  voluntary  exercise;  warmth  to 
muscular  idleness;  and  the  more  the  muscles  are  worked  the 
more  heat  is  produced  in  the  Body. 

3.  Cold  tends  to  produce  involuntary  muscular  move- 
ments, and  so  increased  heat  production;  as  chattering  of 
the  teeth  and  shivering. 

4.  Cold  applied  to  the  skin  increases  the  bodily  chemical 
metamorphoses  and  heat  production.  At  least  the  tem- 
perature in  the  armpit  rises  at  first  on  entering  a  cold  bath, 
though  the  heat  carried  off  from  the  surface  soon  over- 
balances its  increased  production.     The  phenomenon  may, 


484  THE  HUMAN  BODY. 

however,  be  explained  in  another  way,  the  rise  being  at- 
tributed to  a  sudden*  diminution  of  loss  from  more  exposed 
parts  of  the  skin,  dependent  on  contraction  of  the  cutaneous 
arteries.  In  some  cases,  however,  the  temporary  rise  is  accom- 
panied by  an  increased  excretion  of  carbon  dioxide,  which 
would  indicate  that  the  surface  cooling  does  really  increase 
the  oxidations  of  the  Body. 

5.  Certain  drugs,  as  salicylic  acid,  and  perhaps  quinine, 
diminish  the  heat  production  of  the  Body.  Their  mode  of 
action  is  still  obscure. 

On  the  whole,  however,  the  direct  heat-regulating  mech 
anisms    of   the   Human    Body  itself   are    not  very  efficient, 
especially   as   protections    against   excessive   cooling.      Man 
needs  to  supplement  them  by  the  use  of  clothing,  fuel,  and 
exercise. 

Local  Temperatures.  Although,  by  the  means  above 
described,  a  wonderfully  uniform  bodily  temperature  is 
maintained,  and  by  the  circulating  blood  all  parts  are  kept 
at  nearly  the  same  warmth,  variations  in  both  respects  do 
occur.  The  arrangements  for  equalization  are  not  by  any 
means  fully  efficient.  External  parts,  as  the  skin,  the  lungs 
(which  are  really  external  in  the  sense  of  being  in  contact 
with  the  air),  the  mouth,  and  the  nose  chambers,  are  always 
cooler  than  internal;  and  even  all  parts  of  the  skin  have  not 
the  same  temperature,  such  hollows  as  the  armpit  being 
warmer  than  more  exposed  regions.  On  the  other  hand,  a 
secreting  gland  or  a  working  muscle  becomes  warmer,  for 
the  time,  than  the  rest  of  the  Body,  because  more  heat  is 
liberated  in  it  than  is  carried  off  by  the  blood  flowing 
through.  In  such  organs  the  venous  blood  leaving  is  warmer 
than  the  arterial  coming  to  them;  while  the  reverse  is  the 
case  with  parts,  like  the  skin,  in  which  the  blood  is  cooled. 
An  organ  colder  than  the  blood  is  of  course  warmed  by  an 
increase  in  its  circulation,  as  seen  in  the  local  rise  of  temper- 
ature in  the  skin  of  the  face  in  blushing. 

Thermogenic  Nerves.  All  nerves,  such  as  motor  or 
secretory,  which  can  throw  working  tissues  into  activity  are 
in  a  certain  sense  thermogenic  nerves,  since  they  excite  in- 
creased oxidation  and  heat  production  in  the  parts  under 
their  control.  A  true,  purely  thermogenic  nerve  would  be 
one  which  increased  the  heat  production  in  a  tissue  without 
otherwise  throwing  it  into  activity;   and  whether  such  exist 


THE  HEAT  OF  THE  BODY.  485 

is  still  undecided.  Certain  phenomena  of  disease,  however, 
seem  to  render  their  existence  probable.  If  we  return  for  a 
moment  to  our  former  comparison  of  the  working  Body  to 
a  steam-engine,  such  nerves  might  be  regarded  as  agencies 
increasing  its  rate  of  rusting  without  setting  it  at  work. 
The  oxidation  of  the  iron  would  develop  some  heat,  but  by 
processes  useless  to  the  steam-engine,  although  such  are,  in 
moderation,  essential  to  living  cells;  the  vitality  of  these, 
even  when  they  rest,  seems  to  necessitate  a  constant,  if  small, 
breaking  down  of  their  substance.  In  an  amoeboid  cell  no 
doubt  such  processes  occur  quite  independently  of  the  ner- 
vous system;  out  in  more  differentiated  tissues  they  may  be 
controlled  by  it.  Just  as  a  muscle  does  not  normally  con- 
tract unless  excited  through  its  nerve,  although  a  white 
blood-corpuscle  does,  so  may  the  natural  nutritive  processes 
of  the  muscle-fibre  in  its  resting  condition  be  dependent  on 
the  nerves  going  to  it.  If  these  be  abnormally  excited  the 
muscle  will  break  down  its  protoplasm  faster  than  it  con- 
structs it,  and  consequently  waste;  at  the  same  time  the 
increased  chemical  degradation  of  its  substance  will  elevate 
its  temperature.  Febrile  conditions,  in  which  many  tissues 
waste,  without  any  unusual  manifestation  of  their  normal 
physiological  activity,  would  thus  be  readily  accounted  for 
as  due  to  superexcitation  of  the  thermogenic  nerves  and 
nerve-centre. 

The  condition  of  fever  or  pyrexia,  as  an  abnormally  high 
temperature  is  named,  could  conceivably  be  brought  about  by 
increased  heat  production,  decreased  heat  loss,  or  both;  or 
by  a  greater  increase  of  production  than  of  loss.  Direct  ex- 
periments on  animals  prove  that  there  is  always  increased 
production  of  heat,  in  febrile  diseases.  This  is  shown  by  the 
fact  that  the  animal  uses  more  oxygen  and  gives  off  more 
carbon  dioxide  in  a  given  time  than  when  in  health.  It  also 
usually  gives  off  more  heat,  but  not  enough  to  compensate  for 
the  increase  of  oxidative  processes  going  on  in  its  body,  and 
so  it.s  temperature  rises.  The  regulating  mechanism  which 
in  health  keeps  heat  production  and  heat  dissipation  propor- 
tionate is  out  of  gear.  As  regards  the  increased  heat  formation 
in  pyrexia]  conditions,  there  is  some  reason  to  believe  that  it 
is  usually  due  to  excitation  by  morbid  products  of  thermogenic 
centres  lying  in  the  corpora  striata  or  optic  thalami.  Prick- 
ing those  regions  of  the  brain  of  an  animal  causes  greatly  in- 


486  THE  HUMAN  BODY. 

creased  heat  formation  in  its  body.  This  has  been  interpreted 
either  as  due  to  the  excitation  of  thermogenic  nerve-centres 
which  then  stir  up  increased  katabolisms  in  the  tissues  or  to 
injury  and  paralysis  of  inhibitory  centres  which  normally 
hold  tissue  metabolisms  in  check.  The  fact  that  a  similar 
result  may  be  obtained  by  electrical  stimulation  of  this  region 
of  the  brain  is  in  favor  of  the  excitation  theory,  but  the  possi- 
bility of  the  existence  also  of  febrile  paralysis  of  nerve-cells 
which  normally  inhibit  a  heat-production  centre  should  be 
borne  in  mind. 

Clothing.  While  the  majority  of  other  warm-blooded 
animals  have  coats  of  their  own,  formed  of  hairs  or  feathers, 
over  most  of  man's  Body  his  capillary  coating  is  merely  rudi- 
mentary and  has  lost  nearly  all  physiological  importance  as  a 
protection  from  cold;  except  in  tropical  regions  he  has  to 
protect  himself  by  artificial  garments,  which  his  aesthetic 
sense  has  led  him  to  utilize  also  for  purposes  of  adornment. 
Here,  however,  we  must  confine  ourselves  to  clothes  from  a 
physiological  point  of  view.  In  civilized  societies  every  one 
is  required  to  cover  most  of  his  Body  with  something,  and 
the  question  is  what  is  the  best  covering;  the  answer  will 
vary,  of  course,  with  the  climatic  conditions  of  the  country 
dwelt  in.  In  warm  countries,  clothing,  in  general  terms, 
should  allow  free  radiation  or  conduction  of  heat  from  the 
surface;  in  cold  it  should  do  the  reverse;  and  in  temperate 
climates,  with  varying  tern j^eratu res,  it  should  vary  with  the 
season.  If  the  surface  of  the  Body  be  exposed  so  that  cur- 
rents of  air  can  freely  traverse  it  much  more  heat  will  be 
carried  off  (under  those  usual  conditions  in  which  the  air  is 
cooler  than  the  skin)  than  if  a  stationary  layer  of  air  be  main- 
tained in  contact  with  the  surface.  As  every  one  knows,  a 
"  draught"  cools  much  faster  than  air  of  the  same  tempera- 
ture not  in  motion.  All  clothing,  therefore,  tends  to  keep 
up  the  temperature  of  the  Body  by  checking  the  renewal  of 
the  layer  of  air  in  contact  with  it.  Apart  from  this,  how- 
ever, clothes  fall  into  two  great  groups:  those  which  are 
good,  and  those  which  are  bad,  conductors  of  heat.  The 
former  allow  changes  in  the  external  temperature  to  cool  or 
heat  rapidly  the  air  stratum  in  actual  contact  with  the  Body, 
while  the  latter  only  permit  these  changes  to  act  more  slowly. 
Of  the  materials  used  for  clothes,  linen  is  a  good  conductor; 


THE  HEAT  OF  THE  BODY.  487 

calico  not  quite  so  good;  and  silk,  wool,  and  fur  are  bad  con- 
ductors. 

Whenever  the  surface  of  the  Body  is  suddenly  chilled 
the  skin-vessels  are  contracted  and  those  of  internal  parts 
reilexly  dilated ;  hence  internal  organs  tend  to  become  con- 
gested;  this  within  limits  is  a  protective  physiological  pro- 
cess, but  if  excessive  it  readily  passes  into  the  diseased  state 
known  as  inflammation.  When  hot,  therefore,  the  most 
unadvisable  thing  to  do  is  to  sit  in  a  draught,  throw  off  the 
clothing,  or  in  other  ways  to  strive  to  get  suddenly  cooled. 
Moreover,  while  in  the  American  summer  it  is  tolerably  safe 
to  wear  good-conducting  garments,  and  few  people  take  cold 
then,  this  is  by  no  means  safe  in  the  spring  or  autumn,  when 
the  temperature  of  the  air  is  apt  to  vary  considerably  within 
the  course  of  a  day.  A  person  going  out,  clad  only  for  a 
warm  morning,  may  have  to  return  in  a  very  much  colder 
evening;  and  if  his  clothes  be  not  such  as  to  prevent  a  sud- 
den surface  chill,  will  get  off  lightly  if  he  only  "  take  "  one 
of  the  colds  so  prevalent  at  those  seasons.  In  the  great 
majority  of  cases,  no  doubt,  he  suffers  nothing  worse,  but 
persons,  especially  of  the  female  sex,  often  thus  acquire  far 
more  serious  diseases.  When  sudden  changes  of  temperature 
are  at  all  probable,  even  if  the  prevailing  weather  be  warm, 
the  trunk  of  the  Body  should  be  always  protected  by  some 
tolerably  close-fitting  garment  of  non-conducting  material. 
Those  whose  skins  are  irritated  by  anything  but  linen  should 
wear  immediately  outside  the  under-garments  a  jacket  of 
silken  or  woollen  material.  In  mid-winter  comparatively  few 
people  take  cold,  because  all  then  wear  thick  and  noncon- 
ducting clothing  of  some  kind. 


CHAPTER   XXXI. 

SENSATION   AND  SENSE-ORGANS. 

The  Subjective  Functions  of  the  Nervous  System. 
Changes  in  many  parts  of  our  Bodies  are  accompanied  or 
followed  by  those  states  of  consciousness  which  we  call  sen- 
sations. All  such  sensitive  parts  are  in  connection,  direct 
or  indirect,  with  the  brain,  by  certain  afferent  nerve-fibres 
called  sensor//.  Since  all  feeling  is  lost  in  any  region  of  the 
Body  when  this  connecting  path  is  severed,  it  is  clear  that 
all  sensations,  whatever  their  primary  exciting  cause,  are 
finally  dependent  on  conditions  of  the  central  nervous  system. 
Hitherto  we  have  studied  this  as  its  activities  are  revealed 
through  movements  which  it  excites  or  prevents;  we  have 
seen  it,  directly  or  reflexly,  cause  muscles  to  contract,  glands 
to  secrete,  or  the  pulsations  of  the  heart  to  cease;  we  have 
viewed  it  objectively,  as  a  motion-regulating  apparatus.  Now 
we  have  to  turn  to  another  side  and  consider  it  (or  parts  of 
it)  as  influencing  the  states  of  consciousness  of  its  possessor: 
this  study  of  the  subjective  activities  of  the  nervous  system  is 
one  of  much  greater  difficulty. 

It  may  be  objected  that  considerations  concerning  states 
of  feeling  have  no  proper  place  m  a  treatise  on  Anatomy 
and  Physiology;  that,  since  we  cannot  form  the  beginning 
of  a  conception  how  a  certain  state  of  the  nervous  system 
causes  the  feeling  redness,  another  the  feeling  blueness,  and 
a  third  the  emotion  anger,  all  examination  of  mental  phe- 
nomena should  be  excluded  from  the  sciences  dealing  with 
the  structure  and  properties  of  living  things.  But,  although 
we  cannot  imagine  how  a  nervous  state  (neurosis)  gives  rise 
to  a  conscious  state  (psychosis),  we  do  know  this,  that  dis- 
tinct phenomena  of  consciousness  never  come  under  our 
observation  apart  from  a  nervous  system,  and  so  are  pre- 
sumably, in  some  way,  endowments  of  it;  we  are,  therefore, 
justified  in  calling  them  properties  of  the  nervous  system; 

488 


SENSATION  AND  SENSE-OEGANS.  489 

and  their  examination,  especially  with  respect  to  what  nerve- 
parts  are  concerned  with  different  mental  states,  and  what 
changes  in  the  former  are  associated  with  given  phenomena 
in  the  latter,  forms  properly  a  part  of  Physiology.  Whether 
masses  of  protoplasm,  before  the  differentiation  of  definite 
nerve-tissues,  possess  some  ill-defined  sort  of  consciousness, 
as  they  possess  an  indefinite  contractility  before  they  have 
been  modified  into  muscular  fibres,  may  for  the  present  be 
left  undecided:  though  those  who  accept  the  doctrine  of 
evolution  will  be  inclined  to  assent  to  the  proposition. 

While,  however,  the  physiologist  has  a  right  to  be  heard 
on  cpiestions  relating  to  our  mental  faculties,  it  is  neverthe- 
less true  that  many  laws  of  thought  have  been  established 
concerning  which  our  present  knowledge  of  the  laws  of  the 
nervous  system  gives  us  no  clue;  the  science  of  Psychology 
has  thus  a  well-founded  claim  to  an  independent  existence. 
But,  in  so  far  as  its  results  are  confined  merely  to  the  succes- 
sions and  connections  of  mental  states,  as  established  by 
observation,  they  are  merely  descriptions,  and  not  explana- 
tions in  a  scientific  sense:  we  know  that  so  many  mental  phe- 
nomena have  necessary  material  antecedents  and  concomi- 
tants in  nervous  changes,  that  we  are  justified  in  believing 
that  all  have  such,  and  in  continuing  to  seek  for  them.  We 
do  not  know  at  all  how  an  electric  current  sent  round  a  bar 
of  soft  iron  makes  it  magnetic;  we  only  know  that  the  one 
change  is  accompanied  by  the  other;  but  we  say  we  have 
explained  the  magnetism  of  a  piece  of  iron  if  we  have  found 
an  electric  current  circulating  around  it.  Similarly,  we  do 
not  know  how  a  nervous  change  causes  a  mental  state,  but 
we  have  not  explained  the  mental  state  until  we  nave  found 
the  nervous  state  associated  with  it  and  how  that  nervous 
state  was  produced. 

As  yet  it  is  only  with  respect  to  some  of  the  simplest 
states  of  consciousness  that  we  know  much  of  the  necessary 
physiological  antecedents,  and  among  these  our  sensations 
are  the  best  investigated.  As  regards  such  mental  phenom- 
ena as  the  Association  of  Ideas  and  Memory,  physiology 
can  give  us  some  light;  but  so  far  as  others,  such  as  the  Will 
and  the  Emotions,  are  concerned,  it  has  at  present  little  to 
offer.  The  phenomena  of  Sensation,  therefore,  occupy  at 
present  a  much  larger  portion  of  physiological  works  than 
all  other  mental  facts  put  together. 


490  THE  HUMAN  BODY. 

Common  Sensation  and  Organs  of  Special  Sense.  A 
sensory  nerve  is  one  which,  when  stimulated,  arouses,  or  may 
arouse,  a  sensation  in  its  possessor.  The  stimulant  is  in  all 
cases  some  form  of  motion,  molar  (e.g.,  mechanical  pressure) 
or  molecular  (as  ethereal  vibrations  or  chemical  changes). 
Since  all  our  nerves  lie  within  our  Bodies  as  circumscribed 
by  the  skin,  and  are  excited  within  them,  one  might  a  priori 
be  inclined  to  suppose  that  the  cause  of  all  sensations  would 
appear  to  be  within  our  Bodies  themselves;  that  the  thing 
felt  would  be  a  modified  portion  of  the  feeler.  This  is  the 
case  with  regard  to  many  sensations;  a  headache,  toothache, 
or  earache  gives  us  no  idea  of  any  external  object;  it  merely 
suggests  to  each  of  us  a  particular  state  of  a  sensitive  portion 
of  myself.  As  regards  many  sensations,  however,  this  is  not 
so;  they  suggest  to  us  external  causes,  to  properties  of  which, 
and  not  to  states  of  our  Bodies,  we  ascribe  them;  and  so  they 
lead  us  to  the  conception  of  an  external  universe.  A  knife 
laid  on  the  skin  produces  changes  in  it  which  lead  us  to 
think  not  of  a  state  of  our  skin,  but  of  states  of  some  object 
outside  the  skin;  we  believe  we  feel  a  cold  heavy  hard  thing 
in  contact  with  it.  Nevertheless  wre  have  no  sensory  nerves 
going  into  the  knife  and  informing  us  directly  of  its  condi- 
tion; what  we  really  feel  are  the  modifications  of  our  Body 
produced  by  it,  although  we  irresistibly  think  of  them  as 
properties  of  the  knife — of  some  object  that  is  no  part  of  the 
Body,  and  not  of  them  as  states  of  the  latter.  Let  now  the 
knife  cut  through  the  skin;  we  feel  no  more  knife,  but  ex- 
perience pain,  which  we  think  of  as  a  condition  of  ourselves. 
We  do  not  say  the  knife  is  painful,  but  that  our  finger  is,  and 
yet  we  have,  so  far  as  sensation  goes,  as  much  reason  to  call 
the  knife  painful  as  cold.  Applied  one  way  it  produced 
local  changes  arousing  a  sensation  of  cold,  and  in  another 
local  changes  causing  a  sensation  of  pain.  Nevertheless  in 
the  one  case  we  speak  of  the  cold  as  being  in  the  knife,  and 
in  the  other  of  the  pain  as  being  in  the  finger. 

Sensitive  parts,  such  as  the  surface  of  the  skin,  through 
which  wre  get,  or  believe  we  get,  information  about  outer 
things,  are  of  far  more  intellectual  value  to  us  than  sensitive 
parts,  such  as  the  subcutaneous  tissue  into  which  the  knife 
may  cut,  which  give  us  only  sensations  referred  to  conditions 
of  our  Bodies.     The  former  are  called  Sense-organs  proper. 


SENSATION  AND  SENSE-ORGANS.  491 

•or  Organs  of  Special  Sense  ;  the  latter  are  sensitive  parts, 
or  Organs  of  Common  Sensation. 

The  Peripheral  Reference  of  our  Sensations  The  fact 
that  we  refer  certain  sensations  to  external  causes  is  only 
one  case  of  a  more  general  law,  in  accordance  with  which 
we  do  not  ascribe  our  sensations,  as  regards  their  locality,  to 
the  brain,  where  the  neurosis  is  accompanied  by  the  sensa- 
tion, but  to  a  peripheral  part.  With  respect  to  the  brum, 
other  parts  of  the  Body  are  external  objects  as  much  as  the 
rest  of  the  material  universe,  yet  we  locate  the  majority  of 
our  common  sensations  at  the  places  where  the  sensory 
nerves  concerned  are  irritated,  and  not  in  the  brain.  Even 
if  a  nerve-trunk  be  stimulated  in  the  middle  of  its  course, 
we  refer  the  resulting  sensation  to  its  outer  endings.  A  blow 
on  the  inside  of  the  elbow-joint,  injuring  the  ulnar  nerve, 
produces  not  only  a  local  pain,  but  a  sense  of  tingling 
ascribed  to  the  fingers  to  which  the  ends  of  the  fibres  go. 
Persons  with  amputated  limbs  have  feelings  in  their  fingers 
and  toes  long  after  they  have  been  lost,  if  the  nerve-trunks 
in  the  stump  be  irritated.  To  explain  such  facts  we  must 
trench  on  the  ground  of  Psychology,  and  so  they  cannot  be 
fully  discussed  here;  but  they  are  commonly  ascribed  to  the 
results  of  experience.  The  events  of  life  have  taught  us  that 
in  the  great  majority  of  instances  the  sensory  impulses  which 
excite  a  given  tactile  sensation,  for  example,  have  acted  upon 
the  tip  of  a  finger.  The  sensation  goes  when  the  finger  is 
removed,  and  returns  when  it  is  replaced;  and  the  eye  con- 
firms the  contact  of  the  external  object  with  the  finger-tip 
when  we  get  the  tactile  sensation  in  question.  We  thus 
come  firmly  to  associate  a  particular  region  of  the  skin  with 
a  given  sensation,  and  whenever  afterwards  the  nerve-fibres 
coming  from  the  finger  are  stimulated,  no  matter  where  in 
their  course,  we  ascribe  the  origin  of  the  sensation  to  some 
thing  acting  on  the  finger-tip. 

The  Differences  between  Sensations.  In  both  groups 
<>f  sensations,  those  derived  through  organs  of  special  sense 
and  those  due  to  organs  of  common  sensation,  we  distinguish 
kinds  which  are  absolutely  distinct  for  our  consciousness, 
and  tiot  comparable  mentally.  We  can  never  get  confused 
between  a  sight,  a  sound,  and  a  touch,  nor  between  pain, 
hunger,  and  nansea;  nor  can  we  compare  them  with  one 
another:    each   is  sui  generis.     The  fundamental  difference 


492  THE  HUMAN  BODY. 

which  thus  separates  one  sensation  from  another  is  its 
modality.  Sensations  of  the  same  modality  may  differ;  but 
they  shade  imperceptibly  into  one  another,  and  are  com- 
parable between  themselves  in  two  ways.  First,  as  regards 
quality:  while  a  high  and  a  low  pitched  note  are  both 
auditory  sensations,  they  are  nevertheless  different  and  yet 
intelligibly  comparable;  and  so  are  blue,  purple,  and  red  ob- 
jects. In  the  second  place,  sensations  of  the  same  modality  are 
distinguishable  and  comparable  as  to  amount  or  intensity:  we 
readily  recognize  and  compare  a  loud  and  a  weak  sound  of 
the  same  pitch;  a  bright  and  feeble  light  of  the  same  color; 
an  acute  and  a  slight  pain  of  the  same  general  character. 
Our  sensations  thus  differ  in  the  three  aspects  of  modality, 
quali///  within  tJ/e  same  modality,  and  intensity.  Certain 
sensations  also  differ  in  what  is  known  as  the  "  local  signs'* 
a  difference  by  which  we  tell  a  touch  on  one  part  of  the  skin 
from  a  similar  touch  on  another;  or  an  object  exciting  one 
part  of  the  eye  from  an  object  like  it,  but  in  a  different  loca- 
tion in  space  and  exciting  another  part  of  the  visual  surface. 
As  regards  modality,  we  commonly  distinguish  five  senses, 
those  of  sight,  sound,  touch,  taste,  and  smell;  to  these,  tem- 
perature must  be  added.  The  varieties  of  common  sensation 
are  also  several;  for  example,  pain,  hunger,  satiety,  thirst, 
nausea,  malaise,  bien  etre  ("  feeling  good  "),  fatigue.  The 
muscular  sense  stands  on  the  intermediate  line  between 
special  and  common  sensations;  we  gather  by  it  how  much 
our  various  muscles  are  contracted:  and  so  learn  the  position 
of  various  parts  of  the  Body,  on  the  one  hand,  and  the  re- 
sistance opposed  to  bodily  movement  by  external  objects,  on 
the  other.  In  fact,  we  cannot  draw  a  sharp  line  between  the 
special  senses  and  common  sensations:  all  the  Body,  we  con- 
clude from  observations  on  the  lower  animals,  is,  at  an  early 
stage  of  its  development,  sensitive;  very  soon  its  cells  sepa- 
rate themselves  into  an  outer  layer  exposed  to  the  action  of 
external  forces  and  an  inner  layer  protected  from  them :  and 
some  of  the  former  cells  become  especially  sensitive.  From 
them,  as  development  proceeds,  some  are  separated  and 
buried  beneath  the  surface  to  become  the  brain  and  spinal 
cord;  of  those  which  remain  superficial,  some  are  modified 
so  that  they  (in  the  eye)  become  especially  excited  by  ethereal 
vibrations;  others  (in  the  ear)  become  especially  responsive 
to   sound   vibrations;    others  to  slight  chemical  changes  (in 


SENSATION  AND  SENSE-OROANS.  493 

mouth  and  nose),  and  others  (in  the  skin)  to  variations  in 
pressure  or  temperature. 

All  our  sensations  are  thus  modifications  of  one  common 
primary  sensibility,  represented  by  that  of  the  skin,  or  rather 
by  the  primitive  representative  of  the  skin  in  such  an  animal 
as  the  Hydra  (see  Zoology).  The  cutaneous  sensations,  being 
less  differentiated,  shade  off  more  readily  into  the  common 
sensibility  of  the  other  living  tissues  than  do  the  activities  of 
the  highly  differentiated  cells  in  the  eye  and  ear.  We  find, 
accordingly,  that  while  a  powerful  pressure  or  a  high  tem- 
perature acting  on  the  skin  readily  arouses  a  sensation  of 
pain,  that  this  is  not  the  case  with  the  more  specialized  visual 
and  auditory  organs.  Their  super-excitement  may  be  dis- 
agreeable, but  never  passes  into  pain,  in  the  ordinary  sense 
of  the  word.  Similarly  the  special  skin  sensations,  touch 
and  temperature,  may  sometimes  be  confounded,  while  a 
sound  and  a  sight  cannot  be  :  the  modality  of  the  less  modi- 
fied skin-senses  is  less  complete. 

The  study  of  comparative  anatomy  and  development 
shows  that  the  irritable  parts  of  our  sense-organs  are  but 
special  differentiations  of  the  primary  external  layer  of  cells 
which  covered  the  Body  when  it  was  very  young.  Some  of 
these  cells  become  nerve  end-organs  in  the  eye,  others  end- 
organs  in  the  ear,  and  so  on;  while  others,  less  changed,  re- 
main in  the  skin  as  organs  of  touch  and  temperature;  and 
so,  from  a  general  exterior  surface  responding  equally  readily 
to  many  external  natural  forces,  we  get  a  surface  modified  so 
that  its  various  parts  respond  with  different  degrees  of  read- 
iness to  different  external  forces;  and  these  modified  parts 
constitute  the  essential  portions  of  our  organs  of  special  sense. 
Every  sense  organ  thus  comes  to  have  a  special  relationship  to 
some  one  natural  force  or  form  of  energy — is  a  specially 
irritable  mechanism  by  which  such  a  force  is  enabled  to  excite 
sensory  nerves;  and  is,  moreover,  commonly  supplemented  by 
arrangements  which,  in  the  ordinary  circumstances  of  life, 
prevent  other  forces  from  stimulating  the  nerves  connected 
with  it.  Not  all  natural  forces  have  sense-organs  with  ref- 
erence  to  them  developed  in  the  Human  Body;  for  example, 
we  have  no  organ  standing  to  electrical  changes  in  the  same 
relation  that  the  eye  does  to  light  or  the  ear  to  sound. 

The  Essential  Structure  of  a  Sense-organ.  In  every 
.sen.-e-organ  the  fundamental  part  is  one  or  more  end-organs, 


494  THE  IU  MAX  BODT. 

which  are  highly  irritable  tissues  (p.  31),  so  constructed  and 
so  placed  as  to  be  normally  acted  on  by  some  one  of  the 
modes  of  motion  met  with  in  the  external  world.  A  sensory 
apparatus  requires  in  addition  at  least  a  brain-centre  and  a 
sensory  nerve-fibre  connecting  this  with  the  terminal  appa- 
ratus; but  one  commonly  finds  accessory  parts  added.  In 
the  eye,  e.g.,  we  have  arrangements  for  bringing  to  a  focus 
the  light  rays  which  are  to  act  on  the  end  organs  of  the 
nerve-fibres;  and  in  the  ear  are  found  similar  subsidiary 
parts,  to  conduct  sonorous  vibrations  to  the  end  apparatus  of 
the  auditory  nerve. 

Seeing  and  hearing  are  the  two  most  specialized  senses; 
the  stimuli  usually  arousing  them  are  peculiar  and  quite  dis- 
tinct from  the  group  of  general  nerve  stimuli  (Chap.  XIII), 
while  those  most  frequently,  or  naturally,  acting  upon  our 
other  sense-organs  are  not  so  peculiar;  they  are  forces 
which  act  as  general  nerve  stimuli  when  directly  applied  to 
nerve-fibres.  The  end-organs,  however,  as  already  pointed 
out,  so  increase  the  sensitiveness  of  the  parts  containing 
them  that  degrees  of  change  in  the  exciting  forces,  which 
would  be  totally  unable  to  directly  stimulate  the  nerve-fibres, 
are  appreciated.  These  terminal  apparatuses  are  therefore 
as  truly  mechanisms  enabling  changes,  which  would  not 
otherwise  stimulate  nerves,  to  excite  them,  as  are  the  end- 
organs  in  the  eye  or  ear. 

The  Cause  of  trie  Modality  of  our  Sensations.  Seeing 
that  the  external  forces  usually  exciting  our  different  sensa- 
tions differ,  and  that  the  sensations  do  also,  we  might  at  first 
be  inclined  to  believe  that  the  latter  difference  depended  on 
the  former:  that  brightness  differed  from  loudness  because 
light  was  different  from  sound.  In  other  words,  we  are  apt 
to  think  that  each  sensation  derives  its  specific  character 
from  some  property  of  its  external  physical  antecedent,  and 
that  our  sensations  answer  in  some  way  to,  and  represent 
more  or  less  accurately,  properties  of  the  forms  of  energy 
arousing  them.  It  is,  however,  quite  easy  to  show  that  we 
have  no  sufficient  logical  warrant  for  such  a  belief.  Light 
falling  into  the  eye  causes  a  sensation  of  luminosity,  a  feel- 
ing belonging  to  the  visual  group  or  modality:  and,  since 
usually  nothing  else  excites  such  feelings  and  light  entering 
the  healthy  eye  always  does,  we  come  to  believe  that  the 
physical    agent    light   is    something    like   our   sensation    of 


SENSATION  AND  SENSE-ORGANS  495 

luminosity.  Bat,  as  we  have  already  seen,  no  matter 
how  we  stimulate  the  optic  nerve  we  still  get  visual  sensa- 
tions; close  the  eyes  and  press  with  a  finger-nail  on  one  eve- 
lid;  a  sensation  of  touch  is  aroused  where  the  finger  meets 
the  skin;  but  the  pressure  on  the  eyeball  distorts  it  and 
stimulates  the  optic  nerve-fibres  in  it  also,  and  the  result  is 
a  luminous  patch  seen  in  front  of  the  eye  in  such  a  position 
as  a  bright  body  must  occupy  in  space  to  radiate  light  to  the 
stimulated  part  of  the  expansion  of  the  optic  nerve.  Finding, 
then,  the  same  kind  of  sensation,  a  visual  one,  produced  by 
the  totally  different  causes,  pressure  and  light,  we  are  led  to 
doubt  if  the  differences  of  modality  in  our  sensations  depend 
upon  the  differences  of  the  natural  forces  arousing  them; 
and  this  doubt  is  strengthened  when  we  find  still  other  forces 
giving  rise  to  visual  sensations.  But  then,  since  light 
and  pressure,  electricity  and  cutting,  all  cause  visual  sensa- 
tions, we  have  no  valid  reason  for  supposing  that  light,  more 
than  either  of  the  others,  is  really  in  any  way  like  our  sensa- 
tion of  light:  or  that  sight-feeling  differs  from  sound-feeling 
because  objectively  light  differs  from  sound.  The  eye  is  an 
organ  specially  set  apart  to  be  excited  by  light,  and  accord- 
ingly so  fixed  as  to  have  its  nerve-fibres  far  more  often  ex- 
cited by  that  form  of  force  than  by  any  other;  but  the  fact 
that  light  sensations  can  be  otherwise  aroused  shows  plainly 
that  their  kind  or  character  has  nothing  directly  to  do  with 
any  property  of  light.  Just  as  by  pinching  or  heating  or 
galvanizing  a  motor  nerve  we  can  make  the  muscles  attached 
to  it  contract,  and  the  contraction  has  nothing  in  common 
with  the  excitant,  so  the  visual  sensation,  as  such,  is  inde- 
pendent of  the  stimulus  arousing  it  and,  of  itself,  tells  us 
nothing  concerning  the  kind  of  stimulus  which  has  operated. 
Differences  in  kind  between  external  forces  being  thus 
eliminated  as  possible  causes  of  the  modalities  of  our  sensa- 
tions, we  next  naturally  fall  back  upon  differences  in  the 
8ense-oigans  themselves.  They  do  undoubtedly  differ  both 
in  gross  and  microscopic  structure,  and  the  fact  that  pressure 
on  the  closed  eye  arooses  a  touch-feeling  where  the  skin  is 
compressed,  and  a  sight-feeling  where  optic  nerve-fibres  are, 
might  well  be  due  to  the  fad  that  a  peripheral  touch-organ 
wna  different  from  a  peripheral  Bight-organ,  and  the  same 
force  might  therefore  produce  totally  different  effects  on 
them  and   so  cause  different    kinds  of  feelings.     However, 


496  THE  HUMAN  BODY. 

here  also  c.oser  examination  shows  that  we  must  seeK  farther. 
Sensation  is  not  produced  in  a  sense-organ,  but  far  away 
from  it  in  the  brain;  the  organ  is  merely  an  apparatus  for 
generating  nervous  impulses.  Jf  the  optic  nerves  be  divided, 
no  matter  how  perfect  the  eyeballs,  no  amount  of  light  will 
arouse  visual  sensations;  if  the  spinal  cord  be  cut  in  the 
middle  of  the  back  no  pressure  on  the  feet  will  cause  a  tactile 
or  other  feeling;  though  the  skin,  and  its  nerves  and  the 
lower  half  of  tbe  spinal  cord  be  all  intact.  In  all  cases  we 
find  that  if  the  nerve-paths  between  a  sense-organ  and  the 
brain  be  severed  no  stimulation  of  the  organ  will  call  forth  a 
sensation.  The  final  production  of  this  clearly  depends, 
then,  on  something  occurring  in  the  brain,  and  so  the  kind 
of  a  sensation  is  presumably  dependent  upon  brain  events 
rather  than  on  occurrences  in  sense-organs.  Still  it  might 
be  that  something  in  the  sense-organ  caused  one  sensa- 
tion to  differ  from  another.  Each  organ  might  excite  the 
brain  in  a  different  way  and  cause  a  different  sensation,  and 
so  our  sensations  differ  because  our  sense  organs  do.  Such 
a  view  is,  however,  negatived  by  observations  which  show 
that  perfectly  characteristic  sensations  can  be  felt  in  the 
absence  of  the  sense-organs  through  which  they  are  normally 
excited.  Persons  whose  eyeballs  have  been  removed  by  the 
surgeon,  or  completely  destroyed  by  disease,  have  frequently 
afterwards  definite  and  unmistakable  visual  sensations,  quite 
as  characteristic  as  those  which  they  had  while  still  possess- 
ing the  visual  end  organs.  The  tactile  sensations  felt  in  am- 
putated limbs,  already  referred  to,  afford  another  example 
of  the  same  fact.  The  persons  still  feel  things  touching 
their  legs  or  lying  between  their  long-lost  toes;  and  the  sen- 
sations are  distinctly  tactile  and  not  in  any  way  less  different 
from  visual  or  auditory  sensations  than  are  the  touch-feelings 
following  stimulation  of  those  parts  of  the  skin  which  are  still 
possessed.  It  is,  then,  clear  that  the  modality  of  our  sensa- 
tions is  to  be  sought  deeper  than  in  properties  of  the  end- 
organs  of  the  nerves  of  each  sense. 

Properties  of  external  forces  and  properties  of  periph- 
eral nerve-organs  being  excluded  as  causes  of  differences  in 
kind  of  sensation,  we  come  next  to  the  sensory  nerve-fibres 
themselves.  Is  it  because  optic  nerve-fibres  are  different 
from  auditory  nerve-fibres  that  luminous  sensations  are  dif- 
ferent from  sonorous  ?     This  question  must  be  answered  in 


SENSATION  AND  SENSE-ORGANS.  497 

the  negative,  for  we  have  already  seen  reason  to  believe 
that  all  nerve-fibres  are  alike  in  essential  structure  and  that 
their  properties  are  everywhere  the  same;  that  all  they  do  is 
to  transmit  "nervous  impulses"  when  excited,  and  that,  no 
matter  what  the  excitant,  these  impulses  are  molecular  move- 
ments, always  alike  in  kind,  though  they  may  differ  in 
amount  and  in  rate  of  succession.  Since,  then,  all  that  the 
optic  nerve  does  is  to  send  nervous  impulses  to  the  brain, 
and  all  that  the  auditory  and  gustatory  and  tactile  and  olfac- 
tory nerve-fibres  do  is  the  same,  and  these  impulses  are  all 
alike  in  kind,  we  cannot  explain  the  difference  in  quality  of 
visual  and  other  sensations  by  any  differences  in  property  of 
the  nerve-trunks  concerned,  any  more  than  we  could  attempt 
to  explain  the  facts  that,  in  one  case,  an  electric  current  sent 
through  a  thin  platinum  wire  heats  it,  and,  in  another,  sent 
through  a  solution  of  a  salt  decomposes  it,  by  assuming  that 
the  different  results  depend  on  differences  in  the  conducting 
copper  wires,  which  may  be  absolutely  alike  in  the  two  cases. 
We  are  thus  driven  to  conclude  that  our  sensations  pri- 
marily differ  because  different  central  nerve-organs  in  the 
brain  are  concerned  in  their  production.  That  just  as  an 
efferent  nerve-fibre  will,  when  stimulated,  cause  a  secretion  if 
it  go  to  a  gland-cell,  and  a  contraction  if  it  go  to  a  muscle- 
fibre,  so  an  optic  nerve-fibre,  carrying  impulses  to  one  brain 
apparatus  and  exciting  it,  will  cause  a  visual  sensation,  and  a 
gustatory  nerve-fibre,  connected  with  another  brain-centre,  a 
taste  sensation.  In  other  words,  our  kinds  of  sensation 
depend  fundamentally  on  the  properties  of  our  own  cerebral 
nervous  system.  For  each  special  sense  we  have  a  nervous 
apparatus  with  its  peripheral  terminal  organs,  its  nerve-fibres, 
and  its  brain-centres;  and  the  excitement  of  this  apparatus,  no 
matter  in  what  way,  causes  a  sensation  of  a  given  modality, 
determined  by  the  properties  of  its  central  portion.  Usually 
the  apparatus  is  excited  by  one  particular  force  acting  first 
on  its  peripheral  organs,  but  it  may  be  aroused  by  stimulat- 
ing its  nerve-fibres  directly  or,  as  in  certain  diseased  states 
(delirium),  or  under  the  action  of  certain  drugs,  by  direct 
excitation  of  the  centres.  The  sensations  of  dreams,  fre- 
quently bo  vivid,  and  hallucinations,  are  also  probably  in 
many  cases  due  to  direct  excitation  of  the  central  organs  of 
ory  apparatuses,  though  no  doubt  also  often  due  to  periph- 
eral stimulation,     But  no    matter  how  or   where  the  appa 


498  THE  II r  MAX  BODY, 

ratus  is  excited,  provided  a  sensation  is  produced  it.  is  always 
of  the  modality  of  that  sense  apparatus. 

While  in  the  more  specialized  senses  the  modality  of  the 
sensation  can  be  ascribed  only  to  brain  properties  (so  that 
we  may  be  pretty  sure  that  a  man,  the  inner  end  of  whose 
optic  nerve  was  in  physiological  continuity  with  the  outer 
end  of  his  auditory,  and  the  inner  end  of  his  auditory  with 
the  outer  end  of  his  optic,  would  hear  a  picture  and  see  a 
symphony),  yet,  conceivably,  differences  in  the  rhythm  or 
intensity  of  afferent  nervous  impulses  might  cause  differ- 
ences in  modality  in  less  differentiated  senses.  Until  quite 
recently  it. has  been  considered  possible  that  tactile  and  tem- 
perature  sensations  were  but  extremes  of  one  general  kind  of 
feeling;  that  they  were  of  the  same  "  modality; "  and  com- 
parable, for  example,  to  the  sensations  of  yellow  and  blue  in 
the  visual  set  of  feelings.  This  view  has  now  been  definitely 
proved  to  be  inadmissible  (Chap.  XXXY).  The  points  of  the 
skiu  which  arouse  in  us  the  sensations  of  touch,  heat,  and  cold 
are  all  distinct;  each  one  when  stimulated  gives  rise  to  only 
one  kind  of  sensation,  if  any;  and  always  the  same  kind.  A 
heavy  pressure,  gradually  increased,  arouses  sensations  which 
pass  imperceptibly  from  touch  to  pain,  and  this  result  may 
be  due  to  the  fact  that  regular  and  orderly  afferent  impulses, 
determined  through  tactile  nerve-endings,  excite  the  centre 
in  one  way;  while  irregular,  disorderly,  and  violent  impulses, 
originated  when  the  pressure  is  great  enough  to  directly 
excite  nerve-trunks  beneath  the  skin,  may  cause  a  different 
sensation;  much  as  musical  notes  properly  combined  may 
cause  pleasure,  but  all  clashed  together  may  cause  suffering, 
although  the  same  brain-centres  are  stimulated  in  the  two 
cases.  The  pain  from  a  heavy  weight  may,  however,  be  due 
to  the  fact  that  it  excites  a  different  set  of  nerve-fibres  than 
those  connected  with  tactile  feeling,  and  gives  rise  to  impulses 
which  excite  new  centres,  the  modality  of  which  is  a  pain 
sensation  so  great  as  to  cloak  concomitant  touch  sensations. 

However  differences  in  nervous  rhythm  may  account  for 
minor  differences  in  sensation,  it  remains  clear  that  the 
characters  of  our  sensations  are  creations  of  our  own  organ- 
ism;  they  depend  on  properties  of  our  Bodies  and  not  on 
properties  of  external  things,  except  in  so  far  as  these  may 
or  may  not  be  adapted  to  arouse  our  different  sensory  appa- 
ratuses to  activity.     From  the  kind  of  the  sensation  we  can- 


SENSATION  AND   SENSE-ORGANS.  499 

not,  therefore,  argue  as  to  the  nature  of  the  excitant:  we 
have  no  more  warrant  for  supposing  that  light  is  like  our 
sensation  of  light  than  that  the  knife  that  cuts  us  is  like  our 
sensation  of  pain.  All  that  we  know  with  certainty  is  states 
of  our  own  consciousness,  and  although  from  these  we  form 
working  hypotheses  as  to  an  external  universe,  yet,  granting 
it.  ve  have  no  means  of  acquiring  any  real  knowledge  as  to 
the  properties  of  things  about  us.  What  we  want  to  know, 
however,  for  the  practical  purposes  of  life  is,  not  what  things 
are,  but  how  to  use  them  for  our  advantage,  or  to  prevent 
them  from  acting  to  our  disadvantage;  and  our  senses  en- 
able us  to  do  this  sufficiently  well. 

The  Psycho-Physical  Law.  Although  our  sensations 
are,  in  modality  or  kind,  independent  of  the  force  exciting 
them,  they  are  not  so  in  degree  or  intensity,  at  least  within 
certain  limits.  We  cannot  measure  the  amount  of  a  sensa- 
tion and  express  it  in  foot-pounds  or  calories,  but  we  can  get 
a  sort  of  unit  by  determining  how  small  a  difference  in  sensa- 
tion can  be  perceived.  Supposing  this  smallest  perceptible 
difference  to  be  constant  within  the  range  of  the  same  sense 
(which  is  not  proved),  it  is  found  that  it  is  produced  by  dif- 
ferent amounts  of  stimuli,  measured  objectively  as  forces; 
and  that  there  exists  in  some  cases  a  relation  between  the  two 
which  can  be  expressed  in  numbers.  The  increase  of  stimu- 
lus necessary  to  ■produce  the  smallest  perceptible  change  in  a 
sensation  is  proportional  to  the  strength  of  the  stimulus 
already  acting;  for  example,  the  heavier  a  pressure  already 
acting  on  the  skin  the  more  must  it  be  increased  or  dimin- 
ished in  order  that  the  increase  or  diminution  may  be  felt. 
Expressed  in  another  way  the  facts  may  be  put  thus:  sup- 
pose three  degrees  of  stimulation  to  bear  to  one  another  ob- 
jectively the  ratios  10,  100,  1000,  then  their  subjective  ef- 
fects, or  the  amounts  of  sensation  aroused  by  them,  will  be 
respectively  as  1,2,  3:  in  other  words,  the  sensation  increases 
proportionately  to  the  logarithm  of  the  strength  of  the  stimu- 
lus. Examples  of  this,  which  is  known  as  "  Weber's"  or 
"  FceJi tier's  psycho-physical  lam"  will  be  hereafter  pointed 
<»ut,  and  are  readily  observable  in  daily  life;  we  have,  for 
example,  a  luminous  sensation  of  certain  intensity  when  a 
lighted  candle  is  brought  into  a  dark  room;  this  sensation  is 
not  doubled  when  a  second  candle  is  brought,  in;  and  is 
hardly  affected  at  all  by  a  third.     The  law  is  only  true,  how- 


500  THE  HUMAN  BODY. 

ever  (and  then  but  approximately),  for  sensations  of  medium 
intensity;  it  is  applicable,  for  example,  to  light  sensations  of 
all  degrees  between  those  aroused  by  the  light  of  a  candle 
and  ordinary  clear  daylight:  but  it  is  not  true  lor  Luminosi- 
ties so  feeble  as  only  to  be  seen  at  all  with  difficulty,  or  so 
bright  as  to  be  dazzling. 

Besides  their  variations  in  intensity,  dependent  on  varia- 
tions in  the  strength  of  the  stimulus,  our  sensations  also  vary 
with  the  irritability  of  the  sensory  apparatus  itself;  which  is 
not  constant  from  time  to  time  or  from  person  to  person. 
In  the  above  statements  the  condition  of  the  sense-organ  and 
its  nervous  connections  is  presumed  to  remain  the  same 
throughout. 

Perceptions.  In  every  sensation  we  have  to  carefully 
distinguish  between  the  pure  sensation  and  certain  judg- 
ments founded  upon  it;  we  have  to  distinguish  between  what 
we  really  feel  and  what  we  think  we  feel;  and  very  often 
firmly  believe  we  do  feel  when  we  do  not. 

The  most  important  of  these  judgments  is  that  which 
leads  us  to  ascribe  certain  sensations,  those  aroused  through 
organs  of  special  sense,  to  external  objects — that  outer  refer- 
ence of  our  sensations  which  leads  us  to  form  ideas  concern- 
ing the  existence,  form,  position,  and  properties  of  external 
things.  Such  representations  as  these,  founded  on  our  senses, 
are  called  perceptions.  Since  these  always  imply  some 
mental  activity  in  addition  to  a  mere  feeling,  their  full  dis- 
cussion belongs  to  the  domain  of  Psychology.  Physiology, 
however,  is  concerned  with  them  so  far  as  it  can  determine 
the  conditions  of  stimulation  and  neurosis  under  which  a 
given  mental  representation  concerning  a  sensation  is  made. 
It  is  quite  certain  that  we  can  feel  nothing  but  states  of  our- 
selves, but,  as  already  pointed  out,  we  have  no  hesitation  in 
saying  we  feel  a  hard  or  a  cold,  a  rough  or  smooth  body. 
When  we  look  at  a  distant  object  we  usually  make  no  demur 
to  saying  that  we  perceive  it.  What  we  really  feel  is,  how- 
ever, the  change  produced  by  it  in  our  eyes.  There  are  no 
parts  of  our  Bodies  reaching  to  a  tree  or  a  house  a  mile  off — 
and  yet  we  seem  to  feel  all  the  while  that  we  are  looking  at 
the  tree  or  the  house  and  feeling  them,  and  not  merely  ex- 
periencing modifications  of  our  own  eyes  or  brains.  When 
reading  we  feel  that  what  we  really  see  is  the  book;   and  yet 


SENSATION  AND  SENSE-ORGANS.  501 

the  existence  of  the  book  is  a  judgment  founded  on  a  state 
of  our  Body,  which  alone  is  what  we  truly  feel. 

We  have  the  same  experience  in  other  cases,  for  example 
with  regard  to  touch. 

Hairs  are  quite  insensible,  but  are  imbedded  in  the  sensi- 
tive skin,  which  is  excited  when  they  are  moved.  But 
if  the  tip  of  a  hair  be  touched  by  some  external  object  we 
believe  we  feel  the  contact  at  its  insensible  end,  and  not  in 
the  sensitive  skin  at  its  root.  So,  the  hard  parts  of  the  teeth 
are  insensible;  yet  when  we  rub  them  together  we  refer  the 
seat  of  the  sensation  aroused  to  the  points  where  they  touch 
one  another,  and  not  to  the  sensitive  parts  around  the  sockets 
where  the  sensory  nerve  impulse  is  really  started. 

Still  more,  we  may  refer  tactile  sensations,  not  merely  to 
the  distal  ends  of  insensible  bodies  implanted  in  the  skin, 
but  to  the  far  ends  of  things  which  are  not  parts  of  our 
Bodies  at  all;  for  instance,  the  distant  end  of  a  rod  held 
between  the  finger  and  a  table  while  the  finger  is  moved  a  little 
from  side  to  side.  We  then  believe  we  feel  touch  or  pressure 
in  two  places;  one  where  the  rod  touches  our  finger,  and  the 
other  where  it  comes  in  contact  with  the  table.  A  blind 
man  gropes  his  way  along  by  feeling  at  the  end  of  his  stick. 
If  the  rod  is  attached  immovably  to  the  table  we  feel  only 
its  end  next  the  finger.  If  we  could  fix  it  immovably  on  the 
finger  while  the  other  end  was  movable  on  the  table,  we 
would  lose  the  sensation  at  the  finger  and  refer  the  sensa- 
tion of  pressure  to  where  the  rod  touched  the  table.  When  a 
tooth  is  touched  with  a  rod  we  only  feel  the  contact  at  its 
end,  unless  it  is  loose  in  its  socket;  and  then  we  get  two 
sensations  on  touching  its  free  end  with  a  foreign  body. 

This  irresistible  mental  tendency  to  refer  certain  of  our 
states  of  feeling  to  causes  outside  of  our  Bodies,  and  either  in 
contact  with  them  or  separated  from  them  by  a  certain  space, 
is  known  as  the  phenomenon  of  the  extrinsic  reference  of  our 
sensations.  The  discussion  of  its  origin  belongs  properly  to 
Psychology,  and  it  will  suffice  here  to  point  out  that  it  seems 
largely  to  depend  on  the  fact  that  the  sensations  extrinsically 
referral  can  be  modified  by  movements  of  our  Bodies. 
Hanger,  thirst,  and  toothache  all  remain  the  same  whether 
we  turn  to  the  right  or  left,  or  move  away  from  the  place  we 
are  .standing  in.  But  a  Bound  is  altered.  We  may  find  that 
in  a  certain  position  of  the  head  it  is  heard  more  by  the 


502  THE  HUMAN  BODY. 

right  ear  than  the  left;  but  on  turning  round  the  reverse  is 
the  case;  and  half  way  round  the  loudness  in  each  ear  is  the 
same.  Hence  we  are  led,  by  mental  laws  outside  of  the 
physiological  domain,  to  suspeel  thai  its  cause  is  not  in  our 
Body,  but  outside  of  it;  and  depends  not  on  a  condition  of 
the  Body  but  on  something  else  And  this  is  confirmed 
when  going  in  one  direction  we  find'  the  sound  increased, 
and  in  the  other  that  it  is  diminished.  This  implies  that  we 
have  a  knowledge  of  our  movements,  and  this  we  gain 
through  the  muscular  sense.  It  constitutes  the  reactive  side 
of  our  sensory  life,  associated  with  the  changes  Ave  produce 
in  external  things;  and  is  correlated  and  contrasted  with  the 
passive  side,  in  which  other  things  produce  sensations  by  act- 
ing upon  us. 

As  regards  our  common  sensations  we  find  something  of 
the  same  kind.  The  more  readily  they  can  be  modified  by 
movement  the  more  definitely  do  we  localize  them  in  space, 
though  iTi  this  case  within  the  Body  instead  of  outside  it. 
Hunger  and  nausea  can  be  altered  by  pressure  on  the  pit  of 
the  stomach;  thirst  by  moistening  the  throat  with  water; 
the  desire  for  oxygen  (respiration-hunger)  by  movements  of 
the  chest;  and  so  we  more  or  less  definitely  ascribe  these 
sensations  to  conditions  of  those  parts  of  the  Body.  Other 
general  sensations,  as  depression,  anxiety,  and  so  on,  are  not 
modifiable  by  any  particular  movement,  and  so  appear  to  us 
rather  as  mental  states,  pure  and  simple,  than  bodily  sensa- 
tions. 

Sensory  Illusions.  "I  must  believe  my  own  eyes"  and 
"  we  can't  always  believe  our  senses "  are  two  expressions 
frequently  heard,  and  each  expressing  a  truth.  No  doubt  a 
sensation  in  itself  is  an  absolute  incontrovertible  fact:  if  I 
feel  redness  or  hotness  I  do  feel  it,  and  that  is  an  end  of  the 
matter:  but  if  I  go  beyond  the  fact  of  my  having  a  certain 
sensation  and  conclude  from  it  as  to  properties  of  something 
else — if  I  form  a  judgment  from  my  sensation — I  may  be 
totally  wrong;  and  in  so  far  be  unable  to  believe  my  eyes  or 
skin.  Such  judgments  are  almost  inextricably  woven  up 
with  many  of  our  sensations,  and  so  closely  that  we  cannot 
readily  separate  the  two;  not  even  when  we  know  that  the 
judgment  is  erroneous. 

For  example,  the  moon  when  rising  or  setting  appears 
bigger  than   when  high  in    the  heavens — we   seem   to   feel 


SENSATION  AND  SENSE-ORGANS.  503 

directly  that  it  arouses  more  sensation,  and  yet  we  know  cer- 
tainly that  it  does  not.  With  a  body  of  a  given  brightness 
the  amount  of  change  produced  in  the  end  organs  of  the  eye 
will  depend  on  the  size  of  the  image  formed  in  the  eye,  pro- 
vided the  same  part  of  its  sensory  surface  is  acted  upon. 
Now  the  size  of  this  image  depends  on  the  distance  of  the 
object;  it  is  smaller  the  farther  off  it  is  and  greater  the 
nearer,  and  measurements  show  that  the  area  of  the  sensitive 
surface  affected  by  the  image  of  the  rising  moon  is  no  larger 
than  that  affected  by  it  when  overhead.  Why  then  do  we, 
even  after  we  know  this,  see  it  bigger  ?  The  reason  is  that 
when  the  moon  is  near  the  horizon  we  imagine,  unconsciously 
and  irresistibly,  that  it  is  farther  off;  even  astronomers  who 
know  perfectly  well  that  it  is  not,  cannot  help  forming  this 
unconscious  and  erroneous  judgment — and  to  them  the  moon 
appears  in  consequence  larger  when  near  the  horizon,  just  as 
it  does  to  less  well-informed  mortals.  In  fact  we  have  a  con- 
ception of  the  sky  over  which  the  moon  seems  to  travel,  not  as 
a  half  sphere  but  as  somewhat  flattened,  and  hence  when  the 
moon  is  at  the  horizon  we  unconsciously  judge  that  it  is 
farther  off  than  when  overhead.  But  any  body  which  ex- 
cites the  same  extent  of  the  sensitive  surface  of  the  eye  at  a 
great  distance  that  another  does  at  less,  must  be  larger  than 
the  latter;  and  so  we  conclude  that  the  moon  at  the  horizon 
is  larger  than  the  moon  in  the  zenith,  and  are  ready  to  de- 
clare that  we  see  it  so. 

So,  again,  a  small  bit  of  pale  gray  paper  on  a  white 
sheet  looks  gray:  but  placed  on  a  large  bright  green  surface 
it  looks  purple;  and  on  a  bright  red  surface  looks  blue- 
green.  As  the  same  bit  of  gray  paper  is  shifted  from  one  to 
the  other  we  see  it  change  its  color:  it  arouses  in  us  different 
feelings,  or  feelings  which  we  interpret  differently,  although 
objectively  the  light  reflected  from  it  remains  the  same. 
Similarly  a  medium-sized  man  alongside  of  a  very  tall  one 
appears  short,  but  when  walking  with  a  very  short  one,  tall. 

Such  erroneous  perceptions  as  these  are  known  as  sensory 
illusions;  and  we  ought  to  be  constantly  on  guard  against 
them. 


CHAPTER   XXXII. 

THE    EYE    AS  AN   OPTICAL   INSTRUMENT. 

The  Essential  Structure  of  an  Eye.  Every  visual  organ 
consists  primarily  of  a  nervous  expansion,  provided  with  end- 
organs  by  means  of  which  light  is  enabled  to  excite  nervous 
impulses,  and  exposed  to  the  access  of  objective  light;  such 
an  expansion  is  called  a  retina.  By  itself,  however,  a  retina 
would  give  no  visual  sensations  referable  to  distinctly  limited 
external  objects;  it  would  enable  its  possessor  to  tell  light 
from  darkness,  more  light  from  less  light,  and  (at  least  in  its 
highly  developed  forms)  light  of  one  color  from  light  of  an- 
other color;  but  that  would  be  all.  Were  our  eyes  merely 
retinas  we  could  only  tell  a  printed  page  from  a  blank  one  by 
the  fact  that,  being  partly  covered  with  black  letters  (which 
reflect  less  light),  it  would  excite  our  visual  organ  less  power- 
fully than  the  spotless  white  page  would.  In  order  that  dis- 
tinct objects  and  not  merely  degrees  of  luminosity  may  be 
seen,  some  arrangement  is  needed  which  shall  bring  all  light 
entering  the  eye  from  one  point  of  a  luminous  surface  to  a 
focus  again  on  one  point  of  the  sensitive  surface.  If  A  and 
B  (Fig.  139)  be  two  red  spots  on  a  black  surface,  A",  and  rr 
be  a  retina,  then  rays  of  light  diverging  from  A  would  fall 
equally  on  all  parts  of  the  retina  and  excite  it  all  a  little;  so 
with  rays  starting  from  B.  The  sensation  aroused,  suppos- 
ing the  retina  in  connection  with  the  rest  of  the  nervous 
visual  apparatus,  would  be  one  of  a  certain  amount  of  red 
light  reaching  the  eye;  the  red  spots,  as  definite  objects, 
would  be  indistinguishable.  If,  however,  a  convex  glass  lens 
L  (Fig.  140)  be  put  in  front  of  the  retina,  it  will  cause  to 
converge  again  to  a  single  point  all  the  rays  from  A  falling 
upon  it;  so,  too,  with  the  rays  from  B  :  and  if  the  focal  dis- 
tance of  the  lens  be  properly  adjusted  these  points  of  conver- 
gence will  both  lie  on  the  retina,  that  for  rays  from  A  at  a, 
and  that  for  rays  from  B  at  b.  The  sensitive  surface  would 
then  only  be  excited  at  two  limited  and  separated  points  by 

504 


TEE  EYE  AS  AN  OPTICAL  INSTRUMENT. 


505 


the  red  light  emanating  from  the  spots;  consequently  only 
some  of  its  end-organs  and  nerve-fibres  would  be  stimulated 
and  the  result  would  be  the  recognition  of  two  separate  red 


Fig.  129.  —Diagram  illustrating  the  indistinctness  of  vision  with  a  retina  alone. 
K,  a  surface  on  which  are  two  spots,  A  and  B;  r  r,  the  retina.  The  diverging 
lines  represent  rays  of  light  spread  uniformly  over  the  retina  from  each  spot. 

objects.  In  our  eyes  there  are  certain  refracting  media 
which  lie  in  front  of  the  retina  and  take  the  place  of  the  lens 
L  in  Fig.  140.     That  portion  of  physiology  which  treats  of 


Fig.  140.— Ulustrating  the  use  of  a  lens  in  giving  definite  retinal  images.  A,  B, 
K.  r  r,  as  in  Fig.  139.  L,  a  biconvex  lens  so  placed  that  it  brings  to  a  focus  on  the 
points  a  and  b  of  the  retina,  rays  of  light  diverging  from  A  and  H  respectively. 

the  physical  action  of  these  media  or,  in  other  words,  of  the 
eye  as  an  optical  instrument,  is  known  as  the  dioptrics  of  the 
eye. 

The  Appendages  of  the  Eye.  The  eyeball  itself  eon- 
-  of  the  retina  and  refracting  media,  together  with  sup- 
porting and  nutritive  structures  and  other  accessory  appa- 
ratuses as,  for  example,  some  controlling  the  light-converg- 
ing power  of  the  media,  and  others  regulating  the  size  of  the 
aperture  (pupil)  by  which  light  enters.  Outside  the  ball  lie 
muscles  which  bring  about  its  movements,  and  other  parts 
8erviu'_r  to  protect  it. 

Bach  orbit  is  a  pyramidal  cavity  occupied  by  connective 
tissue,  muscles,  hlood-vessels  and  nerves,  and  in  great  part  by 
fat,  which  forms  a  soft  cushion  on  which  the  back  of  the  eye- 
ball lies  and  rolls  during  its  movements.     The  contents  of 


506  THE  HUMAN  BODY. 

the  orbit  being  for  the  most  part  incompressible,  the  eye  can- 
not be  drawn  into  its  socket.  It  simply  rotates  there  as 
the  head  of  the  femur  does  in  the  acetabulum.  When  the 
orbital  blood-vessels  are  gorged,  however,  the  eyeball  may 
protrude  (as  in  strangulation);  and  when  these  vessels  empty 
it  recedes  somewhat,  as  is  commonly  seen  after  death.  The 
front  of  the  eye  is  exposed  for  the  purpose  of  allowing  light 
to  reach  it,  but  can  be  covered  up  by  the  eyelids,  which  are 
folds  of  integument,  movable  by  muscles  and  strengthened 
by  plates  of  fibro-cartilage.  At  the  edge  of  each  eyelid  the 
skin  which  covers  its  outside  is  turned  in,  and  becomes  con- 
tinuous with  a  mucous  membrane,  the  conjunctiva,  which 
lines  the  inside  of  each  lid,  and  also  covers  all  the  front  of 
the  eyeball  as  a  closely  adherent  layer. 

The  upper  eyelid  is  larger  and  more  mobile  than  the 
lower,  and  when  the  eye  is  closed  covers  all  its  transparent 
part.  It  has  a  special  muscle  to  raise  it,  the  levator  palpebrm 
superioris.  The  eyes  are  closed  by  a  flat  circular  muscle, 
the  orbicularis  palpebrarum  which,  lying  on  and  around  the 
lids,  immediately  beneath  the  skin,  surrounds  the  aperture 
between  them.  At  their  outer  and  inner  angles  (cant hi)  the 
eyelids  are  united,  and  the  apparent  size  of  the  eye  depends 
upon  the  interval  between  the  canthi,  the  eyeball  itself  being 
nearly  of  the  same  size  in  all  persons.  Near  the  inner  can- 
thus  the  line  of  the  edge  of  each  eyelid  changes  its  direction 
and  becomes  more  horizontal.  At  this  point  is  found  a  small 
eminence,  the  lachrymal  papilla,  on  each  lid.  For  most  of 
their  extent  the  inner  surfaces  of  the  eyelids  are  in  contact 
with  the  outside  of  the  eyeball,  but  near  their  inner  ends  a 
red  vertical  fold  of  conjunctiva,  the  semilunar  fold  (plica 
semilunaris)  intervenes.  This  is  a  representative  of  the  third 
eyelid,  or  nictitating  membrane,  found  largely  developed  in 
many  animals,  as  birds,  in  which  it  can  be  drawn  all  over  the 
exposed  part  of  the  eyeball.  At  the  inner  or  nasal  corner  is  a 
reddish  elevation,  the  caruncula  lachrymalis,  caused  by  a 
collection  of  sebaceous  glands  imbedded  in  the  semilunar 
fold.  Opening  along  the  edge  of  each  eyelid  are  from 
twenty  to  thirty  minute  compound  sebaceous  glands,  named 
the  Meibomian  follicles.  Their  secretion  is  sometimes  ab- 
normally abundant,  and  then  appears  as  a  yellowish  matter 
along  the  edges  of  the  eyelids,  winch  often  dries  in  the  night 
and  causes  the  lids  to  be    glued   together  in   the  morning. 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  507 

The  eyelashes  are  short  curved  hairs,  arranged  in  one  or  two 
rows  along  each  lid  where  the  skin  joins  the  conjunctiva. 

The  Lachrymal  Apparatus  consists  of  the  tear-gland  in 
each  orbit,  the  ducts  which  carry  its  secretion  to  the  upper  eye- 
lid, and  the  canals  by  which  the  tears,  unless  when  excessive, 
are  carried  off  from  the  front  of  the  eye  without  running  down 
over  the  face.  The  lachrymal  or  tear  gland,  about  the  size 
■of  an  almond,  lies  in  the  upper  and  outer  part  of  the  orbit, 
near  the  front  end.  It  is  a  compound  racemose  gland,  from 
which  twelve  or  fourteen  ducts  run  and  open  in  a  row  at  the 
•outer  corner  of  the  upper  eyelid.  The  secretion  there  poured 
out,  is  spread  evenly  over  the  exposed  part  of  the  eye  by  the 
movements  of  winking,  and  keeps  it  moist;  finally  the  tear  is 
drained  off  by  two  lachrymal  canals,  one  of  which  opens  by  a 
small  pore  (punctum  lachrymalis)  on  each  lachrymal  papilla. 
The  aperture  of  the  lower  canal  can  be  readily  seen  by  ex- 
amining the  corresponding  papilla  by  the  aid  of  a  looking- 
glass.  The  canals  run  inwards  and  open  into  the  lachrymal 
sac,  which  lies  just  outside  the  nose,  in  a  hollow  where  the 
lachrymal  and  superior  maxillary  bones  (L  and  Mr,  Fig. 
30)  meet.  From  the  sac  the  nasal  duct  proceeds  to  open 
into  the  nose-chamber,  below  the  inferior  turbinate  bone 
and  within  the  nostril. 

Tears  are  constantly  being  secreted,  but  ordinarily  in 
•such  quantity  as  to  be  drained  off  into  the  nose,  from  which 
they  flow  into  the  pharynx  and  are  swallowed.  When  the 
lachrymal  ducts  are  stopped  up,  however,  their  continual 
presence  makes  itself  unpleasantly  felt,  and  may  need  the  aid 
of  a  surgeon  to  clear  the  passage.  In  weeping  the  secretion 
is  increased,  and  then  not  only  more  of  it  enters  the  nose, 
but  some  flows  down  the  cheeks.  The  frequent  swallowing 
movements  of  a  crying  child,  sometimes  spoken  of  as  "  gulp- 
ing down  his  passion,"  are  due  to  the  need  of  swallowing  the 
extra  fceare  which  reach  the  pharynx. 

The  Muscles  of  the  Eye  (Fig.  141).  The  eyeball  is 
spheroidal  in  form  and  attached  behind  to  the  optic  nerve,  n, 
somewhat  as  a  cherry  might  be  to  a  thick  stalk.  On  its  ex- 
terior are  inserted  the  tendons  of  six  muscles,  four  straight 
and  two  oblique.  The  straight  muscles  lie,  one  (superior 
rectus),  s,  above,  one  {inferior  rectus)  below,  one  (external 
rectus),  a,  outside,  and  one  (infernal  rectus),  i,  inside  the 
•eyeball.     F]aeh  arises  behind   from  the  bony  margin  of  the 


508 


THE  HUMAN  BODY. 


foramen  through  which  the  optic  nerve  enters  the  orbit.  In 
the  figure,  which  represents  the  orbits  opened  from  above, 
the  superior  rectus  of  the  right  side  lias  been  removed.  The 
superior  oblique  or  pulley  {trochlear)  muscle,  t,  arises  behind 
near  the  straight  muscles  and  forms  anteriorly  a  tendon,  u, 
which  passes  through  a  fibro-cartilaginous  ring,  or  pulley, 
placed  at  the  notch  in  the  frontal  bone  where  it  bounds 
superiorly  the  front  end  of  the  orbit.     The  tendon  then  turns 


Fig.  141.— The  eyeballs  and  their  muscles  as  seen  when  the  roof  of  the  orbit 
has  been  removed  and  ilie  fat  in  the  cavity  has  been  partly  cleared  away.  On  the 
right  side  the  superior  rectus  muscle  h;is  l.een  cut  away,  a,  external  rectus ;  s, 
superior  rectus  ;   t,  internal  rectus:  /.  superior  oblique. 

back  and  is  inserted  into  the  eyeball  between  the  upper  and 
outer  recti  muscles.  The  inferior  oblique  muscle  does  not 
arise,  like  the  rest,  at  the  back  of  the  orbit,  but  near  its  front 
at  the  inner  side,  close  to  the  lachrymal  sac.  It  passes  thence 
outwards  and  backwards  beneath  the  eyeball  to  be  inserted 
into  its  outer  and  posterior  part. 

The  inner,  upper,  and  lower  straight  muscles,  the  inferior 
oblique,  and  the  elevator  of  the  upper  lid  are  supplied  by 
branches  of  the  third  cranial  nerve.  The  sixth  cranial  nerve 
goes  to  the  outer  rectus ;  and  the  fourth  to  the  superior  oblique. 

The  eye  may  be  moved  from  side  to  side;  up  or  down; 
obliquely,  that  is  neither  truly  vertically  nor  horizontally, 
but  partly  both ;  or,  finally,  it  may  be  rotated  on  its  antero- 
posterior axis.     The  oblique  movements  are  always  accom- 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  509 

parried  by  a  slight  amount  of  rotation.  When  the  glance  is 
turned  to  the  left,  the  left  external  rectus  and  the  right  in- 
ternal contract,  and  vice  versa;  when  up,  both  superior  recti; 
when  down,  both  the  inferior.  The  superior  oblique  muscle 
acting  alone  will  roll  the  front  of  the  eye  downwards  and 
outwards  with  a  certain  amount  of  rotation;  the  inferior 
oblique  does  the  reverse.  In  oblique  movements  two  of  the 
recti  are  concerned,  an  upper  or  lower  with  an  inner  or 
outer;  at  the  same  time  one  of  the  oblique  also  always  con- 
tracts.    Movements  of  rotation  rarely,  if  ever,  occur  alone. 

The  natural  combined  movements  of  the  eyes  by  which 
both  are  directed  simultaneously  towards  the  same  point  de- 
pends on  the  accurate  adjustment  of  all  its  nervo-muscular 
apparatus.  When  the  co-ordination  is  deficient  the  person  is 
said  to  squint.  A  left  external  squint  would  be  caused  by 
paralysis  of  the  inner  rectus  of  that  eye,  for  then,  after  the 
eyeball  had  been  turned  out  by  the  external  rectus,  it  would 
not  be  brought  back  again  to  its  median  position.  A  left 
internal  squint  would  be  caused,  similarly,  by  paralysis  of 
the  left  external  rectus;  and  probably  by  disease  of  the  sixth 
cranial  nerve  or  its  brain-centres.  Dropping  of  the  upper 
eyelid  {ptosis)  indicates  paralysis  of  its  special  elevator  muscle 
and  is  often  a  serious  symptom,  pointing  to  disease  of  the 
brain -parts  from  which  it  is  innervated. 

The  Globe  of  the  Eye  is  on  the  whole  spherical,  but 
consists  of  segments  of  two  spheres  (see  Fig.  142),  a  portion 
of  a  sphere  of  smaller  radius  forming  its  anterior  transparent 
part  and  being  set  on  to  the  front  of  its  posterior  segment, 
which  is  part  of  a  larger  sphere.  From  before  back  it 
measures  about  22.5  millimeters  (T9¥  inch),  and  from  side  to 
side  about  25  millimeters  (1  inch).  Except  when  looking  at 
near  objects,  the  antero-posterior  axes  of  the  eyeballs  are 
nearly  parallel,  though  the  optic  nerves  diverge  considerably 
(Fig.  141);  each  nerve  joins  its  eyeball,  not  at  the  centre,  but 
about  2.5  mm.  (T',r  inch)  on  the  nasal  side  of  the  posterior  end 
of  its  antero-posterior  axis.  In  general  terms  the  eyeball  may 
be  described  as  consisting  ot  three  coats  and  three  refracting 
medi". 

The  outer  coat,  1  and  J.  I«ig.  142,  consists  of  the  sclerotic 
and  the  cornea,  the  latter  being  transparent  and  situated  in 
front;  tin-  former  ib  opaque  and  white  and  covers  the  back 
and  .-ides,  of  the  globe  and  part  of  the  front,  where  it  is  seen 


510 


THE  HUMAN  BODY. 


between  the  eyelids  as  the  white  of  the  eve.  Both  are  tough 
and  strong,  being  composed  of  dense  connective  tissue.  The 
white  of  the  eye  and  the  cornea  are  covered  by  a  thin  layer  of 
the  conjunctiva,  4  and  5.  Behind  the  proper  connective- 
tissue  layer,  3,  of  the  cornea  is  a  thin  structureless  membrane, 


Fig.  142.— The  left  eyeball  in  horizontal  section  from  before  back.  1,  sclerotic; 
2,  junction  of  sclerotic  and  cornea;  3,  cornea:  4.  5,  conjunctiva;  6,  posterior 
elastic  layer  of  cornea;  7.  ciliary  muscle:  10,  choroid;  11,  13.  ciliary  proeessi-s; 
14,  iris;  15,  retina;  16,  optic  nerve;  17,  artery  entering  retina  in  optic  nerve;  18, 
fovea  centralis;  19,  region  where  sensory  part  of  retina  ends;  22,  suspensory 
ligament;  -'3  is  placed  in  the  canal  of  Petit  and  the  line  from  25  points  to  it;  24, 
the  anterior  part  of  the  hyaloid  membrane;  20.  27.  2S  are  placed  on  the  lens;  28 
points  to  the  line  of  attachment  around  it  of  the  suspensory  ligament;  29,  vitreous 
humor;  30,  anterior  chamber  of  aqueous  humor;  31,  posterior  chamber  of  aqueous 
humor. 

6,  lined  inside  by  a  single  layer  of  epithelial  cells;  it  is  the 
membrane  of  Descemet,  or  the  posterior  elastic  layer. 

The  second  coat  consists  of  the  choroid,  9,  10,  the  ciliary 
processes,  11,  13,  and  the  iris,  11.  The  choroid  is  made' 
up  of  blood-vessels  supported  by  loose  connective  tissue 
containing  numerous  corpuscles,  which  in  its  inner  layers 
are  richly  filled  with  dark-brown  or  black  pigment  granules. 
Towards  the  front  of  the  eyeball,  where  it  begins  to  diminish 
in  diameter,  the  choroid  is  thrown  into  plaits,  the  ciliary 
processes,  11,  13.  Beyond  these  it  continues  as  the  iris, 
which  forms  the  colored  part  of  the  eye  seen  through  the 
cornea;    and  in  the  centre  of  the  iris  is  a  circular  aperture, 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT  511 

the  pupil :  so  its  second  coat  does  not,  like  the  outer  one, 
completely  envelop  the  eyeball.  In  the  iris  is  a  ring  of  plain 
muscular  tissue  encircling  the  aperture  of  the  pupil:  when  its 
fibres  contract  they  narrow  the  pupil.  Radial  fibres  can  be 
found  passing  from  the  ring  to  the  outer  edge  of  the  iris, 
and  tbey  have  been  supposed  to  be  muscular  and  concerned 
in  dilating  the  pupil.  They  are  probably  merely  elastic  and, 
being  stretched  when  the  circular  muscle  contracts,  by 
mere  physical  elasticity  dilate  the  pupil  when  the  muscle 
relaxes.  The  circular  or  sphincter  muscle  appears  to  be 
normally  in  a  state  of  tonic  contraction;  this  is  increased 
by  impulses  travelling  in  fibres  of  the  third  cranial  nerve 
and  is  diminished  or  inhibited  by  impulses  travelling  along 
fibres  of  the  sympathetic,  which,  however,  have  their  origin  in 
the  medulla  oblongata  and  run  down  the  spinal  cord  to  the 
lower  part  of  the  neck,  where  they  pass  out  in  anterior  spinal 
nerve-roots  to  reach  the  sympathetic.  The  pigment  in  the 
iris  is  yellow,  or  of  lighter  or  darker  brown,  according  to  the 
color  of  the  eye,  and  more  or  less  abundant  according  as  the 
eye  is  black,  brown,  or  gray.  In  blue  eyes  the  pigment  is 
confined  to  the  deeper  layers,  and  modified  in  tint  by  light 
absorption  in  the  anterior  colorless  strata  through  which  the 
light  passes. 

The  third  coat  of  the  eye,  the  retina,  15,  is  its  essential 
portion,  being  the  part  in  which  the  light  produces  those 
changes  that  give  rise  to  impulses  in  the  optic  nerve.  It  is 
a  still  less  complete  envelope  than  the  second  tunic,  extend- 
ing forwards  only  as  far  as  the  commencement  of  the  ciliary 
processes,  at  least  in  its  typical  form.  It  is  extremely  soft 
and  delicate;  and,  when  fresb,  transparent.  Usually  when 
an  eye  is  opened  the  retina  is  colorless;  but  when  the  eye  has 
been  cut  open  in  faint  yellow  light  and  the  exposed  retina 
quickly  examined  in  white  light  it  is  seen  to  be  purple.  The 
coloring  substance  (visual  purple)  very  rapidly  bleaches  when 
a  dead  eye  is  exposed  to  daylight.  On  front  or  inner  surface 
of  the  human  retina  two  special  areas  can  be  distinguished  in 
afresh  eye.  One  is  the  point  of  entry  of  the  optic  nerve,  1G,  the 
fibres  of  which,  penetrating  the  sclerotic  and  choroid,  spread 
out  in  the  retina.  At  this  place  the  retina  is  whiter  than 
elsewhere  and  presents  an  elevation,  the  optic  mound.  The 
other  peculiar  region  is  the  yellow  spot  (macula  lutea),  18, 
which  lies  nearly  at  the  posterior  end  of  the  axis  of  the  eye- 


512  THE  HUMAN  BODY. 

ball  and  therefore  outside  the  optic  mound;  in  its  centre  the 
retina  is  thinner  than  elsewhere  and  so  a  pit  (fovea  cen- 
tralis), 18,  is  formed.  This  appears  black,  the  thinned 
retina  there  allowing  the  choroid  to  be  seen  through  it  more 
clearly  than  elsewhere.  In  Fig.  143  is  represented  the  left 
retina  as  seen  from  the  front,  the  elliptical  darker  patch 
about  the  centre  indicating  the  yellow  spot,  and  the  white  circle 
on  one  side,  the  optic  mound.  The  vessels  of  the  retina 
arise  from  an  artery  (17,  Fig.  142)  which  runs  in  with  the 
optic  nerve  and  from  which  branches  diverge  as  shown  in 
Fig.  143. 

The  Optic  Nerves,  Commissure,  and  Tracts.  The  optic 
nerves  converge  to  meet  in  the  optic  commissure  (in,  Fig. 
141),  from  which  the  optic  tracts  pass  to  the  region  of  the 
midbrain.  They  terminate  mainly  in  the  anterior  corpora 
quadrigemina  (Chap.  XII)  and  in  masses  of  gray  nerve  matter 
lying  to  the  outer  sides  and  in  front  of  these,  and  known  as 
the  corpora  geniculate.  At  the  commissure  (m,  Fig.  141)  many 
fibres  cross  the  middle  line,  so  that  fibres  from  each  optic  nerve 
are  found  in  both  optic  tracts.  In  general,  fibres  from  the 
right  (that  is,  the  outer  or  temporal)  side  of  the  right  retina 
and  the  right  (i.e.  nasal)  side  of  the  left  retina  pass  on  to  the 
brain  in  the  right  optic  tract;  and  similarly  for  the  left  sides 
of  the  two  retinas.  Cutting  the  right  optic  nerve,  therefore, 
causes  total  blindness  of  the  right  eye,  but  cutting  of  the 
right  optic  tract  blindness  of  the  right  half  of  each  retina 
(hemianopia).  It  will  later  be  seen  that  rays  of  light  cross  in 
the  eye  so  that  objects  to  the  left  in  space  form  images  on 
the  right  sides  of  the  retinas;  and  vice  versa  (Figs.  153,  154). 
Consequently  section  or  extensive  disease  of  the  right  optic 
tract  causes  left  hemianopia;  that  is,  blindness  to  objects  on 
the  left  of  the  line  of  vision. 

The  incomplete  crossing  of  the  optic  nerve-fibres  in  man 
is  correlated  with  the  fact  that  his  eyes  are  so  placed  that 
part  of  the  field  of  vision  is  common  to  both.  In  mammals 
whose  eyes  are  so  laterally  placed  that  at  any  given  moment 
the  objects  seen  by  the  two  eyes  are  quite  different,  the  cross- 
ing at  the  commissure  is  complete:  wdien  the  eyes  are  placed 
so  that  some  oojects  can  be  seen  simultaneously  by  the  two 
eyes,  some  fibres  cross,  and  a  greater  number  cross  the  larger, 
the  common  part  of  the  visual  fields.  Even  in  man  more  of 
the  fibres  cross  than  go  direct  to  the  same  side  of  the  brain. 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  513 

The  Microscopic  Structure  of  the  Retina.  A  simpli- 
fied stratum,  continuous  with  the  proper  retina,  and  formed 
of  a  layer  of  nucleated  columnar  cells,  is  continued  over  the 
ciliary  processes;  elsewhere  the  membrane  has  a  very  com- 
plex structure,  and  a  section  taken,  except  at  the  yellow  spot 
or  the  optic  mound,  shows  ten  layers,  partly  sensory  appa- 
ratuses and  nerve-tissues,  and  partly  accessory  structures. 

Beginning  (Fig.  14-4)  on  the  front  side  we  find,  first,  the 
internal  limiting  membrane,  1,  a  thin  structureless  layer. 
Next  comes  the  nerve-fibre  layer,  2,  formed  by  radiating 
fibres  of  the  optic  nerve;  third,  the  nerve-cell  layer,  3;  fourth, 


Fig.  143.— The  right  retina  as  it  would  be  seen  if  the  front  part  of  tho  eyeball 
with  the  lens  aud  vitreous  humor  were  removed. 

the  inner  molecular  layer,  4,  consisting  partly  of  very  fine 
nerve-fibrils,  and  largely  of  connective  tissue;  fifth,  the 
inner  nuclear  layer,  5,  composed  of  nucleated  cells,  with  a 
small  amount  of  protoplasm  at  each  end,  and  a  nucleolus. 
These  cells,  or  at  any  rate  the  majority  of  them,  have  an 
inner  process  running  to  the  inner  molecular  layer  and  an 
outer  running  to,  6,  the  outer  molecular  layer,  which  is 
thinner  than  the  inner.  Then  comes,  seventh,  the  rod  and 
cone  fibre  layer,  7,  or  outer  nuclear  layer;  composed  of  thick 
and  thin  fibres  in  each  of  which  is  a  conspicuous  nucleus 
with  a  nucleolus.  Next  is  the  thin  external  limiting  hh'ih- 
brane,  8,  perforated  by  apertures  through  which  the  rods  and 
bones,  't,  of  the  ninth  layer  join  the  fibres  of  the  seventh. 
Outside  of  all,  next  tint  choroid, is  the  pigmentary  layer,  10; 


514 


THE  111  MAX  BODY. 


the  cells  of  this  layer  send  processes  between  the  rods  and 
cones.  The  processes  contain  dark  pigment  and  in  eyes 
which  have  been  exposed  to  bright  light  reach  a  long  way, 
sometimes  even  as  far  as  the  external  limiting  membrane. 
If,  however,  the  animal  have  been  kept  in  the  dark  for  some 


Fio.  144.— A  section  through  the  retina  from  its  anterior  or  inner  surface,  1,  in 
contact  with  the  hyaloid  membrane,  to  its  outer,  10,  in  contact  with  the  choroid. 
1,  internal  limiting  membrane;  2,  nerve-fibre  layer:  3,  nerve-cell  layer;  4,  inner 
molecular  layer:  5.  inner  nuclear  layer;  6,  outer  molecular  layer;  7,  rod  and  cone 
fibres  or  outer  nuclear  layer;  8,  external  limiting  membrane;  9,  rod  and  cone 
layer;  10,  pigment-cell  layer. 

time  before  its  eye  is  removed,  the  processes  of  the  pigment- 
cells  are  short  and  extend  only  a  short  distance  between  the 
outer  ends  of  the  rods.  In  addition,  certain  fibres  run  verti- 
cally through  the  retina  from  the  inner  to  the  outer  limiting 
membrane;    they  are  known  as   the  radial  fibres  of  Midler 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  515 

and  give  off  lateral  branches,  which  are  especially  numerous 
in  the  molecular  layers.  Like  the  limiting  membranes  they 
are  merely  supporting  tissues. 

On  account  of  the  way  in  which  the  supporting  and  essen- 
tial parts  are  interwoven  in  the  retina  it  is  not  easy  to  track 
the  latter  through  it.  There  is,  however  (Chap.  XXXIII), 
good  evidence  that  light  first  acts  upon  the  rod  and  cone 
layer,  traversing  all  the  thickness  of  inner  strata  of  the  retina 
to  reach  it,  before  starting  those  changes  which  result  in 
visual  sensations;  and  it  is  therefore  probable  that  the  rods 
and  cones  are  in  direct  continuity  with  the  optic  nerve-fibres. 
The  limiting  membranes,  with  the  fibres  of  Muller  and  their 
branches,  are  undoubtedly  merely  accessory  and  supporting. 

Each  rod  and  cone  consists  of  an  outer  and  an  inner  seg- 
ment. The  outer  segments  of  both  tend  to  split  up  trans- 
versely into  disks  and  are  very  similar,  except  that  those  of 
the  rods  are  longer  than  those  of  the  cones  and  do  not  taper 
as  the  latter  do.  Moreover,  the  visual  purple  is  entirely  con- 
fined to  the  outer  segments  of  the  rods,  the  cones  containing 
none  of  it.  The  inner  segments  of  the  cones  are  swollen, 
while  those  of  the  rods  are  narrow  and  nearly  cylindrical. 
Over  most  of  the  retina  the  rods  are  longer  and  much  more 
numerous  than  the  cones,  but  near  the  ciliary  processes  they 
cease  before  the  cones  do;  and  in  the  yellow  spot  elongated 
cones  alone  are  found.  In  this  region  the  whole  retina  is 
modified;  at  its  margin  all  the  layers  are  ihickened  but 
especially  the  nerve-cell  layer,  which  becomes  six  or  seven 
thick,  while  elsewhere  the  cells  are  found  in  but  one  or  two 
strata.  Most  of  the  fibres  run  obliquely,  reaching  in  to  become 
continuous  with  the  cones  of  the  central  pit,  which  are  long, 
slender,  and  very  closely  packed.  In  the  fovea  itself  all  the 
layers,  except  that  ot  the  cones,  thin  away,  and  thus  the  depres- 
sion is  produced.  The  fovea  is  the  seat  of  most  acute  vision; 
when  we  look  at  an  object  we  always  turn  our  eyes  so  that  the 
light  proceeding  from  it  shall  be  focussed  on  the  two  fovea?. 
Where  the  optic  nerve  enters,  all  the  layers  but  the  nerve- 
fibre  layer  (which  is  there  very  thick),  and  the  internal  limit- 
ing membrane,  are  absent. 

The  blood -vessels  of  the  retina  lie  almost  entirely  in  the 
nerve-fibre  and  nerve-cell  layers. 

Tho  Refracting  Media  of  the  Eye  are,  in  succession  from 
before  back,  the  cornea,  the  aqueous  humor,  the  crystalline 
lens,  and  the  vitreous  humor. 


.r>10  THE  III  MAX  BODY. 

The  aqueous  humor  Oils  the  spare  between  the  front  of 
the  lens,  28,  and  the  back  of  the  cornea.  Tins  space  is  in- 
completely divided  by  the  iris  into  an  anterior  chamber,  30, 
and  a  posterior,  31  (Fig.  142).  Chemically,  the  aqueous  humor 

consists  of  water  holding  in  solution  a  small  amount  of  solid 
matters,  mainly  common  salt. 

The  crystalline  lens  (28,  2G,  27)  is  colorless,  transparent, 
and  biconvex,  with  its  anterior  surface  less  curved  than  the 
posterior.  It  is  surrounded  by  a  capsule,  and  the  inner  edge 
of  the  iris  lies  in  contact  with  it  in  front.  In  consistence  it 
is  soft,  but  its  central  layers  are  rather  more  dense  than  the 
outer. 

The  vitreous  humor  is  a  soft  jelly  enveloped  in  a  thin 
capsule,  the  hyaloid  membrane.  In  front,  this  membrane 
splits  into  two  layers,  one  of  which,  22,  passes  on  to  be  fixed 
to  the  lens  a  little  m  front  of  its  edge.  This  layer  is  known 
as  the  suspensory  ligament  of  the  lens;  its  line  of  attachment 
around  that  organ  is  not  straight  but  sinuous  as  represented 
by  the  curved  line  between  28  and  26  in  Fig.  142.  The  spare 
betweeu  the  two  layers  into  which  the  hyaloid  splits  is  the 
canal  of  Petit.  The  vitreous  humor  consists  mainly  of  water 
and  contains  some  salts,  a  little  albumin,  and  some  mucin. 
It  is  divided  up,  by  delicate  membranes,  into  compartments 
in  which  its  more  liquid  portions  are  imprisoned. 

The  Ciliary  Muscle.  Running  around  the  eyeball  where 
the  cornea  joins  the  sclerotic  is  a  lymph-vessel  called  the 
canal  of  Schlemm;  it  is  seen  in  section  at  8  in  Fig.  142. 
Lying  on  the  inner  side  of  this  canal,  just  where  the  iris  and 
the  ciliary  processes  meet,  there  is  some  plain  muscular  tissue, 
imbedded  mainly  in  the  middle  coat  of  the  eyeball  and  form- 
ing the  ciliary  muscle,  which  consists  of  a  radial  and  a 
circular  portion  (Fig.  149).  The  radial  part  is  much  the 
larger,  and  arises  in  front  from  the  inner  surface  of  the  scler- 
otic; the  fibres  pass  back,  spreading  out  as  they  go,  and  are 
inserted  into  the  front  of  the  choroid  opposite  the  ciliary 
processes.  The  circular  part  of  the  muscle  lies  around  the 
outer  rim  of  the  iris.  The  contraction  of  the  ciliary  muscle 
tends  to  pull  forward  (radial  fibres)  and  press  inward  (circu- 
lar fibres)  the  front  part  of  the  choroid,  to  which  the  back 
part  of  the  suspensory  ligament  of  the  lens  is  closely  at- 
tached. When  this  occurs  the  tension  exerted  on  the  margin 
of  the  lens  by  its  ligament  is  diminished. 

The   Properties   of  Light.       Before    proceeding   to    the 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  517 

study  of  the  eye  as  an  optical  instrument,  it  is  necessary  to 
recall  briefly  certain  properties  of  light. 

Light  is  considered  as  a  form  of  movement  of  the  particles 
of  an  hypothetical  medium,  or  ether,  the  vibrations  being  in 
planes  at  right  angles  to  the  line  of  propagation  of  the  light. 
When  a  stone  is  thrown  into  a  pond  a  series  of  circular  waves 
travel  from  that  point  in  a  horizontal  direction  over  the 
water,  while  the  particles  of  water  themselves  move  up  and 
down,  and  cause  the  surface  inequalities  which  we  see  as 
the  waves.  Somewhat  similarly,  light-waves  spread  out  from 
a  luminous  point,  but  in  the  same  medium  travel  equally  in 
all  directions  so  that  the  point  is  surrounded  by  shells  of 
spherical  waves,  instead  of  rings  of  circular  waves  travelling 
in  one  plane  only,  as  those  on  the  surface  of  the  water. 
Starting  from  a  luminous  point  light  would  travel  in  all 
directions  along  the  radii  of  a  sphere  of  which  the  point  is 
the  centre;  the  light  propagated  along  one  such  radius  is 
called  a  ray,  and  in  each  ray  the  ethereal  particles  swing 
from  side  to  side  in  a  plane  perpendicular  to  the  direction  of 
the  ray.  Taking  a  particle  on  any  ray  it  would  swing  aside  a 
certain  distance  from  it,  then  back  to  it  again,  and  across  for 
a  certain  distance  on  the  other  side;  and  then  back  to  its 
original  position  on  the  line  of  the  ray.  Such  a  movement  is 
an  oscillation,  and  takes  a  certain  time;  in  lights  of  certain 
kinds  the  periods  of  oscillation  are  all  the  same,  no  matter 
how  great  the  extent  or  amplitude  of  the  oscillation;  just  as 
a  given  pendulum  will  always  complete  its  swing  in  the  same 
time  no  matter  whether  its  swings  be  great  or  small.  Light 
composed  of  rays  in  which  the  periods  of  oscillation  arc  all 
equal  is  called  monochromatic  or  simple  light,  while  light 
made  of  a  mixture  of  oscillations  of  different  periods  is  called 
mixed  or  compound  light. 

If  monochromatic  light  is  steadily  emitted  from  a  point, 
we  come  at  definite  distances  along  a  ray,  to  particles  in 
the  same  phase  of  oscillation,  say  at  their  greatest  distance 
from  their  position  of  rest;  just  as  in  the  concentric  waves 
seen  on  the  water  after  throwing  in  a  stone  we  would  along 
any  radius  meet,  at  intervals,  with  water  raised  most  above 
its  horizontal  plane  as  the  crest  of  a  wave,  or  depressed  most 
below  it  as  the  hollow  of  a  wave.  The  distance  along  the  ray 
from  crest  to  crest  is  called  a  wave-length  and   is  always  the 

•  •  in  any  given  simple  light;  but  it  is  different  in  simple 


518 


THE   III  MAX  BODY. 


c 

/a 

A 

p 

X 

9^^^/ 

r        1 

D 

lights  of  different  colors;  the  briefer  the  time  of  an  oscillation 
the  less  the  wave-length. 

When  light  falls  on  a  polished  surface  separating  two 
transparent  media,  as  air  and  glass,  part  of  it  is  reflected  or 
turned  back  into  the  first  medium;  part  goes  on  into  the 
second  medium,  and  is  commonly  deviated  from  its  original 
course  or  refracted.  The  original  ray  falling  on  the  surface 
is  the  incident  ray. 

Let  A  B  (Fig.  145)  be  the 
surface  of  separation;  u  x  the 
incident  ray;  and  C  D  the 
perpendicular  or  normal  to  the 
surface  at  the  point  of  inci- 
dence: ax  C  will  then  be  the 
angle  of  incidence.  Then  the 
reflected  ray  makes  an  angle 
of  reflection  with  the  normal 
which  is  equal  to  the  angle  of 
incidence;  and  the  reflected 
ray  lies  in  the  same  plane  as 
the  incident  ray  and  the  nor- 
mal to  the  surface  at  x.  The 
refracted  ray  lies  also  in  the 
same  plane  as  the  normal  and 
the  incident  ray,  but  does  not 
continue  in  its  original  direction,  x  f;  if  the  medium  below 
A  B  be  more  refractive  than  that  above  it,  the  refracted  ray 
is  bent,  as  x  d,  nearer  to  the  normal,  and  making  with  it  an 
angle  of  refraction.  D  xd,  smaller  than  the  angle  of  inci- 
dence, a  x  0.  If,  on  the  contrary,  the  second  medium  is  less 
refracting  than  the  first,  the  refracted  ray  x  g  is  bent  away 
from  the  normal,  and  makes  an  angle  of  refraction,  D  x  g, 
greater  than  the  angle  of  incidence.  The  ratio  of  the  sine  of 
the  angle  of  incidence  to  that  of  the  angle  of  refraction  is 
always  the  same  for  the  same  two  media  with  light  of  the 
same  wave-length.  When  the  first  medium  is  air  the  ratio  of 
the  sine  of  the  angle  of  refraction  to  that  of  the  angle  of  in- 
cidence is  called  the  refractive  index  of  the  second  medium. 
The  greater  this  refractive  index  the  more  is  the  refracted 
ray  deviated  from  its  original  course.  Rays  which  fall  per- 
pendicularly on  the  surface  of  separation  of  two  media  pass 
on  without  refraction. 


Fig.  145.— Diagram  illustrating  the 
refraction  of  light.  A  B,  surface  of 
separation  between  two  transparent 
media:  C  D,  the  perpendicular  to  the 
surface  at  the  point  of  incidence,  x; 
a  x,  incident  ray;  x  d,  refracted  ray, 
if  the  second  medium  be  denser  than 
the  first;  x  g,  refracted  ray,  if  the 
second  medium  is  less  refractive  than 
the  first.  The  reflected  ray  is  not 
represented,  but  would  make  an  angle 
with  C  x,  equal  to  the  angle  n  x  C. 


THE  EYE  AS  AN  OPTICAL-  INSTRUMENT.  519 

The  shorter  the  oscillation  periods  of  light-rays  the  more 
they  are  deviated  by  refraction.     Hence  mixed  light  when 


Fig.  146.— Diagram  illustrating  the  dispersion  of  mixed  light  by  a  prism. 

sent  through  a  prism  is  spread  out,  and  decomposed  into  its 
simple  constituents.  For  let  a  x  (Fig.  146)  be  a  ray  of  mixed 
light  composed  of  a  set  of  short  and  a  set  of  long  ethereal 
waves.  When  it  falls  on  the  surface  A  B  of  the  prism,  that 
portion  which  enters  will  be  refracted  towards  the  normal 
E  D,  but  the  short  waves  more  than  the  longer.  Hence  the 
former  will  take  the  direction  x  y,  and  the  latter  the  direc- 
tion x  z.  On  emerging  from  the  prism  both  rays  will  again 
be  refracted,  but  now  from  the  normals  F  y  and  G  z,  since 
the  light  is  passing  from  a  more  to  a  less  refracting  medium. 
Again  the  ray  x  y,  made  up  of  shorter  waves,  will  be  most 
deviated,  as  in  the  direction  y  v,  and  the  long  waves  less,  in 
the  direction  z  r.  If  a  screen  were  put  at  S  S',  we  would  re- 
ceive on  it  at  separate  points,  v  and  r,  the  two  simple  lights 
which  were  mixed  together  in  the  compound  incident  ray 
a  x.     Such  a  separation  of  light-rays  is  called  dispei'sion. 

Ordinary  white  light,  such  as  that  of  the  sun,  is  composed 
of  ethereal  vibrations  of  every  rate,  mixed  together.  When 
such  light  is  sent  through  a  prism  it  gives  a  continuous  band 
of  light-rays,  known  as  the  solar  spectrum,  reaching  from  the 
least  refracted  to  the  most  refracted  and  shortest  waves.  The 
exceptions  to  this  statement  due  to  Frauenhofer's  lines  (see 
Physics)  are  unessential  for  our  present  purpose.  All  of  the 
simple  lights   into  which  the  compound  solar  light  is   thus 


620  THE  HUMAN  BODY. 

separated  do  not,  however,  excite  in  us  visual  sensations  when 
they  fall  into  the  eye,  hut  only  certain  middle  ones.  If  solar 
light  were  used  with  the  prism,  Fig.  14G,  certain  least  re- 
fracted rays  between  r  and  8'  would  not  be  seen,  nor  the 
most  refracted  between  v  and  S\  while  between  v  and  r 
would  stretch  a  luminous  band  exciting  in  us  the  series  of 
color  sensations  from  red  (due  to  the  least  refracted  visible 
rays),  through  orange,  yellow,  green,  bright  blue,  and  indigo,  to 
violet,  which  latter  is  the  sensation  aroused  by  the  most  re- 
frangible visible  rays.  The  still  shorter  waves  beyond  the 
violet  can  only  be  seen  under  special  conditions;  they  are 
known  mainly  by  their  chemical  effects  and  are  called  the 
actinic  rays;  the  invisible  waves  beyond  the  red  exert  a 
powerful  heating  influence  and  compose  the  dark-heat  rays. 
The  eye,  as  an  organ  for  making  known  to  us  the  existence 
of  ethereal  vibrations,  has,  therefore,  only  a  limited  range. 

Refraction  of  Light  by  Lenses.  In  the  eye  the  refract- 
ing media  have  the  form  of  lenses  thicker  in  the  centre  than 
towards  the  periphery;  and  we  may  here  confine  ourselves 
therefore  to  such  converging  lenses.  If  simple  light  from  a 
point  A,  Fig.  140,  fall  on  such  a  lens  its  rays,  emerging  on 
the  other  side,  will  take  new  directions  after  refraction  and 
meet  anew  at  a  point,  a,  after  which  they  again  diverge.  If 
a  screen,  r  r,  be  held  at  a  it  will  therefore  receive  an  image 
of  the  luminous  point  A.  For  every  converging  lens  there 
is  such  a  point  behind  it  at  which  the  rays  from  a  given  point 
in  front  of  it  meet:  the  point  of  meeting  is  called  the  conju- 
gate focus  of  the  point  from  which  the  rays  start.  If  instead 
of  a  luminous  point  a  luminous  object  be  placed  in  front  of 
the  lens  an  image  of  the  object  will  be  formed  at  a  certain 
distance  behind  it,  for  all  rays  proceeding  from  one  point  of 
the  object  will  meet  in  the  conjugate  focus  of  that  point  be- 
hind. The  image  is  inverted,  as  can  be  readily  seen  from 
L  Fig.  147.     All  rays  from  the  point 

/V- — ;_^— -^->    -A  of  the  object  meet  at  the  point 
NS?I        v^><y^  a  °^  tne  image;  those  from  B  at  b, 

l^~{      h^^-'-i'-'^^  i\    an(l  those  from  intermediate  points 
^\_/'  '"  :r    at   intermediate  positions.     If  the 

Fio.  147. -Diagram  illustrating     single  lens  were  replaced   by  sev- 

the  formation  of  an  image  by  a  °  .  *  ■* 

converging  lens.  eral    combined    so  as   to  form    an 

optical  system  the  general  result  would  be  the  same,  provided 
the  system  were  thicker  in  the  centre  than  at  the  periphery. 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  521 

The  Camera  Obseura,  as  used  by  photographers,  is  an  in- 
strument which  serves  to  illustrate  the  formation  of  images 
by  converging  systems  of  lenses  It  consists  of  a  box  blackened 
inside  and  Having  on  its  front  face  a  tube  containing  the 
lenses;  the  posterior  wall  is  made  of  ground  glass.  If  the 
front  of  the  instrument  be  directed  on  exterior  objects,  in- 
verted and  diminished  images  of  them  will  be  formed  on  the 
ground  glass;  those  images  only  are  well  defined,  at  any  one 
time,  which  are  at  such  a  distance  in  front  of  the  instrument 
that  the  conjugate  foci  of  points  on  them  fall  exactly  on  the 
glass  behind  the  lens:  objects  nearer  or  farther  off  give  con- 
fused and  indistinct  images;  but  by  altering  the  distance  be- 
tween the  lenses  and  the  ground  glass,  in  common  language 
'•'  focussing  the  instrument,"  either  can  be  made  distinct.  For 
near  objects  the  lenses  must  be  farther  from  the  surface  on 
which  the  image  is  to  be  received,  and  for  distant  nearer. 
The  reason  of  this  may  readily  be  seen  from  Fig.  148.  If  the 
system  of  lenses  brings  the  parallel  rays  a  c  and  b  d,  proceed- 
ing from  an  infinitely  distant  object,  to  a  focus  at  x,  then  the 
diverging  rays  f  c  and  f  d,  proceeding  from  a  nearer  point, 
will  be  harder  to  bend  round,  so  to  speak,  and  will  not  meet 
until  a  point  ?/,  farther  behind  the  system  than  x  is.  The 
more  divergent  the  rays,  or  what  amounts  to  the  same  thing, 
the  nearer  the  point  they  proceed  from,  the  farther  behind 
the  refracting  system  will  y  be. 


fO 


Fio.  148.— Diagram  illustrating  the  need  of   "  focussing  "  in  an  optical   instru- 
ment. 

The  refracting  media  of  the  eye  form  a  convergent  optical 
system,  made  up  of  cornea,  aqueous  humor,  lens,  and  vitreous 
humor.  These  four  media  are  reduced  to  three  practically, 
by  the  tact  that  the  indices  of  refraction  of  the  cornea  and 
aqneoue  humor  are  the  same,  bo  that  they  act  together  as  one 
converging  lens.  The  surfaces  at  which  refraction  occurs 
are— (1)  thai  between  the  air  and  the  cornea,  (2)  that  between 
the  aqueous  humor  and  the  f ronl  of  the  lens,  (3)  that  between 
the  vitreous  humor  and  the  back  of  the  lens.     The  refractive 


522  TUE  HUMAN  BODY. 

indices  of  those  media  are— the  air,  1;  the  aqueous  humor, 
1.3379;  the  lens  (average),  1.4.340;  the  vitreous  humor, 
1.3379.  From  the  laws  of  the  refraction  of  light  it  therefore 
follows  that  (Fig.  149)  the  rays  Cd  will  at  the  corneal  surface 
be  refracted  towards  the  normals  N,  X,  and  take  the  course  d  e. 
At  the  front  of  the  lens  they  will  again  be  refracted  towards 
the  normals  to  that  surface  and  take  the  course  e  f ;  at  the 
back  of  the  lens,  passing  from  a  more  refracting  to  a  less  re- 
fracting medium,  they  will  be  bent  from  the  normals  N"  and 
take  the  course///.  If  the  retina  be  there,  these  parallel  rays 
will  therefore  be  brought  to  a  focus  on  it.  In  the  resting 
condition  of  the  natural  eye  this  is  what  happens  to  parallel 
rays  entering  it:  and,  since  distant  objects  send  into  the  eye 
rays  which  are  practically  parallel,  such  objects  are  seen  dis- 
tinctly without  any  effort,  because  all  rays  emanating  from  a 
point  of  the  object  meet  again  in  one  point  on  the  retina. 

Accommodation.  Points  on  near  objects  send  into  the  eye 
diverging  rays:  these  therefore  would  not  come  to  a  focus  on 
the  retina  but  behind  it,  and  would  not  be  seen  distinctly, 
did  not  some  change  occur  in  the  eye;  since  we  can  see  them 


Fig.  149.— Diagram  illustrating  the  surfaces  at  which  light  is  refracted  in  theeye. 

quite  plainly  if  we  choose  (unless  they  be  very  near  indeed), 
there  must  exist  some  means  by  which  the  eye  is  focussed  or 
accommodated  for  looking  at  objects  at  different  distances. 
That  some  change  does  occur  one  can,  also,  readily  prove  by 
observing  that  we  cannot  see  distinctly,  at  the  same  moment, 
both  near  and  distant  objects.     For  example  standing  behind 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  523 

a  lace  curtain,  at  a  window,  we  can  as  we  choose  look  at 
the  threads  of  the  lace  or  at  the  houses  across  the  street;  but 
when  we  look  at  the  one  we  see  the  other  only  indistinctly; 
and  if,  after  looking  at  the  more  distant  object,  we  look  at  the 
nearer  we  experience  a  distinct  sense  of  effort.  It  is  clear, 
then,  that  something  in  the  eye  is  different  in  the  two  cases. 
The  resting  eye,  suited  for  distinctly  seeing  distant  objects, 
might  conceivably  be  accommodated  for  near  vision  in  several 
ways.  The  refracting  indices  of  its  media  might  be  in- 
creased; that  of  course  does  not  happen;  the  physical  prop- 
erties of  the  media  are  the  same  in  both  cases:  or  the  dis- 
tance of  the  retina  from  the  refracting  surfaces  might  be  in- 
creased, for  example  by  compression  of  the  eyeball  by  the 
muscles  around  it;  however,  experiment  shows  that  changes 
of  accommodation  can,  by  stimulating  the  third  cranial  nerve, 
be  brought  about  in  the  fresh  excised  eyes  of  animals  from 
which  the  muscles  lying  outside  the  eyeball  have  been  re- 
moved, in  which  no  such  compression  is  possible;  we  are  thus 
reduced  to  the  third  explanation,  that  the  refracting  surfaces, 
or  some  of  them,  become  more  curved,  and  so  bring  diverging 
rays  sooner  to  a  focus;  for  a  lens  of  smaller  curvature  is  more 
converging  than  one  of  greater  curvature  composed  of  the 
same  material.  Observation  shows  that  this  is  what  actually 
happens:  the  corneal  surface  remains  unchanged  when  a  near 
object  is  looked  at  after  a  distant  one,  but 
the  anterior  surface  of  the  lens  becomes  con- 
siderably more  convex  and  the  posterior 
slightly  so.  As  already  pointed  out,  when 
light  meets  the  separating  surface  of  two 
media  some  is  reflected  and  some  refracted. 
If,  therefore,  a  person  be  taken  into  a 
dark  room  and  a  candle  be  held  on  one  side 
of  his  eye  while  he  looks  at  a  distant  object,  a|'J!Gs-  o^'a^ancUe" 
an  observer  can  see  three  images  of  the  flame  ^"ted  from^the  re- 
in bis  pupil,  due  to  that  portion  of  the  light  tracting  media  of  the 
reflected  from  the  surfaces  between  the 
media.  One  image  (a,  Fig.  150)  is  erect  and  bright,  reflected 
from  the  convei  mirror  Formed  by  the  cornea;  the  next,  b,  is 
dimmer  and  also  erect;  it  comes  from  the  front  of  the  lens. 
The  third,  c,  is  dim  and  inverted,  being  reflected  from  the 
concave  mirror  (see  Physics)  formed  by  the  back  of  the  lens. 
When  the  curvature  of  a  curved   mirror  is  altered  the  size  of 


524 


THE  HUMAN  BODY. 


the  image  reflected  from  it  is  also  altered,  becoming  smaller 
when  the  radius  of  curvature  of  the  mirror  is  lessened  and 
vice  versa.  If  the  three  images  be  carefully  watched  while 
the  observed  eye  looks  at  a  near  object  in  I  he  same  line  as  the 
distant  point  previously  looked  at,  it  is  seen  that  the  image 
due  to  corneal  reflection  remains  unchanged;  that  due  to  light 
from  the  front  of  the  lens  becomes  smaller  and  brighter;  the 
image  from  the  back  of  the  lens  also  becomes  very  slightly 
smaller.  The  change  in  the  curvature  of  the  front  of  the 
lens  can  be  calculated  from  the  change  in  size  of  the  image 
reflected  from  it  when  the  eye  changes  from  distant  to  near 
accommodation.  When  a  distant  object  is  looked  at  the 
radius  of  curvature  is  10  mm.  (f  inch),  when  a  very  near 
about  6  mm.  (/-  inch),  and  this  change  is  sufficient  to  ac- 
count for  the  range  of  accommodation  of  the  normal  eye. 

When  the  eye  is  focussed  for  seeing  a  near  object  the  cir- 
cular muscle  of  the  iris  contracts,  narrowing  the  pupil,  but  this 
has  nothing  directly  to  do  with  the  accommodation. 

Accommodation  is  brought  about  mainly  by  the  ciliary 
muscle  (Fig.  151).  In  the  resting  eye  it  is  relaxed  and  the 
suspensory  ligament  of  the  lens  is  taut,  and,  pulling  on  its 
edge,  drags  it  out  laterally  a  little  and  flattens  its  surfaces, 


Fig.  151. — Diagram  to  illustrate  the  mechanism  of  accommodation;  on  the  right 
half  of  the  figure  for  a  near,  on  the  Wt  for  a  distant,  object:  ex.  canal  of  Schlemm; 
c/,  circular  portion  of  ciliary  muscle:  ?•/.  radial  portion  of  ciliary  muscle;  ch.  ciliary 
process  of  choroid;  .s7,  suspensory  ligament;  i,  iris. 

especially  the  anterior,  since  the  ligament  is  attached  a  little 
in  front  of  the  edge.  To  see  a  nearer  object  the  ciliary  muscle 
is  contracted,  and  according  to  the  degree  of  its  contraction 
slackens  the  suspensory  ligament,  and  then  the  elastic  lens, 
relieved  from  the  lateral  drag,  bulges  out  a  little  in  the  centre. 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  525 

Short  Sight  and  Long  Sight.     In  the  eye  the  range  of 
accommodation  is  very  great,  allowing  the  rays  from  points 

infinitely  distant  up  to  those  from      

points  ahout  eight  inches  in  front      a 
of  the  eye  to   be  brought  to  a      — 
focus  on  the  retina.     In  the  nor- 
mal eye  parallel  rays  meet  on  the      — 
retina  when  the  ciliary  muscle  is 
completely  relaxed  (A,  Fig.  152). 

Such   eyes  are   emmetropic.     In      

other  eyes  the  eyeball  is  too  long 
from  before  back;  in  the  resting 


state  parallel  rays  meet  in  front      FlG  153._Diagram  uhlstI.atin?  the 

Of  the  retina  (B).       Persons  With     Path  of  parallel  rays  after  entering 
\      '  an   emmetropic   (A),  a  myopic  (B), 

SUcll    eves,    therefore,  CailUOt,    See    and  a  hypermetropic  (C)  eye. 

distant  objects  distinctly  without  the  aid  of  diverging  (con- 
cave) spectacles;  they  are  short-sighted  or  myopic.  Or  the 
eyeball  may  be  too  short  from  before  back;  then,  in  the  rest- 
ing state,  parallel  rays  are  brought  to  a  focus  behind  the 
retina  (C).  To  see  even  infinitely  distant  objects,  such  per- 
sons must  therefore  use  their  accommodating  apparatus  to 
increase  the  converging  power  of  the  lens;  and  when  objects 
are  near  they  cannot,  with  the  greatest  effort,  bring  the  di- 
vergent rays  proceeding  from  them  to  a  focus  soon  enough. 
To  get  distinct  retinal  images  of  near  objects  they  therefore 
need  converging  (convex)  spectacles.  Such  eyes  are  called 
hypermetropic,  or  in  common  language  long-sighted. 

Hygienic  Remarks.  Since  muscular  effort  is  needed  by 
the  normal  eye  to  see  near  objects,  it  is  clear  why  the  pro- 
longed contemplation  of  such  is  more  fatiguing  than  looking 
at  more  distant  things.  If  the  eye  be  hypermetropic  still 
more  is  this  apt  to  be  the  case,  for  then  the  ciliary  muscle 
has  no  res!  when  the  eye  is  used,  and  to  read  a  book  at  a  dis- 
tance such  th.it  enough  light  is  reflected  from  it  into  the  eye 
in  onlcr  to  enable  the  letters  to  be  seen  at  all,  requires  an  ex- 
traordinary effort  of  accommodation.  Such  persons  complain 
thai  they  can  read  well  enough  for  a  time,  but  soon  fail  to  be 
able  to  see  distinctly.  This  kind  of  weak  sighl  should  always 
lead  to  examination  of  the  eyes  by  an  oculist,  to  see  if 
glasses  are  needed-  otherwise  severe  neuralgic  pains  about 
the  eyes  are  apt  to  come  on,  and  the  overstrained  organ 
may  be  permanently  injured.     Old   persons  are  apt  to  have 


526  THE  HUMAN  BODY. 

such  eyes;  but  young  children  frequently  also  possess  them, 
aud  if  so  should  at  ouce  be  provided  with  spectacles. 

►Short-sighted  eyes  appear  to  be  much  more  commou  now 
thau  formerly,  especially  in  those  giveu  to  literary  pursuits. 
Myopia  is  rare  among  those  who  cannot  read  or  who  live 
mainly  out  of  doors.  It  is  not  so  apt  to  lead  to  permanent 
injury  of  the  eye  as  is  the  opposite  condition,  but  the  effort 
to  see  distinctly  objects  a  little  distant  is  apt  to  produce  head- 
aches and  other  symptoms  of  nervous  exhaustion.  If  the 
myopia  become  gradually  worse  the  eyes  should  be  rested  for 
several  months.  Short-sighted  persons  are  apt  to  have,  or 
acquire,  peculiarities  of  appearance:  their  eyes  are  often 
prominent,  indicative  of  the  abnormal  length  of  the  eyeball. 
They  also  get  a  habit  of  "screwing"  up  the  eyelids,  probably 
an  indication  of  an  effort  to  compress  the  eyeball  from  before 
back  so  that  distant  objects  may  be  better  seen.  They  often 
stoop,  too,  from  the  necessity  of  getting  their  eyes  near  ob- 
jects they  want  to  see.  The  acquirement  of  such  habits  may 
be  usually  prevented  by  the  use  of  proper  glasses.  On  the 
other  hand  "it  is  said  that  myopia  even  induces  peculiarities 
of  character,  and  that  myopes  are  usually  unsuspicious  and 
easily  pleased;  being  unable  to  observe  many  little  matters  in 
the  demeanor  or  expression  of  those  with  whom  they  con- 
verse, which,  being  noticed  by  those  of  quicker  sight,  might 
induce  feelings  of  distrust  or  annoyance." 

In  old  age  the  lens  loses  some  of  its  elasticity  and  becomes 
more  rigid.  This  leads  to  the  long-sightedness  of  old  people, 
known  as  presbyopia.  The  stiffer  lens  does  not  become  as 
convex  as  it  did  in  early  life,  when  the  ciliary  muscle  con- 
tracts and  the  suspensory  ligament  is  relaxed.  A  special 
effort  of  accommodation  is  therefore  needed  in  order  to  adapt 
the  eye  to  see  near  objects  distinctly;  and  convex  glasses  are 
required. 

In  all  forms  of  deficient  accommodation  too  strong  glasses 
will  injure  the  eyes  irreparably,  increasing  the  defects  they 
are  intended  to  relieve.  Skilled  advice  should  therefore  be 
invariably  obtained  in  their  selection,  except  perhaps  in  the 
long-sightedness  of  old  age,  when  the  sufferer  may  tolerably 
safely  select  for  himself  any  glasses  that  allow  him  to  read 
easily  a  book  about  30  centimeters  (12  inches)  from  the  eye. 
As  age  advances  stronger  lenses  must  usually  be  obtained. 

Optical  Defects  of  the  Eye.     The  eye,  though  it  answers 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  527 

admirably  as  a  physiological  instrument,  is  by  no  means  per- 
fect optically;  not  nearly  so  good,  for  example,  as  a  good 
microscope  objective.     The  main  defects  in  it  are  due  to — 

1.  Chromatic  Aberration.  As  already  pointed  out,  the 
rays  at  the  violet  end  of  the  solar  spectrum  are  more  refran- 
gible than  those  at  the  red  end.  Hence  they  are  brought  to  a 
focus  sooner.  The  light  emanating  from  a  point  on  a  white 
object  does  not,  therefore,  all  meet  in  one  point  on  the  retina; 
but  the  violet  rays  come  to  a  focus  first,  then  the  indigo,  and 
so  on  to  the  red,  farthest  back  of  all.  If  the  eye  is  accommo- 
dated so  as  to  bring  to  a  focus  on  the  retina  parallel  red  rays, 
then  violet  rays  from  the  same  source  will  meet  half  a  milli- 
meter in  front  of  it,  and  crossing  and  divergiug  there  make 
a  little  violet  circle  of  diffusion  around  the  red  point  on  the 
retina.  In  optical  instruments  this  defect  is  remedied  by 
combining  together  lenses  made  of  different  kinds  of  glass; 
such  compound  lenses  are  called  achromatic. 

The  general  result  of  chromatic  aberration,  as  may  be  seen 
in  a  bad  opera-glass,  is  to  cause  colored  borders  to  appear 
around  the  edges  of  the  images  of  objects.  In  the  eye  we 
usually  do  not  notice  such  borders  unless  we  especially  look 
for  them;  but  if,  while  a  white  surface  is  looked  at,  the  edge 
of  an  opaque  body  be  brought  in  front  of  the  eye  so  as  to 
cover  half  the  pupil,  colorations  will  be  seen  at  its  margin. 
If  accommodation  be  inexact  they  appear  also  when  the 
boundary  between  a. white  and  a  black  surface  is  observed. 
The  phenomena  due  to  chromatic  aberration  are  much  more 
easily  seen  if  light  containing  only  red  and  violet  rays  be  used 
instead  of  white  light  containing  all  the  rays  of  intermediate 
refrangibility.  Ordinary  blue  glass  only  lets  through  these  two 
kinds  of  rays.  If  a  bit  of  it  be  placed  over  a  very  small  hole 
in  an  opaque  shutter  and  sunlight  be  admitted  through  the 
hole,  it  will  be  found  that  with  one  accommodation  (that  for 
the  red  rays)  a  red  point  is  seen  with  a  violet  border,  and 
with  another  (that  at  which  violet  rays  are  brought  to  a  focus 
on  the  retina)  a  violet  point  is  seen  with  a  red  aureole. 

:.'.  Spherical  Aberration.  It  is  not  quite  correct  to  state 
that  ordinary  lenses  bring  to  a  focus  in  one  point  behind  them 
proceeding  from  a  point  in  front,  even  when  these  are 
all  of  the  game  refrangibility.  Convex  lenses  whose  surfaces 
are  segments  of  spheres,  as  are  those  of  the  eve,  bring  to  a 
focus  sooner  tin-  rays  which  pass  through  their  marginal  than 


528 


THE  HUMAN  BODY. 


those  passing  through  their  central  parts.  If  rays  proceeding 
from  a   point  and  traversing  the  lateral  part  <»!'  a  lens  lie 

broughl  to  a  Incus  at  any  point,  t lien  those  passing  through 
the  centre  of  the  lens  will  not  meet  until  a  little  beyond  that 
point.  If  the  retina  receive  the  image  formed  by  tin-  periph- 
eral raVs  the  others  will  form  around  this  a  small  luminous 
circle  of  light — such  as  would  be  formed  by  sections  of  the 
cones  of  converging  rays  in  Fig.  140.,  taken  a  little  in  front 
of  ;•  r.  This  defect  exists  in  all  glass  lenses,  as  it  is  found 
impossible  in  practice  to  grind  them  of  the  non-spherical 
curvatures  necessary  to  avoid  it.  In  our  eyes  its  effect  is  to 
a  large  extent  corrected  in  the  following  ways — (a)  The 
opaque  iris  cuts  off  many  of  the  external  and  more  strongly 
refracted  rays,  preventing  them  from  reaching  the  retina. 
(//)  The  outer  layers  of  the  lens  are  less  refracting  than  the 
central;  hence  the  rays  passing  through  its  peripheral  parts 
are  less  refracted  than  those  passing  nearer  its  axis. 

3.  Irregularities  in  Curvature,  The  refracting  surfaces 
of  our  eyes  are  not  even  truly  spherical;  this  is  especially  the 
case  with  the  cornea,  which  is  very  rarely  curved  to  the  same 
extent  in  its  vertical  and  horizontal  diameters.  Suppose  the 
vertical  meridian  to  be  the  most  curved;  then  the  rays  pro- 
ceeding from  points  along  a  vertical  line  will  be  brought  to  a 
focus  sooner  than  those  from  points  on  a  horizontal  line.  If 
the  eye  is  accommodated  to  see  distinctly  the  vertical  line,  it 
will  see  indistinctly  the  horizontal  and  vice  versa.  Few 
people  therefore  see  equally  clearly  at  once  two  lines  crossing 
one  another  at  right  angles.  The  phenomenon  is  most  obvi- 
ous, however,  when  a  series  of 
concentric  circles  (Fig.  153)  is 
looked  at :  then  when  the  lines 
appear  sharp  along  some  sec- 
tors, they  are  dim  along  the 
rest.  When  this  defect,  known 
as  astigmatism,  is  marked  it 
causes  serious  troubles  of  vis- 
ion and  requires  peculiarly 
shaped  glasses  to  counteract 
it. 

4.     Opaque   /ladies   in   the 
fig.  153.  Refract inii  Media.      In    dis- 

eased   eyes   the   lens    may    be    opaque    (cataract)    and  need 


THE  EYE  AS  AN  OPTICAL  INSTRUMENT.  529 

removal;  or  opacities  from  ulcers  or  wounds  may  exist 
on  the  cornea.  But  even  in  the  best  eye  there  are  apt  to  be 
small  opaque  bodies  in  the  vitreous  humor  causing  muscce 
volitantes;  that  is,  the  appearance  of  minute  bodies  floating 
in  space  outside  the  eye,  but  changing  their  position  when 
the  position  of  the  eye  changes,  by  which  fact  their  origin  in 
internal  causes  may  be  recognized.  Many  persons  never  see 
them  until  their  attention  is  called  to  their  sight  by  some 
weakness  of  it,  and  then  they  think  they  are  new  phenomena. 
Visual  phenomena  due  to  causes  in  the  eye  itself  are  called 
entoptic;  the  most  interesting  are  those  due  to  the  retinal 
blood-vessels  (Chap.  XXXIII.).  Tears,  or  bits  of  the  secre- 
tion of  the  Meibomian  glands,  on  the  front  of  the  eyeball 
often  cause  distant  luminous  objects  to  look  like  ill-defined 
luminous  bands  or  patches  of  various  shape.  The  cause  of 
such  appearances  is  readily  recognized,  since  they  disajrpear 
or  are  changed  after  winking. 


CHAPTER  XXXIII. 
THE   EYE   AS   A   SENSORY   APPARATUS. 

The  Excitation  of  the  Visual  Apparatus. — The  excitable 
visual  apparatus  for  each  eye  consists  of  the  retina,  the  optic 
nerve,  and  the  brain-centres  connected  with  the  latter;  how- 
ever stimulated,  if  intact,  it  causes  visual  sensations.  In  the 
great  majority  of  cases  its  excitant  is  objective  light,  and  so 
we  refer  all  stimulations  of  it  to  that  cause,  unless  we  have 
special  reason  to  know  the  contrary.  As  already  pointed 
out  pressure  on  the  eyeball  causes  a  luminous  sensation 
(phosphene),  which  suggests  itself  to  us  as  dependent  on  a 
luminous  body  situated  in  space  where  such  an  object  must 
be  in  order  to  excite  the  same  part  of  the  retina.  Since  all 
rays  of  light  penetrating  the  eye,  except  in  the  line  of  its 
long  axis,  cross  that  axis,  if  Ave  press  the  outer  side  of  the 
eyeball  we  get  a  visual  sensation  referred  to  a  luminous  body 
on  the  nasal  side;  if  we  press  below  we  see  the  luminous 
patch  above,  and  so  on. 

Of  course  different  rays  entering  the  eye  take  different 
paths  through  it,  but  on  general  optical  principles,  which 
cannot  here  be  detailed,  we  may  trace  all  oblique  rays  through 
the  organ  by  assuming  that  they  meet  and  leave  the  optic 
axis  at  what  are  known  as  the  nodal  points  of  the  system; 
these  {kk',  Fig.  154)  lie  near  together  in  the  lens.  If  we 
want  to  find  where  rays  of  light  from  A  will  meet  the 
retina  (the  eye  being  properly  accommodated  for  seeing  an 
object  at  that  distance)  we  draw  a  line  from  A  to  k  (the  first 
nodal  point)  and  then  another,  parallel  to  the  first,  from  k' 
(the  second  nodal  point)  to  the  retina.  The  nodal  points  of 
the  eye  lie  so  near  together  that  for  practical  purposes  we 
may  treat  them  ;is  one  (k,  Fig.  155),  placed  near  the  back  of 
the  lens.  By  manifold  experience  we  have  learnt  that  a 
luminous  body  (A,  Fig.  155)  which  we  see,  always  lies  on  the 
prolongation  of  the  line  joining  the  excited  part  of  the  retina, 

530 


THE  EYE  AS  A   SENSORY  APPARATUS. 


531 


a,  and  the  nodal  point  h.  Hence  any  excitation  of  that  part 
of  the  retina  makes  us  think  of  a  luminous  body  somewhere 
on  the  line  a  A,  and,  similarly,  any  excitation  of  b,  of  a  body 


Ftg.  154. — Diagram  illustrating  the  points  at  which  incident  rays  meet  the  retina. 
xx.  optic  axis  ;  k,  first  nodal  point;  k\  second  nodal  point;  b,  point  where  the  im- 
age of  B  would  be  formed,  were  the  eye  properly  accommodated  for  it  ;  a,  the 
retinal  point  where  the  image  of  A  would  be  formed. 

on  the  line  b  B  or  its  prolongation.  It  is  only  other  conflict- 
ing experiences,  as  that  with  the  eyes  closed  external  bodies 
do  not  excite  visual  sensations,  and  the  constant  connection 


Fig.  155.— Diagrammatic  section  through  the  eyeball,    xx,  optic  axis  ;  fc,  nodal 
point. 

of  the  pressure  felt  on  the  eyelid  with  the  visual  sensation, 
that  enable  us  when  we  press  the  eyeball  to  conclude  that,  in 
spite  of  what  we  seem  to  sec,  the  luminous  sensation  is  not 
due  to  objective  light  from  outside  the  eye. 

The  Idio-Retinal  Light.— The  eyelids  are  not  by  any 
means  perfectly  opaque  ;  in  ordinary  daylight  they  still  allow 
a  considerable  quantity  of  light  to  penetrate  the  eye,  as  any 
one  may  observe  by  passing  hia  hand  in  front  of  the  closed 
Bui  even  in  a  dark  room  with  the  eyes  completely 
red  Hi.  bo  that  no  objective  light  can  enter  them,  there  is 
still  experienced  a  small  amount  of  visual  sensation  due  to 


532  THE  HUMAN  BODY. 

internal  causes.  The  field  of  vision  is  not  absolutely  dark 
but  slightly  luminous,  with  brighter  fleeting  patches  travers- 
ing it.  These  are  especially  noticeable,  for  example,  in  try- 
ing to  see  and  grope  one's  way  with  the  eyes  open  up  a  per- 
fectly dark  staircase.  Then  the  luminous  patches  attract 
special  attention  because  they  are  apt  to  be  taken  for  the 
signs  of  objective  realities;  they  become  very  manifest  when 
any  sudden  jar  of  the  Body,  due  for  example  to  knocking 
against  something,  occurs;  and  have  no  doubt  given  rise  to 
many  ghost  stories.  These  visual  sensations  felt  in  the  ab- 
sense  of  all  external  stimulation  of  the  eyes,  may  for  conveni- 
ence be  spoken  of  as  due  to  the  idio-retiual  light. 

The  Excitation  of  the  Visual  Apparatus  by  Light. — 
Light  only  excites  the  retina  when  it  reaches  its  nerve  end 
organs,  the  rods  and  cones.     The  proofs  of  this  are  several. 


Fig.  156. 

1.  Light  does  not  arouse  visual  sensations  when  it  falls 
directly  on  the  fibres  of  the  optic  nerve.  Where  this  nerve 
enters  there  is  a  retinal  part  possessing  only  nerve-fibres, 
and  this  part  is  blind.  Close  the  left  eye  and  look  steadily 
with  the  right  at  the  cross  in  Fig.  15G,  holding  the  book  verti- 
cally in  front,  of  the  face,  and  moving  it  to  and  fro.  It  will 
be  found  that  at  about  25  centimeters  (10  inches)  off  the 
white  circle  disappears  ;  but  when  the  page  is  nearer  or 
farther,  it  is  seen.  During  the  experiment  the  gaze  must  be 
kept  fixed  on  the  cross.  There  is  thus  in  the  field  of  vision  a 
blind  spot,  and  it  is  easy  k>  show  by  measurement  that  it  lies 
where  the  optic  nerve  enters. 

When  the  right  eye  is  fixed  on  the  cross,  it  is  so  directed 
that  rays  from  this  fall  on  the  yellow  spot  (_?/,  Fig.  157). 
The  rays  from  the  circle  then  cross  the  visual  axis  at  the 
nodal  point,  n,  and  meet  the  retina  at  o.  If  the  distance  of 
the  nodal  point  of   the  eye  from  the  paper  be/,  and  from 


THE  EYE  AS  A  SENSORY  APPARATUS. 


533 


the  retina  (which  is  15mm.)  be  F,  then  the  distance,  on 
the  paper,  of  the  cross  from  the  circle  will  be 
to  the  distance  of  y  from  o  as/  is  to  F.  Meas- 
urements made  in  this  way  show  that  the  circle 
disappears  when  its  image  is  thrown  on  the 
entry  of  the  optic  nerve,  which  lies  to  the  nasal 
side  of  the  yellow  spot. 

2.  The  above  experiment  having  shown  that 
light  does  not  act  directly  on  the  optic  nerve- 
fibres  any  more  than  it  does  on  any  other  nerve- 
fibres,  we  have  next  to  see  in  what  part  of  the 
retina  those  changes  do  first  occur  which  form 
the  link  between  light  and  nervous  impulses. 
They  occur  in  the  outer  part  of  the  retina,  in 
the  rods  and  cones.     This  is  proved  by  what  is 
called  Purkinje's  experiment.  Take  a  candle  in- 
to a  dark  room  and  look  at  a  surface  not  covered 
with  any  special  pattern,  say  a  whitewashed  wall 
or  a  plain  window-shade.     Hold  the  candle  to  the  side  of  one 
eye  and  close  to  it,  but  so  far  back  that  no  light   enters   the 
pupil  from  it;  that  is  so  far  back  that  the  flame  just  can- 
not be  seen,  but  so  that  a  strong  light  is  thrown  on  the  white 
of  the  eye  as  far  back  as  possible.     Then  move  the  candle  a 
little  to  and  fro.     The  surface  looked  at  will  appear  luminous 
with  reddish-yellow  light,  and  on  it  will  be  seen  dark  branch- 
ing lines  which  are  the  shadows  of  the  retinal  vessels.     Now 
in  order  that  these  shadows  may  be  seen  the  parts  on  which 
the  light  acts  must  be  behind  the  vessels,  that  is  in  the  outer 
layers  of  the  retina  since  the  blood  vessels  lie  in  its  inner 
strata.     The  experiment  may  be  more  satisfactorily  performed 
by  getting  another  person  to  focus  with  a  lens  the  light  of 
1 1n-  candle  as  a  bright  spot  as  far  back  as  possible  on  the  white 
of  the  observer's  eye. 

If  the  light  be  kept  steady  the  vascular  shadows  soon  dis- 
appear  ;  in  order  to  continue  to  see  them  the  candle  must  be 
kept  moving.  The  explanation  of  this  fact  may  readily  be 
made  clear  by  fixing  the  eyes  for  ten  or  fifteen  seconds  on  the 
dot  of  an  "i"  somewhere  about  the  middle  of  this  page:  at 
first  the  distinction  between  the  slightly  luminous  black 
letters  and  the  highly  luminous  white  page  is  very  obvious; 
in  other  words,  the  different  sensations  arising  from  the 
o ugly  and  the  feebly  excited  areas  of  the  retina.     P>ut  if 


534  THE  HUMAN  BODY. 

the  glance  do  not  be  allowed  to  wander,  very  soon  the  letters 
become  indistinct  and  at  last  disappear  altogether  ;  the  whole 
page  looks  uniformly  grayish.  The  reason  of  this  is  that  the 
powerful  stimulation  of  the  retina  by  the  light  reflected  from 
the  white  part  of  the  page  soon  fatigues  the  part  of  the  visual 
apparatus  it  acts  upon  ;  and  as  this  fatigue  progresses  the 
stimulus  produces  less  and  less  effect.  The  parts  of  the 
retina,  on  the  other  hand,  which  receive  light  only  from  the 
black  letters  are  but  little  stimulated  and  retain  much  of  their 
original  excitability,  so  that,  at  last,  the  feebler  excitation  act- 
ing upon  these  more  irritable  parts  produces  as  much  sensa- 
tion as  the  stronger  stimulus  acting  upon  the  fatigued  parts; 
and  the  letters  become  indistinguishable.  To  see  them  con- 
tinuously we  must  keep  shifting  the  eyes  so  that  the  parts  of 
the  visual  apparatus  are  alternately  fatigued  and  rested,  and 
the  general  irritability  of  the  whole  is  kept  about  the  same. 
So,  in  Purkinje's  experiment,  if  the  position  of  the  shadows 
remain  the  same,  the  shaded  part  of  the  retina  soon  becomes 
more  irritable  than  the  more  excited  unshaded  parts,  and  its 
relative  increase  of  irritability  makes  up  for  the  less  light 
falling  on  it,  so  that  the  shadows  cease  to  be  perceived.  It  is 
for  this  reason  that  we  do  not  see  the  retinal  vessels  under  ordi- 
nary circumstances.  When  light,  as  usual,  enters  the  eye 
from  front  through  the  pupil  the  shadows  always  fall  on  the 
same  parts  of  the  retina,  and  these  parts  are  thus  kept  suffi- 
ciently more  excitable  than  the  rest  to  make  up  for  the  less  light 
reaching  them  through  the  vessels.  To  see  the  latter  we 
must  throw  the  light  into  the  eye  in  an  unusual  direction, 
not  through  the  pupil  but  laterally  through  the  sclerotic.  If 
a  £'  j    v,  Fig.  158,  be  the  section  of  a  retinal 

vessel,  ordinarily  its  shadow  will  fall 
at  some  point  on  a  line  prolonged 
through  it  from  the  centre  of  the  pupil. 
If  a  candle  flame  be  held  opposite  b  it 
illuminates  that  part  of  the  sclerotic 
and  from  there  light  radiates  and  illu- 
mines the  interior  of  the  eye.  The 
resulting  sensation  we  refer  to  light 
entering  the  eye  in  the  usual  manner 
through  the  pupil,  and  accordingly  see 
the  surface  we  look  at  as  if  it  were  illuminated.  The  shadow 
of  v,  is  now  cast  on  an  unusual  spot  c,  and  we  see  it  as  if  at  the 


THE  EYE  AS  A   SENSORY  APPARATUS.  535 

point  d  on  the  wall,  on  the  prolongation  of  the  line  joining 
the  nodal  point,  k,  of  the  eye  with  c.  If  the  candle  be  moved 
so  as  to  illuminate  the  point  V  of  the  sclerotic,  the  shadow  of 
v  will  be  cast  on  c'  and  will  accordingly  seem  on  the  wall  to 
move  from  d  to  d'.  It  is  clear  that  if  we  know  how  far  b  is 
from  b' ,  how  far  the  wall  is  from  the  eye,  and  how  far  the 
nodal  point  is  from  the  retina  (15  mm.  or  O.G  inch),  and 
measure  the  distance  on  the  wall  from  d  to  d' ,  we  can  calcu- 
late how  far  c  is  from  c' :  and  then  how  far  the  vessel  throwing 
the  shadow  must  be  in  front  of  the  retinal  parts  perceiving 
it.  In  this  way  it  is  found  that  the  part  seeing  the  shadow, 
that  is  the  layer  on  which  light  acts,  is  just  about  as  far  be- 
hind the  retinal  vessels  as  the  main  vascular  trunks  of  the 
retina  are  in  front  of  the  rod  and  cone  layer.  It  is,  there- 
fore, in  that  layer  that  the  light  initiates  those  changes  which 
give  rise  to  nervous  impulses  ;  which  is  further  made  obvious 
by  the  fact  that  the  seat  of  most  acute  vision  is  the  fovea  cen- 
tralis, where  only  this  layer  and  the  cone-fibres  diverging 
from  it  are  present.  When  we  want  to  see  anything  dis- 
tinctly we  always  turn  our  eyes  so  that  its  image  shall  fall  on 
the  centres  of  the  yellow  spots. 

The  Vision  Purple.  How  light  acts  in  the  retina  so  as  to 
produce  nerve  stimuli  is  still  uncertain.  Eecent  observations 
show  that  it  produces  chemical  changes  in  the  rod  and  cone 
layer,  and  seemed  at  first  to  indicate  that  its  action  was  to 
produce  'substances  which  were  chemical  excitants  of  nerve- 
fibres  ;  but  although  there  can  be  little  doubt  that  these 
chemical  changes  play  some  important  part  in  vision,  what 
their  role  may  be  is  at  present  quite  obscure.  If  a  perfectly 
fresh  retina  be  excised  rapidly,  its  outer  layers  will  be  found 
of  a  rich  purple  color.  In  daylight  this  rapidly  bleaches,  but 
in  the  dark  persists  even  when  putrefaction  has  set  in.  In 
pure  yellow  light  it  also  remains  unbleached  a  long  time,  but 
in  other  lights  disappears  at  different  rates.  If  a  rabbit's  eye 
be  fixed  immovably  and  exposed  so  that  an  image  of  a  window 
is  focused  on  the  same  part  of  its  retina  for  some  time,  and 
then  the  eye  be  rapildy  excised  in  the  dark  and  placed  in 
.solution  of  potash  alum,  a  colorless  image  of  the  window  is 
found  on  the  retina,  surrounded  by  the  visual  purple  of  the 
which  is,  through  the  alum,  fixed  or  rendered  incapable 
of  change  by  light.  Photographs,  or  optograms,  are  thus  ob- 
tained which  differ  from  the  photographer's  in  that  he  uses 


.r>36  THE  HUMAN  BODY. 

light  to  produce  chemical  changes  which  give  rise  to  colored 
bodies,  while  here  the  reverse  is  the  case.  If  the  eye  be  not 
rapidly  excised  and  put  in  the  alum  after  its  exposure,  the 
optogram  will  disappear  ;  the  vision  purple  being  rapidly  re- 
generated at  the  bleached  part.  This  reproduction  of  it  is 
due  mainly  to  the  cells  of  the  pigmentary  layer  of  the  retina, 
which  in  living  eyes  exposed  to  light  thrust  long  processes 
between  the  rods  and  cones.  Portions  of  frogs'  retinas  raised 
from  this,  bleach  more  rapidly  than  those  left  in  contact  with 
it,  but  become  soon  purple  again  if  let  fall  back  upon  the 
pigment-cells.  Experiments  show,  however,  that  animals 
(frogs)  exposed  for  a  long  time  to  a  bright  light  may  have 
their  retinas  completely  bleached  and  still  see  very  well;  they 
can  still  unerringly  catch  flies  that  come  within  their  reach  ; 
and  they  can  also  distinguish  colors,  or  at  least  some  colors, 
as  green.  Moreover,  the  vision  purple  is  only  found  in  the 
outer  segments  of  the  rods  ;  there  is  none  in  the  cones,  and 
yet  these  alone  exist  in  the  yellow  spot  of  the  human  eye, 
which  is  the  seat  of  most  acute  vision;  and  animals,  such  as 
snakes,  which  have  only  cones  in  the  retina,  possess  no  vision 
purple  and  nevertheless  see  very  well. 

It  may  be  that  other  bodies  exist  in  the  retina  which  are 
also  chemically  changed  by  light,  but  the  changes  of  which 
are  not  accompanied  by  alterations  in  color  which  we  can  see; 
and.  in  the  absence  of  the  vision  purple,  seeing  might  be 
carried  on  by  means  of  these,  which  may  be  less  quickly 
destroyed  by  light  and  so  still  persist  in  the  bleached  retinas 
of  the  frogs  above  mentioned.  For  the  present,  however,  the 
question  of  the  part,  if  any,  played  in  vision  by  such  bodies 
must  be  left  an  open  one  :  and  the  possibility  that  the  rods 
and  cones  form  an  apparatus  which  directly  converts  ethereal 
vibrations  into  nerve  stimuli  without  any  intervening  chemi- 
cal process  must  be  borne  in  mind. 

The  Intensity  of  Visual  Sensations.  Light  considered  as 
a  form  of  energy  may  vary  in  quantity  ;  physiologically,  also, 
we  distinguish  quantitative  differences  in  light  as  degrees  of 
brightness,  but  the  connection  between  the  intensity  of  the 
sensation  excited  and  the  quantity  of  energy  represented  by 
the  stimulating  light  is  not  a  direct  one.  In  the  first  place, 
some  rays  excite  our  visual  apparatus  more  powerfully  than 
others  :  a  given  amount  of  energy  in  the  form  of  yellow  light, 
for  example,  causes  more  powerful  visual  sensations  than  the 


THE  EYE  AS  A   SENSORY  APPARATUS.  537 

same  quantity  of  energy  in  the  form  of  violet  light;  and  ultra- 
violet rays  only  become  visible,  and  then  very  faintly,  when 
all  others  are  suppressed;  but  if  they  be  passed  through  some 
fluorescent  substance  (see  Physics),  such  as  an  acid  solution 
of  quinine  sulphate,  which,  without  altering  the  amount  of 
energy,  turns  it  into  ethereal  oscillations  of  a  longer  period, 
then  the  light  becomes  readibly  perceptible. 

Even  with  light-rays  of  the  same  oscillation  period  our  sen- 
sation is  not  proportional  to  the  amount  of  energy  in  the 
light;  to  the  amount  of  heat,  for  example,  to  which  it  would 
give  rise  if  all  transformed  into  it.  If  objective  light  increase 
gradually  in  amount  oar  sensation  increases  also,  up  to  a  limit 
beyond  which  it  does  not  go,  no  matter  how  strong  the  light 
becomes;  but  the  increase  of  sensation  takes  place  far  more 
slowly  than  that  of  the  light,  in  accordance  with  the  psycho- 
physical law  already  mentioned.  If  we  call  the  amount  of 
light  given  out  by  a  single  candle  a,  then  that  emitted  by  two 
candles  will  be  2a;  and  so  on.  If  the  amount  of  sensation 
excited  by  the  single  candle  be  A,  then  that  due  to  two  can- 
dles will  not  be  2A,  and  that  by  three  will  be  far  less  than  3A. 
If  a  white  surface,  P,  Fig  159,  be  illuminated  by  a  candle  at 
c  and  another  elsewhere,  and  a  rod,  u,  be  placed  so  as  to  in- 
tercept the  light  from  c,  but  not 
that  from  the  other  candle,  we  see 
clearly  a  shadow,  since  our  eyes 
recognize  the  difference  in  luminos-  /*~ 

ity  of  this  part  of  the  paper,  reflect-    / 
ins  light  from  one  candle  onlv,  from 

Fig.  159 

that  of  the  rest  which  is  illuminated 

by  two:  that  is  we  can  tell  the  sensation  due  to  the  stimulus 
a  from  that  due  to  the  stimulus  2a.)  If  now  a  bright  lamp  be 
brought  in  and  placed  alongside,  and  its  light  be  physically 
equal  to  that  of  10  candles,  we  cease  to  perceive  the  shadows. 
\\  e  find  the  sensation  aroused  by  objective  light  ==  12a  (due 
to  the  lamp  and  two  candles)  cannot  be  told  from  that  due  to 
light  =  lia;  although  the  difference  of  objective  light  i3  still 
la  as  before.  Most  persons  must  have  observed  illustrations 
of  this.  Sitting  in  a  room  with  three  lights  not  unfrequently 
some  object  so  intercepts  the  light  from  two  as  to  cast  on  the 
wall  two  shadows  which  partly  overlap.  Where  the  shadows 
overlap  the  wall  gets  light  only  from  the  thin)  candle;  around 
chat,  where  each  shallow  is  separate,  it  is  illuminated  by  this 


538  THE  111  MAN  BODY. 

and  one  other  candle;  and  the  wall  in  the  neighborhood  of 
the  shadows  by  all  three.  Objectively,  therefore,  the  differ- 
ence between  the  deep  shadow  and  half  shadow  is  that 
between  the  light  of  one  candle  and  that  of  two.  The  differ- 
ence between  the  half  shadows  and  the  wall  around  is  that 
between  the  light  of  two  and  three  candles.  But  as  a  matter 
of  sensation  the  difference  between  the  half  shadow  and  the 
full  shadow  seems  much  greater  than  that  between  the 
half  shadow  and  the  rest  of  the  wall;  in  other  words  the 
difference,  a,  between  a  and  2a,  is  a  more  efficient  stimulus 
than  the  same  difference,  a,  between  2a  and  3a.  When  the 
total  stimulus  increases  the  same  absolute  difference  is  less 
felt  or  may  be  entirely  unperceived.  An  example  of  this 
which  every  one  will  recognize  is  afforded  by  the  invisibility 
of  the  stars  in  daytime. 

On  the  other  hand,  as  the  total  stimulus  increases  or  de- 
creases the  same  fractional  difference  of  the  whole  is  per- 
ceived with  the  same  ease;  i.e.,  excites  the  same  amount  of 
sensation.  In  reading  a  book  by  lamplight  we  perceive 
clearly  the  difference  between  the  amount  of  light  reflected 
from  the  black  letters  and  the  white  page.  If  we  call  the 
total  lamplight  reflected  by  the  blank  parts  10a  and  that  by 
the  letters  2a,  we  may  say  we  perceive  with  a  certain  distinct- 
ness a  luminous  difference  equal  to  one  fifth  of  the  whole. 
If  we  now  take  the  book  into  the  daylight  the  total  light  re- 
flected from  the  letters  and  the  imprinted  part  of  the  page 
increases,  but  in  the  same  proportion.  Say  the  one  now  is 
50a  and  the  other  10a;  although  the  absolute  difference  be- 
tween the  two  is  now  40a  instead  of  8a  we  do  not  see  the 
letters  any  more  plainly  than  before.  The  smallest  difference 
in  luminous  intensity  which  we  can  perceive  is  about  j^m  of 
the  whole,  for  all  the  range  of  lights  we  use  in  carrying  on 
our  ordinary  occupations.  For  strong  lights  the  smallest  per- 
ceptible fraction  is  considerably  greater;  finally  we  reach  a 
limit  where  no  increase  in  brightness  is  felt.  For  weak 
illumination  the  sensation  is  more  nearly  proportioned  to  the 
total  differences  of  the  objective  light.  Thus  in  a  dark  room 
an  object  reflecting  all  the  little  light  that  reaches  it  appears 
almost  twice  as  bright  as  one  reflecting  only  half;  in  a 
stronger  light  it  would  so  appear.  Bright  objects  in  general 
obscurity  thus  appear  unnaturally  bright  when  compared 
with  things  about  them,  and  indeed  often  look  self-luminous. 


THE  EYE  AS  A  SENSORY  APPARATUS.  539 

A  cat's  eyes,  for  example,  are  said  to  "shine  in  the  dark"; 
and  painters  to  produce  moonlight  effects  always  make  the 
bright  parts  of  a  picture  relatively  brighter,  when  compared 
with  things  about  them,  than  would  be  the  case  if  a  sunny 
scene  were  to  be  represented ;  by  a  relatively  excessive  use  of 
white  pigment  they  produce  the  relatively  great  brightness  of 
those  things  which  are  seen  at  all  in  the  general  obscurity  of 
a  moonlight  landscape. 

The  Duration  of  Luminous  Sensations. — This  is  greater 
than  that  of  the  stimulus,  a  fact  taken  advantage  of  in  mak- 
ing fireworks :  an  ascending  rocket  produces  the  sensation  of 
a  trail  of  light  extending  far  behind  the  position  of  the  bright 
part  of  the  rocket  itself  at  the  moment,  because  the  sensation 
aroused  by  it  in  a  lower  part  of  its  course  still  persists.  So, 
shooting  stars  appear  to  have  luminous  tails  behind  them. 
By  rotating  rapidly  before  the  eye  a  disk  with  alternate  white 
and  black  sectors  we  get  for  each  point  of  the  retina  on 
which  a  part  of  its  image  falls,  alternating  stimulation  (due 
to  the  passage  of  white  sector)  and  rest  (when  a  black  sector 
is  passing).  If  the  rotation  be  rapid  enough  the  sensation 
aroused  is  that  of  a  uniform  gray,  such  as  would  be  produced 
if  the  white  and  black  were  mixed  and  spread  evenly  over 
the  disk.  In  each  revolution  the  eye  gets  as  much  light  as  if 
that  were  the  case,  and  is  unable  to  distinguish  that  this 
light  is  made  up  of  separate  portions  reaching  it  at  intervals: 
the  stimulation  due  to  each  lasts  until  the  next  begins  and  so 
all  are  fused  together.  If,  while  looking  at  the  flame,  one 
turns  out  suddenly  the  gas  in  a  room  containing  no  other 
light,  the  image  of  the  flame  persists  a  short  time  after  the 
flame  itself  is  extinguished. 

The  Localizing  Power  of  the  Retina. — As  already  pointed 
out  a  necessary  condition  of  seeing  definite  objects,  as  distin- 
guished from  the  power  of  recognizing  differences  of  light 
and  darkness,  is  that  all  light  entering  the  eye  from  one  point 
of  an  object  shall  be  focused  on  one  point  of  the  retina. 
This,  however,  would  not  be  of  any  use  had  we  not  the  faculty 
of  distinguishing  the  stimulation  of  one  part  of  the  retina 
from  that  of  another  part.  This  power  the  visual  apparatus 
possesses  in  a  very  high  degree;  while  with  the  skin  we  can- 
not distinguish  from  one.  two  points  touching  it  less  than  1 
mm.  (.J*  inch)  apart,  with  our  eyes  we  can  distinguish  two 
points   whose   retinal   images    are    not    more  than  .004  mm. 


540 


THE  III  MAX    BODY. 


(.U001G  inch)  apart.  The  distance  between  the  retinal  images 
of  two  points  is  determined  by  the  "visual  angle''  under 
which  they  are  seen;  this  angle  is  that  included  between 
lines  drawu  from  them  to  the  nodal  point  of  the  eye.  Jf  a 
and  b  (Fig.  1G0)  are  luminous  points,  the  image  of  a  wil)  b*. 


formed  at  a'  on  the  prolongation  of  the  line  a  n  joining  a 
with  the  node,  n.  Similarly  the  image  of  b  will  be  formed 
at  //.  If  a  and  b  still  remaining  the  same  distance  apart,  be 
moved  nearer  the  eye  to  c  and  d,  then  the  visual  angle 
under  which  they  are  seen  will  be  greater  and  their  retinal 
images  will  be  farther  apart,  at  c'  and  d' .  If  a  and  b  are  the 
highest  and  lowest  parts  of  an  object,  the  distance  between 
their  retinal  images  will  then  depend,  clearly,  not  only  on  the 
size  of  the  object,  but  on  its  distance  from  the  eye;  to  know 
the  discriminating  power  of  the  retina  we  must  therefore 
measure  the  visual  angle  in  each  case.  In  the  fovea  centralis 
two  objects  seen  under  a  visual  angle  of  50  to  70  seconds  can 
be  distinguished  from  one  another;  this  gives  for  the  distance 
between  the  retinal  images  that  above  mentioned,  and  corre- 
sponds pretty  accurately  to  the  diameter  of  a  cone  in  that 
part  of  the  retina.  We  may  conclude,  therefore,  that  when 
two  images  fall  on  the  same  cone  or  on  two  contiguous  cones 
they  are  not  discriminated;  but  that  if  one  or  more  unstimu- 
lated cones  intervene  between  the  stimulated,  the  points  may 
be  perceived  as  distinct.  The  diameter  of  a  rod  or  cone,  in 
fact,  marks  the  anatomical  limit  np  to  which  we  can  by  prac- 
tice raise  our  acuteness  of  visual  discrimination;  and  in  the 
yellow  spot  which  we  constantly  use  all  our  lives  in  looking 
at  things  which  we  want  to  see  distinctly,  we  have  educated 
the  visual  apparatus  up  to  about  its  highest  power.  Else- 
where on  the  retina  our  discriminating  power  is  much  less 
and  diminishes  as  the  distance  from  the  yellow  spot  increases. 
This  is  partly  due,  no  doubt,  to  a  less  sensibility  of  those  reti- 
nal regions,  such  as,  by  other  facts,  is  proved  to  exist,  but  in 
part,  no  doubt  is  also  due  to  a  want  of  practice.     The  more 


THE  EYE  AS  A  SENSORY  APPARATUS.  541 

peripheral  the  retinal  region  the  less  we  have  used  it  for  such 
purposes.  It  is  probable,  therefore,  that  outlying  portions  of 
the  retina  are  capable  of  education  to  a  higher  discriminating 
power,  just  as  we  shall  find  the  skin  to  be  for  tactile  stimuli. 

While  Ave  can  tell  the  stimulation  of  an  upper  part  of  the 
retina  from  a  lower,  or  a  right  region  from  a  left,  it  must  be 
borne  in  mind  that  we  have  no  direct  knowledge  of  which  is 
upper  or  lower  or  right  vr  left  in  the  ocular  image.  All  our 
visual  sensations  tell  us  is  that  they  are  aroused  at  different 
points,  and  nothing  at  all  about  the  actual  positions  of  these 
on  the  ratina.  There  is  no  other  eye  behind  the  retina  look- 
ing at  it  to  see  the  inversion  of  the  image  formed  on  it. 
Suppose  I  am  looking  at  a  pane  in  a  second-story  window 
of  a  distant  house:  its  image  will  then  fall  on  the  fovea  cen- 
tralis ;  the  line  joining  this  with  the  pane  is  called  the  visual 
axis.  The  image  of  the  roof  will  be  formed  on  a  part  of  the 
retina  below  the  fovea,  and  that  of  the  front  door  above  it.  I 
distinguish  that  the  images  of  all  these  fall  on  different  parts- 
of  the  retina  in  certain  relative  positions,  and  have  learnt,  by 
the  experience  of  all  my  life,  that  when  the  image  of  any- 
thing arouses  the  sensation  due  to  excitation  of  part  of  the 
retina  below  the  fovea  the  object  is  above  my  visual  axis,  and 
vice  versa  ;  similarly  with  right  and  left.  Consequently  1  in- 
terpret the  stimulation  of  lower  retinal  regions  as  meaning 
high  objects,  aud  of  right  retinal  regions  as  meaning  left  ob- 
jects, and  never  get  confused  by  the  inverted  retinal  image 
about  which  directly  I  know  nothing.  A  new-born  child, 
even  supposing  it  could  use  its  muscles  perfectly,  could  not, 
except  by  mere  chance,  reach  towards  an  object  which  it  saw; 
it  would  grasp  at  random,  not  yet  having  learnt  that  to  reach 
an  object  exciting  a  part  of  the  retina  above  the  fovea  needed 
movement  of  the  hand  towards  a  position  in  space  below  the 
visual  axis  ;  but  very  soon  it  learns  that  things  near  its  brow, 
that  is  up,  excite  certain  visual  sensations,  and  objects  below 
its  eyes  others,  and  similarly  with  regard  to  right  and  left;  in 
time  it  learns  to  interpret  retinal  stimuli  so  as  to  localize 
accurately  the  direction,  with  reference  to  its  eyes,  of  outer 
objects,  and  never  thenceforth  gets  puzzled  by  retinal  inver- 
sion. 

Color  Vision. — Sunlight  reflected  from  snow  gives  us  a 
sensation  which  we  call  white.  The  same  light  sent  through 
a  prism  and  reflected  from  a  white  surface   excites  in  us  no> 


542  THE  HUMAN  BODY. 

white  sensation  but  a  number  of  color  sensations,  gradating 
insensibly  from  red  to  violet,  through  orange,  yellow,  green, 
blue-green,  blue,  and  indigo.  The  prism  separatee  from  one 
another  light-rays  of  differenl  periods  of  oscillation  and  each 
ray  excites  in  us  a  colored  visual  sensation,  while  all  mixed 
together,  as  in  sunlight,  they  arouse  the  entirely  different 
sensation  of  white.  If  the  light  fall  on  a  piece  of  black 
velvet  we  get  still  another  sensation,  that  of  black;  in  this 
case  the  light-rays  are  so  absorbed  thai  but  \\-\v  are  reflected 
to  the  eye  and  the  visual  apparatus  is  left  at  rest.  Physically 
black  represents  nothing:  it  is  a  mere  zero — the  absence  of 
ethereal  vibrations;  but,  in  consciousness,  it  is  as  definite  a 
sensation  as  white,  red,  or  any  other  color.  We  do  not  feel 
blackness  or  darkness  except  over  the  region  of. the  possible 
visual  field  of  our  eyes.  In  a  perfectly  dark  room  we  only 
feel  the  darkness  in  front  of  our  eyes,  and  in  the  light  there 
is  no  such  sensation  associated  with  the  hack  of  our  heads  or 
the  palms  of  our  hands,  though  through  these  we  get  no 
visual  sensations.  It  is  obvious,  therefore,  that  the  sensation 
of  blackness  is  not  due  to  the  mere  absence  of  luminous 
stimuli,  but  to  the  unexcited  state  of  the  retinas,  which  are 
alone  capable  of  being  excited  hy  such  stimuli  when  present. 
This  fact  is  a  very  remarkable  one,  and  is  not  paralleled  in  any 
other  sense.  Physically,  complete  stillness  is  to  the  ear  what 
darkness  is  to  the  eye;  but  silence  impresses  itself  on  us  as 
the  absence  of  sensation,  while  darkness  causes  a  definite 
feeling  of  "  blackness." 

Young's  Theory  of  Color  Vision. — Our  color  sensations 
insensibly  fade  into  one  another;  starting  with  black  we  can 
insensibly  pass  through  lighter  and  lighter  shades  of  gray 
to  white:  or  beginning  with  green  through  darker  and  darker 
shades  of  it  to  black  or  through  lighter  and  lighter  to  white: 
or  beginning  with  red  we  can  by  imperceptible  steps  pass  to 
orange,  from  that  to  yellow  and  so  on  to  the  end  of  the  solar 
spectrum:  and  from  the  violet,  through  purple  and  carmine, 
we  may  gel  hark  again  to  red.  Black  and  white  appeal-  to  be 
fundamental  color  sensations  mixed  up  with  all  the  rest:  we 
never  imagine  a  color  but  as  light  or  dark,  that  is  as  more  or 
less  near  white  or  black;  and  it  is  found  thai  as  the  light 
thrown  on  any  given  colored  surface  weakens,  the  shade  be- 
comes deeper  until  it  passes  into  black;  and  if  the  illumina- 
tion he  increased,  the  color  becomes  "lighter"  until  it  passes 


T1IL  EYE  AS  A   SENSORY  APPARATUS.  543 

into  white.  Of  all  the  colors  of  the  spectrum  yellow  most 
easily  passes  into  white  with  strong  illumination.  Black  and 
white,  with  the  grays  which  are  mixtures  of  the  two,  thus 
seem  to  stand  apart  from  all  the  rest  as  the  fundamental 
visual  sensations,  and  the  others  alone  are  in  common  par- 
lance named  "  colors."  It  has  even  been  suggested  that  the 
power  of  differentiating  them  in  sensation  has  only  lately 
been  acquired  by  man,  and  a  certain  amount  of  evidence  has 
been  adduced  from  passages  in  the  Iliad  to  prove  that  the 
Greeks  in  Homer's  time  confused  together  colors  that  are 
very  different  to  most  modern  eyes;  at  any  rate  there  seems 
to  be  no  doubt  that  the  color  sense  can  be  greatly  improved 
by  practice;  women  whose  mode  of  dress  causes  them  to  pay 
more  attention  to  the  matter,  have,  as  a  general  rule,  a  more 
acute  color  sense  than  men. 

Leaving  aside  black,  white,  gray,  and  the  various  browns 
(which  are  only  dark  tints  of  other  colors),  we  may  enumer- 
ate our  color  sensations  as  red,  orange,  yellow,  green,  blue, 
violet,  or  purple;  between  each  there  are,  however,  numerous 
transition  shades,  as  yellow-green,  blue-green,  etc.,  so  that 
the  number  which  shall  have  definite  names  given  to  them  is 
to  a  large  extent  arbitrary.  Of  the  above,  all  but  purple  are 
found  in  the  spectrum  given  when  sunlight  is  separated  by  a 
prism  into  its  rays  of  different  refrangibility;  rays  of  a  cer- 
tain wave-length  or  period  of  oscillation  cause  in  us  the  feel- 
ing red;  others  yellow,  and  so  on;  for  convenience  we  may 
speak  of  these  as  red,  yellow,  blue,  etc.,  rays;  all  together,  in 
anout  equal  proportions,  they  arouse  the  sensation  of  white. 
A  remarkable  fact  is  that  most  color  feelings  can  be  aroused 
in  Beveral  ways.  White,  for  example,  not  only  by  the  above 
general  mixture,  but  red  and  blue-green  rays,  or  orange  and 
blue,  or  yellow  and  violet,  taken  in  pairs  in  certain  propor- 
tions, and  acting  simultaneously  or  in  very  rapid  succession 
on  the  same  part  of  the  retina,  cause  the  sensation  of  white: 
h  colors  are  called  complementary  to  one  another.  The 
mixture  may  be  made  in  several  ways;  as,  for  example,  by 
causing  the  red  and  blue-green  parts  of  the  spectrum  to 
overlap,  or  by  painting  red  and  blue-green  sectors  on  a  disk 
and  rotating  it  rapidly;  they  cannot  be  made,  however,  by 
mixing  pigments,  since  what  happens  in  such  cases  is  a  very 
complex  phenomenon.  Painters,  tor  example,  are  accustomed 
to  produce  green  by  mixing  blue  and  yellow  paints,  and  some 


544  THE  HUMAN  BODY. 

may  be  inclined  to  ridicule  the  statement  that  yellow  and 
blue  when  mixed  give  white.  When,  however,  we  mix  the 
pigments  we  do  not  combine  the  sensations  of  the  same  name, 
which  is  the  matter  in  question.  Blue  paint  is  blue  because  it 
absorbs  all  the  rays  of  the  sunlight  except  the  blue  and  some 
of  the  green;  yellow  is  yellow  because  it  absorbs  all  but  the 
yellow  and  some  of  the  green,  and  when  blue  and  yellow  are 
mixed  the  blue  absorbs  all  the  distinctive  part  of  the  yellow 
and  the  yellow  does  the  same  for  the  blue;  and  so  only  the 
green  is  left  over  to  reflect  light  to  the  eye,  and  the  mixture 
has  that  color.  Grass-green  has  no  complementary  color  in 
in  the  solar  spectrum;  but  with  purple,  which  is  made  by 
mixing  red  and  blue,  it  gives  white.  Several  other  colors 
taken  three  together,  give  also  the  sensation  of  white.  If 
then  we  call  the  light-rays  which  arouse  in  us  the  sensation 
red,  a,  those  giving  us  the  sensation  orange  b,  yellow  c,  and 
so  on,  we  find  that  we  get  the  sensation  white  with  a,  b,  c,  d, 
e,  f,  and  g  all  together;  or  with  b  and  e,  or  with  c  and/,  or 
with  a,  d,  and  e  ;  our  sensation  white  has  no  determinate  re- 
lation to  ethereal  oscillations  of  a  given  period,  and  the  same 
is  true  for  several  other  colors;  yellow  feeling,  for  example, 
may  be  excited  by  ethereal  vibrations  of  one  given  wave- 
length (spectral  yellow),  or  by  a  light  containing  only  such 
waves  as  taken  separately  cause  the  sensations  red  and  grass- 
green  ;  in  other  words  a  physical  light  in  which  there  are  no 
waves  of  the  "yellow"  length  may  cause  in  us  the  sensation 
yellow,  which  is  only  one  more  instance  of  the  general  fact 
that  our  sensations,  as  such,  give  us  no  direct  information  as 
to  the  nature  of  external  forces;  they  are  but  signs  which  we 
have  to  interpret.  The  doctrine  of  specific  nerve  energies 
makes  it  highly  improbable  that  our  different  color  sensa- 
tions can  all  be  due  to  different  modes  of  excitation  of  exactly 
the  same  nerve-fibres;  a  fibre  which  when  excited  alone  gives 
us  the  sensation  red  will  always  give  us  that  feeling  when 
so  excited.  The  simplest  method  of  explaining  our  color 
sensations  would  therefore  be  to  assume  that  for  each  there 
exists  in  the  retina  a  set  of  nerve-fibres  with  appropriate 
terminal  organs,  each  excitable  by  its  own  proper  stimu- 
lus. But  we  can  distinguish  so  innumerable  and  so  finely 
graded  colors,  that,  on  such  a  supposition,  there  must  be  an 
almost  infinite  number  of  different  end  organs  in  the  retina, 
and  it  is  more  reasonable  to  suppose  that  there  are  a  limited 


THE  EYE  AS  A   SENSORY  APPARATUS.  545 

number  of  primary  color  sensations,  and  that  the  rest  are  due 
to  combinations  of  these.  That  a  compound  color  sensation 
may  be  very  different  from  its  components  when  these  are 
regarded  apart,  is  clearly  shown  by  the  sensation  white 
aroused  either  by  what  we  may  call  red  and  blue-green,  or 
green  and  purple,  stimuli  acting  together;  or  of  yellow  due 
to  grass-green  and  red.  To  account  for  our  various  color  sen- 
sations we  may,  therefore,  assume  a  much  smaller  number  of 
primary  sensations  than  the  total  number  of  color  sensations 
we  experience;  all  can  in  fact  be  explained  by  assuming  any 
three  primary  color  sensations  which  together  give  white,  and 
regarding  all  the  rest  as  due  to  mixtures  of  these  in  various 
proportions;  there  may  be  more  than  three,  but  three  will 
account  for  all  the  phenomena,  black  being  a  sensation  expe- 
rienced when  all  visual  stimuli  are  absent.  This  is  known  as 
Young's  theory  of  color  vision,  and  is  that  at  present  most 
commonly  accepted.  The  selection  of  the  three  primary  sen- 
sations is  decided  by  the  phenomena  of  color-blindness,  which 
show  that  if  this  theory  of  color  vision  be  correct  red  must 
be  one  of  the  primary  color  sensations:  if  so,  then  green 
and  violet  must  be  the  other  two.  The  theory  further 
assumes  that  all  kinds  of  light  stimulating  the  end  appa- 
ratuses give  rise  to  all  three  sensations,  but  not  necessarily  in 
the  same  proportion.  When  all  are  equally  aroused  the  sen- 
sation is  white  or  some  shade  of  gray  when  the  red  and  green 
are  tolerably  powerfully  excited  and  the  violet  little,  the  sen- 
sation is  yellow;  when  the  green  powerfully  and  the  red  and 
violet  little,  the  sensation  is  green,  and  so  on.  In  this  way 
we  can  also  explain  the  fact  that  all  colored  surfaces  when 
intensely  illuminated  pass  into  white.  A  red  light,  for  ex- 
ample, excites  the  primary  red  sensation  most,  but  green  and 
violet  a  little;  as  the  light  becomes  stronger  a  limit  is 
reached  beyond  which  the  red  sensation  cannot  go,  but  the 
green  and  violet  go  on  increasing  with  the  intensity  of  the 
light,  until  they  too  reach  their  limits;  and  all  three  primary 
sensations  being  then  equally  aroused,  the  sensation  white  is 
produced. 

Color  Blindness.  Some  persons  fail  to  distinguish  colors 
which  are  to  others  quite  different ;  when  such  a  deficiency  is 
well  marked  it  is  known  as  "colorblindness,"  and,  assuming 
Young's  theory  to  be  correct,  it  may  be  explained  by  an  ab- 
sence of  one  or  more  of  the  three  primary  color  sensations; 


546  Til K  HUMAN  HODY. 

observation  of  color-blind  persons  thus  helps  in  deciding 
■which  these  are.  The  most  common  form  is  red  color  blind- 
ness; persons  afflicted  with  it  confuse  reds  and  greens.  lied 
to  the  normal  eve  is  red  because  it  excites  red  sensation 
much,  green  some,  and  violet  less;  and  a  white  page  white, 
because  it  excites  red,  green,  and  violet  sensations  about 
equally.  In  a  person  without  red  sensation  a  red  object 
would  arouse  only  some  green  and  violet  sensation  and  so  would 
be  indistinguishable  from  a  bluish  green;  in  practice  it  is 
found  that  many  persons  confound  these  colors.  Cases  of 
green  and  violet  color  blindness  are  also  met  with,  but  they 
are  much  rarer  than  the  red  color  blindness  or  "  Daltonism." 

The  detection  of  color  blindness  is  often  a  matter 
of  considerable  importance,  especially  in  sailors  and  railroad 
officials,  since  the  two  colors  most  commonly  confounded,  red 
and  green,  are  those  used  in  maritime  and  railroad  signals. 
Persons  attach  such  different  names  to  colors  that  a  deci.-ion 
as  to  color  blindness  cannot  be  safely  arrived  at  by  simply 
showing  a  color  and  asking  its  name.  The  best  plan  is  to 
take  a  heap  of  worsted  of  all  tints,  select  one,  say  a  red,  and 
tell  the  man  to  put  alongside  it  all  those  of  the  same  color, 
whether  of  a  lighter  or  a  darker  shade;  if  red  blind  he  will 
select  not  only  the  reds  but  the  greens,  especially  the  paler 
tints.  About  one  man  in  eight  is  more  or  less  red  blind. 
The  defect  is  much  rarer  in  women. 

Fatigue  of  the  Retina.  The  nervous  visual  apparatus  is 
easily  fatigued.  Usually  we  do  not  observe  this  because  its 
restoration  is  also  rapid,  and  in  ordinary  life  our  eyes,  when 
open,  are  never  at  rest;  we  move  them  to  and  fro,  so  that 
parts  of  the  retina  receive  light  alternately  from  brighter  and 
darker  objects  and  are  alternately  excited  and  rested.  How 
constant  and  habitual  the  movement  of  the  eyes  is  can  be 
readily  observed  by  trying  to  fix  for  a  short  time  a  small  spot 
without  deviating  the  glance;  to  do  so  for  even  a  few  seconds 
is  impossible  without  practice,  if  any  small  object  is  steadily 
"  fixed  "  for  twenty  or  thirty  seconds  it  will  be  found  that  the 
whole  field  of  vision  becomes  grayish  and  obscure,  because 
the  parts  of  the  retina  receiving  most  light  get  fatigued,  and 
arouse  no  more  sensation  than  those  less  fatigued  and  stimu- 
lated by  light  from  less  illuminated  objects.  Or  look  steadily 
at  a  block  object,  say  a  blot  on  a  white  page,  for  twenty 
seconds,  and  then  turn  the  eye  on   a  white  wall:  the  latter 


THE  EYE  AS  A  SEXSORY  APPARATUS.  547 

will  seem  dark  gray,  with  a  white  patch  on  it;  an  effect  due 
to  the  greater  excitability  of  the  retinal  parts  previously 
rested  by  the  black,  when  compared  with  the  sensation 
aroused  elsewhere  by  light  from  the  white  wall  acting  on  the 
jjreviously  stimulated  parts  of  the  visual  surface.  All  persons 
will  recall  many  instances  of  such  phenomena,  which  are  es- 
pecially noticeable  soon  after  rising  in  the  morning.  Similar 
things  may  be  noticed  with  colors;  after  looking  at  a  red 
patch  the  eye  turned  on  a  white  wall  sees  a  blue-green  patch; 
the  elements  causing  red  sensations  having  been  fatigued,  the 
white,  mixed  light  from  the  wall  now  excites  on  that  region 
of  the  retina  only  the  other  primary  color  sensations.  The 
blending  of  colors  so  as  to  secure  their  greatest  effect  depends 
on  this  fact;  red  and  green  go  well  together  because  each 
rests  the  parts  of  the  visual  apparatus  most  excited  by  the 
other,  and  so  each  apj^ears  bright  and  vivid  as  the  eye  wan- 
ders to  and  fro;  while  red  and  orange  together,  each  exciting 
and  exhausting  mainly  the  same  visual  elements,  render  dull, 
or  in  popular  phrase  "  kill,"  one  another. 

Contrasts.  If  a  well-defined  black  surface  be  looked  at  on 
a  larger  white  one  the  parts  of  the  latter  close  to  the  black  look 
whiter  than  the  rest,  and  the  parts  of  the  black  near  the 
white  blacker  than  the  rest;  so,  also,  if  a  green  patch  be 
looked  at  on  a  red  surface  each  color  is  heightened  near  where 
they  meet.  This  phenomenon  is  largely  due  to  fatigue  and 
deficient  fixation:  the  retinal  parts  not  excited  and  fatigued 
by  the  black  or  the  green  are  brought  by  a  movement  of  the 
organ  so  as  to  receive  light  from  the  white  or  red  surface; 
phenomena  due  to  this  cause  are  known  as  those  of  successive 
emit  fast.  Even  in  the  case  of  perfect  fixation,  however,  some- 
thing of  the  same  kind  is  seen;  black  looks  blacker  near 
white  and  green  greener  near  red  when  the  eye  has  not 
moved  in  the  least  from  one  to  the  other.  A  small  piece  of 
light  gray  paper  put  on  a  sheet  of  red,  which  latter  is  then 
red  accurately  with  a  sheet  of  semi-transparent  tissue- 
paper,  assumes  the  complementary  color  of  the  red,  i.e.,  "iooks 
bluish  green;  and  gray  on  a  green  sheet  under  similar  cir~ 
cu instances  looks  pink.  Such  phenomena  are  known  as  those 
of  simultaneous  contrast,  and  are  explained  on  psychological 
grounds  by  those  who  accept  Young's  theory  of  color  vision. 
Ju-t  as  a  medium-sized  man  looks  short  beside  a  tall  one,  so, 
it  is  said,  a  black  Burface  looks  blacker  near  a  white  one,  or  a 


548  THE  1 1  (MAX  BODY. 

gray  (slightly  luminous  white)  surface,  which  feebly  excites 
red,  green,  and  violet  sensations,  looks  deficient  in  red  (and  so 
bluish  green)  near  a  deeper  red  surface.  There  are,  however, 
certain  phenomena  of  simultaneous  contrast  which  cannot  be 
satisfactorily  so  explained,  and  these  have  led  to  other  theories 
of  color  vision,  the  most  important  of  which  is  that  described 
in  the  next  paragraph. 

Hering's  Theory  of  Vision.  Contrasts  can  be  seen  with 
the  eyes  closed  and  covered.  If  we  look  a  short  time  at  a 
bright  object  and  then  rapidly  exclude  light  from  the  eye,  we 
see  for  a  moment  a  positive  after-image  of  the  object,  e.g.f 
a  window  with  its  frame  and  panes  after  a  glance  at  it  and 
then  closing  the  eyes.  In  these  positive  after-images  the 
bright  and  dark  parts  of  the  object  which  was  looked  at  retain 
their  original  relationship;  they  depend  on  the  persistence  of 
retinal  excitement  after  the  cessation  of  the  stimulus  and 
usually  soon  disappear.  If  an  object  be  looked  at  steadily  for 
some  time,  say  twenty  seconds,  and  the  eyes  be  then  closed,  a 
negative  after-image  is  seen.  In  this  the  lights  and  shades  of 
the  object  looked  at  are  reversed.  Frequently  a  positive 
after-image  becomes  negative  before  disappearing.  The 
negative  images  are  explained  commonly  by  fatigue;  when  the 
eye  is  closed  some  light  still  enters  through  the  lids  and  ex- 
cites less  those  parts  of  the  retina  previously  exhausted  by 
prolonged  looking  at  the  brighter  parts  of  the  field  of  vision; 
or,  when  all  light  is  rigorously  excluded,  the  self  stimula- 
tion of  the  visual  apparatus  itself,  causing  the  idio-retinal 
light,  affects  less  the  exhausted  portions,  and  so  a  negative 
image  is  produced.  If  we  fix  steadily  for  thirty  seconds  a 
point  between  two  white  squares  about  4  mm.  (£  inch) 
apart  on  a  large  black  sheet,  and  then  close  and  cover  our 
eyes,  we  get  a  negative  after-image  in  which  are  seen  two 
dark  squares  on  a  brighter  surface;  this  surface  is  brighter 
close  around  the  negative  after-image  of  each  square,  and 
brightest  of  all  between  them.  This  luminous  boundary  is 
called  the  corona,  and  is  explained  usually  as  an  effect  of 
simultaneous  contrast;  the  dark  after-image  of  the  square  it 
is  said  makes  us  mentally  err  in  judgment  and  think  the 
clear  surface  close  to  it  brighter  than  elsewhere;  and  it  is 
brightest  between  the  two  dark  squares,  just  as  a  middlo-sized 
man  between  two  tall  ones  looks  shorter  than  if  alongside  one 
only.     If,  however,  the  after-image  be  watched  it  will   often 


THE  EYE  AS  A   SENSORY  APPARATUS.  549 

be  noticed  not  only  that  the  light  band  between  the  squares 
is  intensely  white,  much  more  so  than  the  normal  idio-retinal 
light,  but,  as  the  image  fades  away,  often  the  two  dark  after- 
images of  the  squares  disappear  entirely  with  all  of  the 
corona,  except  that  part  between  them  which  is  still  seen  as  a 
bright  band  on  a  uniform  grayish  field.  Here  there  is  no 
contrast  to  produce  the  error  of  judgment,  and  from  this  and 
other  experiments  Hering  concludes  that  light  acting  on  one 
part  of  the  retina  produces  inverse  changes  in  all  the  rest, 
and  that  this  has  an  important  part  in  producing  the  phe- 
nomena of  contrasts.  Similar  phenomena  may  be  observed 
with  colored  objects;  in  their  negative  after-images  each  tint 
is  represented  by  its  complementary,  as  black  is  by  white  in 
colorless  vision. 

Endeavoring  to  exclude  such  loose  general  explanations  as 
" errors  of  judgment,"  Hering  proposes  a  theory  of  vision 
which  can  only  be  briefly  stated  here.  We  may  put  all- 
our  colorless  sensations  in  a  continuous  series,  passing  through 
grays  from  the  deepest  black  to  the  brightest  white;  some- 
where half-way  between  will  be  a  neutral  gray  which  is  as 
black  as  it  is  white.  We  may  do  something  similar  with  our 
color  sensations;  as  in  gray  we  see  black  and  white  so  in 
purple  we  see  red  and  blue,  and  all  colors  containing  red  and 
blue  may  be  put  in  a  series  of  which  one  end  is  pure  red,  the 
other  pure  blue.  So  with  red  and  yellow,  blue  and  green, 
yellow  and  green.  If  we  call  to  mind  the  whole  solar  spec- 
trum from  yellow  to  blue,  through  the  yellow-greens,  green, 
and  blue-greens,  we  get  a  series  in  which  all  but  the  ter- 
minals have  this  in  common  that  they  contain  some  green. 
Green  itself  forms,  however,  a  special  point;  it  differs  from 
all  tints  on  one  side  of  it  in  containing  no  yellow,  and  from 
all  on  the  other  in  containing  no  blue.  In  ordinary  language 
this  is  recognized:  we  give  it  a  definite  name  of  its  own  and 
call  it  green.  Its  simplicity  compared  with  the  doubleness 
of  its  immediate  neighbors  entitles  it  to  a  distinct  place  in 
the  color-sensation  series.  There  are  three  other  color  sensa- 
tions which  like  green  are  simple  and  must  have  specific 
names  of  their  own;  they  are  red,  blue,  and  yellow.  Green 
may  be  pure  green  or  yellow-greon  or  blue-green,  but  never 
yellow  and  bluish  at  once,  or  reddish.  Red  may  be  pure  or 
yellowish  or  bluish,  hut  never  greenish.  Red  and  green  are 
thus  mutually  exclusive;  yellow  and  blue  stand  in  a  similar 


.rM<»  THE  HUMAN  BODY. 

relationship.     All  other  color  sensations,  as  orange,  sug^ 
two  of  the  above,  and  may  be  described  as  mixtures  of  them; 

but  thuy  themselves  stand  out  as  fundamental  color  sensa- 
tions. Moreover,  it  follows  from  the  above,  that  more  than 
two  simple  color  sensations  are  never  combined  in  a  com- 
pound color  sensation. 

Since  red  always  excludes  green,  and  yellow  blue,  we  may 
call  them  anti-colors  (the  complementary  colors  of  Young's 
theory),  and  are  led  to  suspect  that  in  the  visual  organ  there 
must  occur,  in  the  production  of  each,  processes  which  pre- 
vent the  simultaneous  production  of  the  other,  since  there  is 
no  a  priori  reason  in  the  nature  of  things  why  we  should  not 
see  red  and  green  simultaneously,  as  well  as  red  and  yellow. 
Along  with  our  color  sensations  there  is  always  some  color- 
less from  the  black- white  series;  which  we  recognize  in  speak- 
ing of  lighter  and  darker  shades  of  the  same  color. 

Tiering  assumes,  then,  in  the  retina  or  some  part  of  the 
nervous  visual  apparatus,  three  substances  answering  to  the 
black-white,  red-green,  and  yellow-blue  sensational  series,  the 
construction  of  each  substance  being  attended  with  one  sen- 
sation of  its  pair,  and  its  destruction  with  the  other.  Thus, 
when  construction  of  the  black-white  substance  exceeds  de- 
struction, we  get  a  blackish-gray  sensation;  when  the  pro- 
cesses are  equal  the  neutral  gray;  when  destruction  exceeds 
construction  a  light-gray,  and  so  on.  In  the  other  color 
series  similar  things  would  occur;  when  construction  of  red- 
green  substance  exceeded  destruction  in  any  point  of  the 
retina  Ave  would  get,  say,  a  red  feeling;  if  so,  then  excess  of 
destruction  would  give  green  sensation.  The  intensity  of 
any  given  simple  sensation  would  depend  on  the  ratio  of  the 
difference  between  the  construction  and  destruction  of  the 
corresponding  substance,  to  the  sum  of  all  the  constructions 
and  destructions  of  visual  substances  going  on  in  that  part  of 
the  visual  apparatus  :  in  this  way  anabolic  and  katabolic 
nutritive  processes  would  be  the  material  basis  of  visual  sen- 
sations. The  intensity  of  a  mixed  color  sensation  would  be 
the  sum  of  the  intensities  of  its  factors,  and  its  tint  and 
shade  dependent  on  the  relative  proportion  of  these  factors. 
"When  the  construction  and  destruction  of  the  red-green  sub- 
stance are  equal  no  color  sensation  is  aroused  by  it;  and  we 
get  gray,  due  to  those  simultaneouslv  occurring  changes  in 
the  black- white  substance  which  are  always  present,  but  were 


THE  EYE  AS  A   SENSORY  APPARATUS.  551 

previously  more  or  less  cloaked  by  the  results  of  the  changes 
in  the  red-green  substance.  Eed  and  green  in  certain  pro- 
portions cause  then  a  white  or  gray  sensation,  not  because 
they  supplement  one  another,  as  on  Young's  theory,  but  be- 
cause they  mutually  cancel;  and  so  for  other  complementary 
colors. 

Moreover,  according  to  Hering,  destruction  of  a  visual  sub- 
stance going  on  in  one  region  of  the  retina  promotes  con- 
struction and  accumulation  of  that  substance  elsewhere,  but 
especially  in  the  neighborhood  of  the  excited  spot.  Hence, 
when  a  white  square  on  a  black  ground  is  looked  at,  destruc- 
tion of  the  black-white  substance  overbalances  construction 
in  the  place,  on  which  the  image  of  the  square  falls,  but 
around  this  construction  occurs  in  a  high  degree.  When  the 
eyes  are  shut,  this  latter  reti?ial  region,  with  its  great  accumu- 
lation of  decomposable  material,  is  highly  irritable  and, 
under  the  internal  stimuli  causing  the  idio-retinal  light, 
breaks  down  comparatively  fast,  causing  the  corona,  which 
may  be  intensely  luminous;  for  with  the  closed  eye  the  total 
constructive  and  destructive  processes  in  the  visual  apparatus 
are  small,  and  so  the  excess  of  destruction  in  the  coronal 
region  bears  a  large  ratio  to  the  sum  of  the  whole  processes. 
The  student  must  apply  this  theory  for  himself  to  the  other 
phenomena  of  contrasts  and  negative  images,  as  also  to  the 
gradual  disappearance  of  differences  between  light  and  dark 
objects  when  looked  at  for  a  time  with  steady  fixation;  the 
general  key  being  the  principle  that  anything  leading  to  the 
accumulation  of  a  visual  substance  increases  its  decomposi- 
tions under  given  stimulation,  and  vice  versa.  The  main 
value  of  Hering's  theory  is  that  it  attempts  to  account 
physiologically  for  phenomena  previously  indefinitely  ex- 
plained psychologically  by  such  terms  as  "errors  of  judg- 
ment," which  really  leave  the  whole  matter  where  it  was, 
since  if  (as  we  must  believe)  mind  is  a  function  of  brain,  the 
errors  of  judgment  have  still  to  be  accounted  for  on  physio-  ' 
logical  grounds,  as  due  to  conditions  of  the  nervous  system. 

The  three  visual  substances,  the  anabolisms  and  katabol- 
i.-ms  of  which  according  to  Hering  give  rise  to  color  sensa- 
tions, need  not  necessarily  be  in  the  retina  itself:  they  may 
be  in  the  central  nerve  portions  of  the  visual  apparatus, 
being  excited  through  different  nerve  fibres  excited  by  dif- 
ferent lights  falling  on  the  retina. 


552  THE  HUMAN  BODY. 

There  are  difficulties  in  the  way  of  the  full  acceptance  of 
either  the  Young  (often  called  the  Yonng-Helmholtz)  theory 
or  the  theory  of  Hering,  and  the  whole  doctrine  of  color 
vision  is  still  in  a  very  unsettled  state. 

Visual  Perceptions.  The  sensations  which  light  excites 
in  us  we  interpret  as  indications  of  the  existence,  form,  and 
position  of  external  objects.  The  conceptions  which  we 
arrive  at  in  this  way  are  known  as  visual  perceptions.  The 
full  treatment  of  perceptions  belongs  to  the  domain  of 
Psychology,  but  Physiology  is  concerned  with  the  conditions 
under  which  they  are  produced. 

The  Visual  Perception  of  Distance.  "With  one  eye  our 
perception  of  distance  is  very  imperfect,  as  illustrated  by  the 
common  trick  of  holding  a  ring  suspended  by  a  string  in 
front  of  a  person's  face,  and  telling  him  to  shut  one  eye  and 
pass  a  rod  from  one  side  through  the  ring.  If  a  pen-holder 
be  held  erect  before  one  eye.  while  the  other  is  closed,  and 
an  attempt  be  made  to  touch  it  with  a  finger  moved  across 
towards  it,  an  error  will  nearly  always  be  made.  (If  the 
finger  be  moved  straight  on  towards  the  pen  it  will  be 
touched  because  with  one  eye  we  can  estimate  direction  accu- 
rately and  have  only  to  go  on  moving  the  finger  in  the  proper 
direction  till  it  meets  the  object.)  In  such  cases  we  get  the 
only  clue  from  the  amount  of  effort  needed  to  "  accommo- 
date "  the  eye  to  see  the  object  distinctly.  When  we  use 
both  eyes  our  perception  of  distance  is  much  better;  when 
we  look  at  an  object  with  two  eyes  the  visual  axes  are  con- 
verged on  it,  and  the  nearer  the  object  the  greater  the  con- 
vergence. We  have  a  pretty  accurate  knowledge  of  the 
degree  of  muscular  effort  required  to  converge  the  eyes  on 
all  tolerably  near  points.  When  objects  are  farther  off,  their 
apparent  size,  and  the  modifications  of  their  retinal  images 
brought  about  by  aerial  perspective,  come  in  to  help.  The 
relative  distance  of  objects  is  easiest  determined  by  moving 
the  eyes;  all  stationary  objects  then  appear  displaced  in  the 
opposite  direction  (as  for  example  when  we  look  out  of 
the  window  of  a  railway  car)  and  those  nearest  most  rapidly; 
from  the  different  apparent  rates  of  movement  we  can  tell 
which  are  farther  and  nearer.  We  so  inseparably  and  uncon- 
sciously bind  up  perceptions  of  distance  with  the  sensations 
aroused  by  objects  looked  at,  that  we  seem  to  see  distance; 
it   seems  at   first   thought  as   definite  a  sensation  as  color. 


THE  EYE  AS  A   SENSORY  APPARATUS.  553 

That  it  is  not  is  shown  by  cases  of  persons  born  blind,  who 
have  had  sight  restored  later  in  life  by  surgical  operations. 
Such  persons  have  at  first  no  visual  perceptions  of  distance: 
all  objects  seem  spread  out  on  a  flat  surface  in  contact  with 
the  eyes,  and  they  only  learn  gradually  to  interpret  their 
sensations  so  as  to  form  judgments  about  distances,  as  the 
rest  of  us  did  unconsciously  in  childhood  before  we  thought 
about  such  things. 

The  Visual  Perception  of  Size.  The  dimensions  of  the 
retinal  image  determine  primarily  the  sensations  on  which 
conclusions  as  to  size  are  based;  and  the  larger  the  visual 
angle  the  larger  the  retinal  image:  since  the  visual  angle  de- 
pends on  the  distance  of  an  object  the  correct  perception  of 
size  depends  largely  upon  a  correct  perception  of  distance; 
having  formed  a  judgment,  conscious  or  unconscious,  as  to 
that,  we  conclude  as  to  size  from  the  extent  of  the  retinal 
region  affected.  Most  people  have  been  surprised  now  and 
then  to  find  that  what  appeared  a  large  bird  in  the  clouds 
was  only  a  small  insect  close  to  the  eye;  the  large  apparent 
size  being  due  to  the  previous  incorrect  judgment  as  to  the 
distance  of  the  object.  The  presence  of  an  object  of  toler- 
ably well-known  height,  as  a  man,  also  assists  in  forming 
conceptions  (by  comparison)  as  to  size;  artists  for  this  pur- 
pose frequently  introduce  human  figures  to  assist  in  giving 
an  idea  of  the  size  of  other  objects  represented. 

The  Visual  Perception  of  a  Third  Dimension  of  Space. 
This  is  very  imperfect  with  one  eye;  still  we  can  thus  arrive 
at  conclusions  from  the  distribution  of  light  and  shade  on  an 
object,  and  from  that  amount  of  knowledge  as  to  the  relative 
distance  of  different  points  which  is  attainable  monocularly; 
the  different  visual  angles  under  which  objects  are  seen  also 
assist  us  in  concluding  that  objects  are  farther  and  nearer, 
and  so  are  not  spread  out  on  a  plane  before  the  eye,  but 
occupy  depth  also.  Painters  depend  mainly  on  devices  of 
these  kinds  for  representing  solid  bodies,  and  objects  spread 
over  the  visual  field  in  the  third  dimension  of  space. 

Single  Vision  with  Two  Eyes.  When  we  look  at  a  flat 
object  with  both  eyes  we  get  a  similar  retinal  image  in  each. 
Under  ordinary  circumstances  we  see,  however,  not  two  ob- 
jects hut  one.  In  the  habitual  use  of  the  eyes  we  move  them 
so  that  the  images  of  the  object  looked  at  fall  on  the  two 
yellow  ypots.     A  point  to  the  left  of  this  object  forms  its 


554  TEE  111  MA. X   BODY. 

image  on  the  inner  (right)  side  of  the  left  eye  and  tli<'  outer 
(right)  side  of  the  right.  An  object  vertically  above  that 
looked  at  would  form  an  image  straighi  below  tin'  yellow 
spot  of  each  eye;  an  object  to  the  left  and  above, its  image 

to  the  inner  side  and  below  in  the  left  eye  and  to  the 
outer  side  and  below  in  the  right  eye:  and  so  on.  We 
have  learned  that  similar  simultaneous  excitations  of  these 
corresponding  points  mean  single  objects,  and  so  interpret 
our  sensations.  This  at  least  is  the  theory  of  the  experi- 
ential or  empirical  school  of  psychologists,  though  others  be- 
lieve we  have  a  sort  of  intuition  on  the  subject.  When  the 
eyes  do  not  work  together,  as  in  the  muscular  incoordination 
of  one  stage  of  intoxication,  then  they  are  not  turned  so  that 
images  of  the  same  objects  fall  on  corresponding  retinal 
points,  and  the  person  sees  double.  When  a  squint  comes 
on,  as  from  paralysis  of  the  external  rectus  of  one  eye,  the 
sufferer  at  first  sees  double  for  the  same  reason,  but  after  a 
time  he  makes  new  associations  of  corresponding  retinal 
points,  and  this  is  in  favor  of  the  empirical  theory. 

When  a  given  object  is  looked  at,  lines  drawn  from  it 
through  the  nodal  points  reach  the  fovea  centralis  in  each 
eye.  Lines  so  drawn  at  the  same  time  from  a  more  distant 
object  diverge  less  and  meet  each  retina  on  the  inner  side  of 
its  fovea;  but  as  above  pointed  out  the  corresponding  points 
for  each  retinal  region  on  the  inside  of  the  left  eye,  are  on 
the  outside  of  the  right,  and  vice  versa.  Hence  the  more 
distant  object  is  seen  double.  So,  also,  is  a  nearer  object,  be- 
cause the  more  diverging  lines  drawn  from  it  through  the 
nodal  points  lie  outside  of  the  fovea  in  each  eye.  Most 
people  go  through  life  unobservant  of  this  fact;  we  only  pay 
attention  to  what  we  are  looking  at,  and  nearly  always  this 
makes  its  images  on  the  two  fovese.  That  the  fact  is  as 
above  stated  may,  however,  be  readily  observed.  Hold  one 
finger  a  short  way  from  the  face  and  the  other  a  little  farther 
off;  looking  at  one,  observe  the  other  without  moving  the 
eyes;  it  will  be  seen  double.  For  every  given  position  of  the 
eyes  there  is  a  surface  in  space,  all  objects  on  which  produce 
images  on  corresponding  points  of  the  two  retinas:  this  sur- 
face is  called  the  horopter  for  that  position  of  the  eyes:  all 
objects  in  it  are  seen  single;  all  others  in  the  visual  field, 
double. 

The  Perception  of  Solidity.     When    a    solid    object   is 


THE  EYE  AS  A   SENSORY  APPARATUS. 


555 


lookea  at  the  two  retinal  images  are  different.  If  a  truncated 
pyramid  be  held  in  front  of  one  eye  its  image  will  be  that 
represented  at  P,  Fig.  161.     If,  however,  it  be  held  midway 


b 
a 

/, 

\ 

d 

/ 

\ 

d 

>/ 

p 

/ 

/ 

c 

c 

a 

Fig.  161. 

between  the  eyes,  and  looked  at  with  both,  then  the  left-eye 
image  will  be  that  in  the  middle  of  the  figure,  and  the  right- 
eye  image  that  to  the  right.  The  small  surface,  b  d  c  a,  in 
one  answers  to  the  large  surface,  V  d'  c'  a',  in  the  other. 
This  may  be  readily  observed  by  holding  a  small  cube  in 
front  of  the  nose  and  alternately  looking  at  it  with  each  eye. 
In  such  cases,  then,  the  retinal  images  do  not  correspond, 
and  yet  we  combine  them  in  consciousness  so  as  to  see  one 
solid  object.  This  is  known  as  stereoscopic  vision,  and  the 
illusion  of  the  common  stereoscope  depends  on  it.  Two 
photographs  are  taken  of  the  same  object  from  two  different 
points  of  view,  one  as  it  appears  when  seen  from  the  left,  and 
the  other  when  seen  from  the  right.  These  are  then  mounted 
for  the  stereoscope  so  that  each  is  looked  at  by  its  proper  eye, 
and  the  object  appears  in  distinct  relief,  as  if,  instead  of  flat 
pictures,  solid  objects,  occupying  three  dimensions  of  space, 
Avere  looked  at.  Of  course  in  many  stereoscopic  views  the  dis- 
tribution of  light  and  shade,  etc.,  assist,  but  these  are  quite 
unessential,  as  may  be  readily  observed  by  copying  the  draw- 
ings of  Fig.  161  and  mounting  them  on  a  card  the  size  of  a 
stereoscopic  slide,  and  placing  it  in  the  instrument.  A  solid 
pyramid  standing  out  into  space  will  be  distinctly  perceived; 
if  t  ho  pictures  be  reversed  the  pyramid  appears  hollow.  The 
pictures  must  not  be  too  different,  or  their  combination  to  give 
tin-  idea  of  a  Binffle  solid  body  will  not  take  place.  Many 
persons,  indeed,  fail  entirely  to  get  the  illusion  with  ordinary 
stereoscopic  slides.  The  phenomena  of  stereoscopic  vision 
militate  strongly  against  the  view  that  there  are  any  anatom- 
ically prearranged  corresponding  points  in  the  two  retinas. 
The  Perception  of  Shine.     When  we  look   at  a  rippled 


556  THE  HUMAN  BODY. 

lake  in  the  moonlight,  we  get  the  perception  of  a  "shiny"  or 
brilliant  surface.  The  moonlight  is  reflected  from  the  waves 
to  the  eyes  in  a  number  of  bright  points:  these  are  not  ex- 
actly the  same  for  both  eyes,  since  the  lines  of  light-reflection 
from  the  surface  of  the  water  to  each  are  different.  The 
perception  of  brilliancy  seems  largely  to  depend  on  this 
slight  non-agreement  of  the  light  and  dark  points  on  the  two 
retinas.  A  rapid  change  of  luminous  points,  to  and  fro  be- 
tween neighboring  points  on  one  retina,  seems  also  to  pro- 
duce it. 


CHAPTER  XXXIV. 

THE  EAR   AND   HEARING. 

The  External  Ear.  The  auditory  organ  in  man  consists 
of  three  portions,  known  respectively  as  the  external  ear,  the 
middle  ear  or  tympanum,  and  the  internal  ear  or  labyrinth; 
the  latter  contains  the  end  organs  of  the  auditory  nerve. 
The  external  ear  consists  of  the  expansion  seen  on  the  ex- 
terior of  the  head,  called  the  concha,  M,  Fig.  162,  and  a  pas- 
sage leading  in  from  it,  the  external  auditory  meatus,  G. 


Fio.  103.— Semidiagrammatic  section  through  the  right  ear  (Czermak).  M, 
concha;  G,  externa!  auditory  meatus;  '/',  tympanic  membrane;  /',  tympanic 
cavity  ;  o,  oval  foramen  ;  r,  round  foramen  ;  B,  pharyngeal  opening  or  Eusta- 
chian tube  ;  i'.  vestibule  ;  /<'.  a  semicircular  canal  ;  8,  the  cochlea  ;  Vt,  scala  ves- 
tibllli  ;  J't,  scala  tyinpaui  ;  A,  auditory  nerve. 

This  passage  is  closed  at  its  inner  end  hy  the  tympanic  or 
drum  membrane,  T.  It  is  lined  by  skin,  through  which 
numerous  small  glands,  Becreting  the  wax  of  the  ear,  open. 

The  Tympanum  (/',  Fig.    162)  is   an    irregular  cavity  in 
the  temporal  bone,  closed  externally  by  the  drum  membrane. 

557 


558  THE  HUMAN  BODY. 

From  its  inner  side  the  Eustachian  tube  (R)  proceeds  to  the 
pharynx,  and  the  mucous  membrane  of  that  cavity  is  con- 
tinued up  the  tube  to  line  the  tympanum;  the  proper  tym- 
panic membrane  composed  of  connective  tissue  is  therefore 
covered  by  mucous  membrane  on  its  inner,  as  it  is  by  very 
thin  skin  on  its  outer,  side.  In  the  bony  inner  wall  of  the 
tympanum  are  two  small  apertures,  the  oval  and  round  fora- 
mens, o  and  r,  which  lead  into  the  labyrinth.  During  life  the 
round  aperture  is  closed  by  the  lining  mucous  membrane,  and 
the  oval  in  another  way,  to  be  described  presently.  The  tym- 
panic membrane,  T,  stretched  across  the  outer  side  of  the 
tympanum,  forms  a  shallow  funnel  with  its  concavity  out- 
wards. It  is  pressed  by  the  external  air  on  its  exterior,  and 
by  air  entering  the  tympanic  cavity  through  the  Eustachian 
tube  on  its  inner  side.  If  the  tympanum  were  closed  the 
pressures  on  the  inner  and  outer  sides  of  the  drum  membrane 
would  not  be  always  equal  when  barometric  pressure  varied, 
and  the  membrane  would  be  bulged  in  or  out  according  as 
the  external  or  internal  pressure  on  it  were  the  greater.  On 
the  other  hand,  were  the  Eustachian  tube  always  open  the 
sounds  of  our  own  voices  would  be  loud  and  disconcerting,  so 
it  is  usually  closed;  but  every  time  we  swallow  it  is  opened, 
and  thus  the  air-pressure  in  the  cavity  is  kept  equal  to  that 
in  the  external  auditory  meatus.  By  holding  the  nose,  keep- 
ing the  mouth  shut,  and  forcibly  expiring,  air  may  be  forced 
under  pressure  into  the  tympanum,  and  will  be  held  in  part 
imprisoned  there  until  the  next  act  of  swallowing.  On 
making  a  balloon  ascent  or  going  rapidly  down  a  deep  mine, 
the  sudden  and  great  change  of  aerial  pressure  outside  fre- 
quently causes  painful  tension  of  the  drum  membrane,  which 
may  be  greatly  alleviated  by  frequent  swallowing  movements. 
The  Auditory  Ossicles.  Three  small  bones  lie  in  the 
tympanum  forming  a  chain  from  the  drum  membrane  to  the 
oval  foramen.  The  external  bone  (Fig.  163)  is  the  malleus 
or  hammer;  the  middle  one,  the  incus  or  anvil;  and  the 
internal,  the  stapes  or  stirrup.  The  malleus,  M,  has  an 
upper  enlargement  or  head,  which  carries  on  its  inner 
side  an  articular  surface  for  the  incus;  below  the  head  is 
a  constriction,  the  neck,  and  below  this  two  processes  com- 
plete the  bone;  one,  the  long  or  slender  process,  is  im- 
bedded in  a  ligament  which  reaches  from  it  to  the  front 
wall-  of    the   tympanum;    the    other    process,    the     handle, 


THE  EAR  AND  HEARING.  559 

reaches    down    between    the   mucous   membrane  lining   the 

inside  of  the  drum  membrane 

and    the    membrane    proper, 

and  is  firmly  attached  to  the 

latter  near  its  centre  and  keeps 

the  membrane  dragged  in  there 

so  as  to  give   it   its   peculiar 

concave   form,   as  seen    from 

the  outside.     The  incus  has  a 

body  and  two  processes,  and  is 

much  like  a  molar  tooth  with 

two  fangs.     On  its  body  is  an 

articular  hollow  to  receive  the  Mm 

head  of  the  malleus;  its  short     FlG  i63.-The  auditory  ossicles  of  the 

process  (Jb)  is  attached  byliga-  ^T^^^tsT^heToi 

ment  to  the  back  wall  of  the  *Z£2^&fi*£g%;Jft£t, 

tympanum:  the  long-  process  and  •*}-  long:  process  of  incus;  Jpi,  os 

J      1 .        .  .  °    x  orbwulare ,"  Sep,  head  ot  stapes. 

(Jl)  is  directed  inwards  to  the 

stapes;  on  the  tip  of  this  process  is  a  little  knob,  which  rep- 
resents a  bone  (os  orbiculare)  distinct  in  early  life.  The 
stapes  (S)  is  extremely  like  a  stirrup,  and  its  base  (the  foot- 
piece  of  the  stirrup)  fits  into  the  oval  foramen,  to  the  margin 
of  which  its  edge  is  united  by  a  fibrous  membrane,  allowing 
of  a  little  play  in  and  out. 

From  the  posterior  side  of  the  neck  of  the  malleus  a  liga- 
ment passes  to  the  back  wall  of  the  tympanum :  this,  with 
the  ligament  imbedding  the  slender  process  and  fixed  to  the 
front  wall  of  the  tympanum,  forms  an  antero-posterior  axial 
ligament,  on  which  the  malleus  can  slightly  rotate,  so  that  the 
handle  can  be  pushed  in  and  the  head  out  and  vice  versa. 
If  a  pin  be  driven  through  Fig.  163  just  below  the  neck  of 
the  malleus  and  perpendicular  to  the  paper  it  will  very  fairly 
represent  this  axis  of  rotation.  Connected  with  the  malleus 
is  ;i  tiny  muscle,  called  the  tensor  tympani;  it  is  inserted  on 
the  handle  of  the  bone  below  the  axis  of  rotation,  and  when 
it  contracts  pulls  the  handle  in  and  tightens  the  drum  mem- 
brane. Another  muscle  (the  stapedius)  is  inserted  into  the 
outer  end  of  tin?  stapes,  and  when  it  contracts  fixes  the  bone 
so  as  to  limit  its  range  of  movement  in  and  out  of  the  fenestra 
oval  is. 

Tho  Internal  Ear.  The  labyrinth  consists  primarily  of 
chambers  and  tubes  hollowed  out  in  the  temporal  bone  and 


560 


THE  HUMAN  BODY. 


inclosed  by  it  on  nil  sides,  excepl  for  the  oval  and  round 
foramens  on  its  exterior,  and  certain  apertures  on  its  inner 
side  by  which  blood-vessels  and  branches  of  the  auditory 
nerve  enter;  during  life  all  these  are  closed  water-tight  in  one 
way  or  another.     Lying  in   the  bony  labyrinth  thus  con.-ti- 


Fio.  164.—  Casts  of  the  bony  labyrinth.  A,  left  labyrinth  seen  from  the  outer 
side;  B,  right  labyrinth  from  the  inner  side  ;  C.  left  labyrinth  from  above;  /•'<•. 
round  foramen  ;  Fv.  oval  foramen  :  A,  horizontal  semicircular  canal  ;  ha,  its 
ampulla  ;  vaa,  ampulla  of  anterior  vertical  semicircular  canal;  rpa.  ampulla  of 
posterior  vertical  semicircular  canal  ;  re,  conjoined  portion  of  the  two  vertical 
canals. 

tuted,  are  membranous  parts,  of  the  same  general  form  but 
smaller,  so  that  between  the  two  a  space  is  left;  this  is  filled 
with  a  watery  fluid,  called  the  perilymph;  and  the  mem- 
branous internal  car  is  filled  by  a  similar  liquid,  the  endo- 
lymph. 

The  Bony  Labyrinth.  The  bony  labyrinth  is  described 
in  three  portions,  the  rest  Untie,  the  semicircular  canals,  and 
the  cochlea  ;  easts  of  its  interior  are  represented  from  differ- 
ent aspects  in  Fig.  164.  The  vestibule  is  the  central  part 
and  has  on  its  exterior  the  oval  foramen  (Fv)  into  which  the 
base  of  the  stirrup-bone  fits.  Behind  the  vestibule  are  three 
bony  semicircular  canals,  communicating  with  the  back  of 
the  vestibule  at  each  end,  and  dilated  near  one  end  to  form 
an  ampulla  {vpa,  ran.  and  ha).  The  horizontal  canal  lies  in 
the  plane  which  its  name  implies,  and  has  its  ampulla  at  the 
front  end.  The  two  other  canals  lie  vertically,  the  anterior 
at  right  angles,  and  the  posterior  parallel,  to  the  median 
antero-posterior  vertical  plane  of  the  head.  Their  ampulhiry 
ends  are  turned  forwards  and  open  close  together  into  the 
vestibule;  their  posterior  ends  unite  (vc)  and  have  a  common 
vestibular  opening. 

The  bony  cochlea  is  a  tube  coiled  on  itself  somewhat  like 
a  snail's  shell,  and  lying  in  front  of  the  vestibule. 


THE  EAR  AND  HEARING. 


561 


^^^3> 


Fig  165. — A  section  through  the  cochlea 
ill  the  lisie  of  its  axis. 


The  Membranous  Labyrinth.  The  membranous  vesti- 
bule, lying  in  the  bony,  consists  of  two  sacs  communicating 
by  a  narrow  aperture.  The  posterior  is  called  the  utricidus, 
and  into  it  the  membran- 
ous semicircular  canals 
open.  The  anterior,  called 
the  sacculus,  communi- 
cates by  a  tube  with  the 
membranous  cochlea.  The 
membranous  semicircular 
canals  much  resemble  the 
bony,  and  each  has  an 
ampulla;  in  most  of  their 
extent  they  are  only  united 
by  a  few  irregular  connec- 
tive-tissue bands  with  the 
periosteum  lining  the  bony  canals;  but  in  the  ampulla  one 
side  of  the  membranous  tube  is  closely  adherent  to  its  bony 
protector;  at  this  point  nerves  enter  the  former.  The  rela- 
tions of  the  membranous  to  the  bony  cochlea  are  more  com- 
plicated. A  section  through  this  part  of  the  auditory  appa- 
ratus (Fig.  165)  shows  that  its  osseous  portion  consists  of  a 
tube  wound  two  and  a  half  times  (from  left  to  right  in  the 
right  ear  and  vice  versa)  around  a  central  bony  axis,  the 
modiolus.  From  the  axis  a  shelf,  the  lamina  spiralis,  pro- 
jects and  partially  subdivides  the  tube,  extending  farthest 
across  in  its  lower  coils.  Attached  to  the  outer  edge  of  this 
bony  plate  is  the  membranous  cochlea  (scala  media),  a  tube 
triangular  in  cross-section  and  attached  by  its  base  to  the 
outer  side  of  the  bony  cochlear  spiral.  The  spiral  lamina 
and  the  membranous  cochlea  thus  subdivide  the  cavity  of  the 
bony  tube  (Fig.  16G)  into  an  upper  portion,  the  scala  vesti- 
buli,  8Vt  and  a  lower,  the  scala  tympani,  ST.  Between  these 
lie  the  lamina  spiralis  (ho)  and  the  membranous  cochlea  (CO), 
the  latter  being  bounded  above  by  the  membrane  of  Reissner 
(//)  and  below  by  the  basilar  membrane  (b).  The  free  edge 
of  the  lamina  spiralis  is  thickened  and  covered  with  con- 
nective  tissue  which  is  hollowed  out  so  as  to  form  a  spiral 
groove  (the  sulcus  spiralis,  ss)  along  the  whole  length  of  the 
membranous  cochlea.  The  latter  does  not  extend  to  the  tip 
of  the  bony  cochlea;  above  its  npex  the  scala  vestibuli  and 
scala  tympani  join;    both  are  lilled  with  perilymph,  and  the 


M\2 


THE  HUMAN  BODY. 


former  communicates  below  with  the  perilymph  cavity  of  the 
vestibule,  while  the  scala  tympani  abuts  below  on  the  round 
foramen,  which,  as  has  already  been  pointed  out,  is  closed  by 
a   membrane.       The    membranous    cochlea   contains  certain 


Fig.  166.— Section  of  one  coil  of  the  cochlea,  magnified,  SV,  scala  vestibnli ; 
fi,  membrane  of  Reissner;  CC,  membranous  cochlea  {.scala  media)'.  Us.  limbus 
lamince  spiralis;  t.  tectorial  membrane;  .ST,  scala  tympani;  also,  spiral  lamiua  ; 
Co,  rods  of  Corti  ;  b,  basilar  membrane. 

solid  structures  seated  on  the  basilar  membrane  and  forming 
the  organ  of  Corti  ;  the  rest  of  its  cavity  is  filled  with  endo- 
lymph,  which  has  free  passage  to  that  in  the  sacculus. 

The  Organ  of  Corti.  This  contains  the  end  organs  of 
the  cochlear  nerves.  Lining  the  sulcus  spiralis  are  cuboid al 
cells;  on  the  inner  margin  of  the  basilar  membrane  the  cells 
become  columnar,  and  then  are  succeeded  by  a  row  which  bear 
on  their  upper  ends  a  set  of  short  stiff  hairs,  and  constitute 
A  B 


Fio.  167. — The  rods  of  Oorti.  A,  a  pair  of  rods  separated  from  the  rest  ;  B,  a 
bit  ut  tin-  basnar  membrane  with  several  rods  on  it,  snowing  how  they  cover  in 
the  tunnel  of  Corti  ;  i.  inner,  and  e,  outer  rods  ;  6,  basilar  membrane  ;  r.  reticular 
membrane. 

the  inner  hair-cells,  which  are  fixed  below  by  a  narrow  apex 
to  the  basilar  membrane  ;  nerve-fibres  enter  them.  To  the 
inner  hair-cells  succeed  the  rods  of  Corti  {Co,  Fig.  106), 
Avhich  are  represented  much  maun  died  in  Fig.  167.  These 
rods  are  stiff  and  arranged  side  by  side  in  two  rows,  leaned 


THE  EAR  AXD  HEARING. 


563 


against  one  another  by  their  upper  ends  so  as  to  cover  in  a 
tunnel;  they  are  known  respectively  as  the  inner  and  outer 
rods,  the  former  being  nearer  the  lamina  spiralis.  Each 
has  a  somewhat  dilated  base,  firmly  fixed  to  the  basilar  mem- 
brane; an  expanded  head  where  it  meets  its  fellow  (the  inner 
rod  presenting  there  a  concavity  into  which  the  rounded 
head  of  the  outer  fits);  and  a  slender  shaft  uniting  the  two, 
slightly  curved  like  an  italic  f.  The  inner  rods  are  more 
slender  and  more  numerous  than  the  outer,  the  numbers 
being  about  60U0  and  4500  respectively.  Attached  to  the 
external  sides  of  the  head  of  the  outer  rods  is  the  reticular 
membrane  (r,  Fig.  167),  which  is  stiff  and  perforated  by 
holes.  External  to  the  outer  rods  come  four  rows  of  outer 
hair-cells,  connected  like  the  inner  row  with  nerve-fibres; 
their  bristles  project  into  the  holes  of  the  reticular  mem- 
brane. Beyond  the  outer  hair-cells  is  ordinary  columnar 
epithelium,  which  passes  gradually  into  cuboidal  cells  lining 
most  of  the  membranous  cochlea.  The  ujoper  lip  of  the 
sulcus  spiralis  is  uncovered  by  epithelium,  and  is  known  as 
the  limbus  lamina  spiralis;  from  it  projects  the  tectorial 
membrane  (t,  Fig.  16G)  which  extends  over  the  rods  of  Corti 
and  the  hair-cells. 

Nerve-Endings   in   the    Semicircular    Canals    and.  the 
Vestibule.     Medullated  fibres  ( /,  Fig.  168)  from  the  vestib- 
ular branch  of  the  auditory  nerve  are  distributed  along  a  line 
across   the    ampulla   of   each 
semicircular  canal.    They  lose 
their  medullary  sheath    close 
to  the   basement   membrane, 
a,  which    the   axis   cylinders 
pierce.      The   axis    cylinders 
branch  among  the  epithelium 
cells,  which  at  this  place  are 
several  rows  thick,  but   have 
not  yet  been  traced  into  direct  b- 
continuity  with  any  of  them. 
The  cells  of   the   epithelium 
are    <>f    two   varieties.      The 
columnar  cells  or  hair  cells,  ct 

do    not     reach     the    basement  nervous  region  of  ampulla  of  a  semicir- 
cular canal. 

membrane,   are   nucleated  or 

slightly  granular:  from  the  free  end  of  each  projects  a  rigid 


564  77/ a;  HUMAN  body. 

hair  process,  d.  The  remaining  cells,  rod  cells,  l>,  are  in 
several  rows:  each  has  a  slender  inner  process  extending  to 
the  basement  membrane  and  an  outer  which  reaches  to  the 
bases  of  the  columnar  cells  and  appears  there  to  end  in  a 
rigid  membrane,  e,  winch  is  perforated  for  the  passage  of  the 
bail's.  They  probably  are  mere  supporting  structures  an- 
swering somewhat  to  the  fibres  of  Midler  of  the  retina. 
After  death  the  hairs  lend  to  break  up  into  a  bunch  of  fila- 
ments, and  they  are  found  imbedded  in  a  sticky  mucus-like 
material,  which  is  probably  a  post-mortem  product:  it  has 
been  named  the  cupula  terminalis.  In  some  parts  of  the 
utricle  and  saccule  is  a  region  of  epithelium  very  similar  to 
that  above  described,  and  also  supplied  with  nerve-iibres.  In 
connection  with  them  are  found  minute  calcareous  particles, 
■ — otoliths  or  ear-stones. 

The  Loudness,  Pitch,  and  Timbre  of  Sounds.  Sounds, 
as  sensations,  fall  into  two  groups — notes  and  noises.  Physi- 
cally, sounds  consist  of  vibrations,  and  these,  under  most 
circumstances,  when  they  first  reach  our  auditory  organs,  are 
alternating  rarefactions  and  condensations  of  the  air.  <>r 
aerial  waves.  When  the  waves  follow  one  another  uni- 
formly, or  periodically,  the  resulting  sensation  (if  any)  is  a 
note;  when  the  vibrations  are  aperiodic  it  is  a  noise.  In 
notes  we  recognize  (1)  loudness  or  intensity;  (2)  pitch;  (3) 
quality  or  timbre,  or,  as  it  has  been  called,  tone  color;  a  note 
of  a  given  loudness  and  pitch  produced  by  a  flute  and  by  a 
violin  has  a  different  character  or  individuality  in  each  case; 
this  quality  is  its  timbre.  Before  understanding  the  work- 
ing of  the  auditory  mechanism  we  must  get  some  idea  of  the 
physical  qualities  in  objective  sound  of  which  the  subjective 
differences  of  auditory  sensations  are  signs. 

The  loudness  of  a  sound  depends  on  the  force  of  the  aerial 
waves;  the  greater  the  intensity  of  the  alternating  condensa- 
tions and  rarefactions  of  these  in  the  external  auditory 
meatus,  the  louder  the  sound.  The  pitch  of  a  note  depends 
on  the  length  of  the  waves,  that  is  the  distance  from  one 
point  of  greatest  condensation  to  the  next,  or  (what  amounts 
to  the  same  thing)  on  the  number  of  waves  reaching  the  ear 
in  given  time,  say  a  second.  The  shorter  the  waves  the 
more  rapidly  they  follow  one  another,  and  the  higher  the 
pitch  of  the  note.  When  audible  vibrations  bear  the  ratio 
1:2  to  one  another,  we   hear  the  musical  interval  called  an 


THE  EAR  AND  HEARING.  565 

octave.  The  note  c  on  the  unaccented  octave  is  due  to  132 
vibrations  in  a  second.  The  note  c',  the  next  higher  octave 
of  this,  is  produced  by  2G4  vibrations  in  a  second;  the  next 
lower  octave  (great  octave,  C),  by  6G;  and  so  on.  Sound 
vibrations  may  be  too  rapid  or  too  slow  in  succession  to  pro- 
duce sonorous  sensations,  just  as  the  ultra-violet  and  ultra- 
red  rays  of  the  solar  spectrum  fail  to  excite  the  retina.  The 
highest-pitched  audible  note  answers  to  about  38,016  vibra- 
tions in  a  second,  but  it  differs  in  individuals;  many  persons 
cannot  hear  the  cry  of  a  bat  nor  the  chirp  of  a  cricket,  which 
lie  near  this  upper  audible  limit.  On  the  other  hand,  sounds 
of  vibrational  rate  about  40  per  second  are  not  well  heard, 
and  a  little  below  this  become  inaudible.  The  highest  note 
used  in  orchestras  is  the  dv  of  the  fifth  accented  octave,  pro- 
duced by  the  piccolo  flute,  due  to  4752  vibrations  in  a  second; 
and  the  lowest-pitched  is  the  E1}  of  the  contra  octave,  pro- 
duced by  the  double  bass.  Modern  grand  pianos  and  organs 
go  down  to  C,  in  the  contra  octave  (33  vibrations  per  second) 
or  even  A",  (27A),  but  the  musical  quality  of  such  notes  is 
imperfect;  they  produce  rather  a  "  hum  "  than  a  true  tone 
sensation,  and  are  only  used  along  with  notes  of  higher 
octaves  to  which  they  give  a  character  of  greater  depth. 

Pendular  Vibrations.  Since  the  loudness  of  a  tone  de- 
pends on  the  vibrational  amplitude  of  its  physical  antece- 
dent, and  its  pitch  on  the  vibrational  rate,  we  have  still  to 
seek  the  cause  of  timbre;  the  quality  by  which  we  recognize 
the  human  voice,  the  violin,  the  piano,  and  the  flute,  even 
when  all  sound  the  same  note  and  of  the  same  loudness. 
The  only  quality  of  periodic  vibrations  left  to  account  for 
this,  is  what  we  may  call  ion  re-form.  Think  of  the  movement 
of  a  pendulum;  starting  slowly  from  its  highest  point,  it 
sweeps  .'aster  and  faster  to  its  lowest,  and  then  slower  and 
slower  to  its  highest  point  on  the  opposite  side;  and  then 
repeats  the  movements  in  the  reverse  direction.  Graphically 
we  may  represent  such  vibrations  by  the  outer  continuous 
curved  line  in  Fig.  1G9.  Suppose  the  lower  end  of  the  pen- 
dulum to  bear  a  writing  point  which  marked  on  a  sheet  of 
paper  travelling  down  uniformly  behind  it,  and  at  such  a  rate 
as  to  travel  the  distance  0-1  in  two  seconds.  If  the  pendu- 
lum were  at  rest  the  straight,  vertical  line  would  be  drawn. 
But  if  the  pendulum  were  swinging  we  would  get  a  curved 
line,  compounded  of  the  vertical  movement  of  the  paper  and 


566 


THE  HUMAN  BODY. 


the  to-and-fro  movement  of  the  pendulum,  writing  sometimes 
on  one  side  of  the  line  0-1-2  and  sometimes  on  the  other. 
Starting  at  a  moment  when  the  pendulum  crosses  the  middle, 
u,  we  would  get  described  the  curve  0,  ata2at)  at  first  sepa- 
rating fust  from  the  vertical  line,  then  slower,  then  return- 
ing, at  first  gradually,  then  faster,  until  it  crossed  the  vertical 
again  at  the  end  of  a  second,  and  commenced  a  similar  ex- 
cursion on  its  other  side,  at  the  end  of  which  it  would  he 
back  at  1,  and  in  just  the  same  position,  and  ready  to  repeat 
exactly  the  swing,  with  which  we  commenced.  A  pendulum 
thus  executes  similar  movements  in  equal 
periods  of  time,  or  its  vibrations  are  periodic. 
A  full  swing  on  each  side  of  the  position  of 
rest  constitutes  a  complete  vibration,  so  the 
vibrational  period  of  a  seconds  pendulum  is 
two  seconds:  at  the  end  of  that  time  it  is 
precisely  where  it  was  two  seconds  before, 
and  moving  in  the  same  direction  and  at  the 
same  rate.  It  is  clear  that  by  examining 
such  a  curve  we  could  tell  exactly  how  the 
pendulum  moved,  and  also  in  what  period,  if 
we  knew  the  rate  at  which  the  paper  on 
which  its  point  wrote  was  moving.  The 
vertical  line  0-1-2  is  called  the  abscissa; 
perpendiculars  drawn  from  it  and  meeting 
the  curve  are  ordinates  :  equal  lengths  on 
the  abscissa  represent  equal  times;  where  an 
ordinate  from  a  given  point  of  the  abscissa 
meets  the  curve,  there  the  writing  point  was 
at  that  moment;  where  successive  ordinates 
increase  or  decrease  rapidly  the  pendulum 
moved  fast  from  or  towards  its  position  of 
rest,  and  vice  versa.  Similarly,  any  other 
periodic  movement  may  be  perfectly  repre- 
sented by  curves;  and  since  the  form  of  the 
Fig.  169.  curve  tells  us  all  about  the  movement,  it  is 

common  to  speak  of  the  "  form  of  a  vibration,"  meaning  the 
form  of  the  curve  which  indicates  its  characters.  Periodic 
vibrations  (Fig.  169),  whose  ordinates  at  first  grow  fast,  then 
more  slowly,  next  diminish  slowly  and  then  faster,  and 
represented  by  a  symmetrical  curve  on  one  side  the  abscissa, 


THE  EAR  AND  HEARING.  567 

which  is  repeated  exactly  on  the  other  side  of  the  abscissa, 
are  known  as  pendular  vibrations. 

The  Composition  of  Vibrations.  The  vibrations  of  a 
seconds  pendulum  set  the  air-particles  in  contact  with  it  in 
similar  movement,  but  the  aerial  waves  succeed  one  another 
too  slowly  to  produce  in  us  the  sensation  of  a  musical  note. 
If,  for  the  pendulum,  we  substitute  a  tuning-fork  (the  prongs 
of  which  move  in  a  like  way),  and  the  fork  vibrates  132  times 
per  1",  then  Vol  aerial  waves  will  fall  on  the  tympanic  mem- 
brane in  that  time,  and  we  will  hear  the  note  c  of  the  unac- 
cented octave.  If  the  larger  continuous  curve  in  Fig.  169 
represent  the  aerial  vibrations  in  this  case,  the  distance  0  to 
1  on  the  abscissa  will  represent  T^  of  a  second.  Let,  simul- 
taneously, the  air  be  set  in  movement  by  a  fork  of  the  next 
higher  octave,  c' ,  making  2G4  vibrations  per  1";  under  the 
influence  of  this  second  fork  alone,  the  aerial  particles  would 
move  as  represented  by  the  line  0,  bl,  b3,  and  so  on,  the 
waves  being  half  as  long  and  cutting  the  abscissa  twice  as 
often.  But  when  both  forks  act  together  the  aerial  move- 
ment will  be  the  algebraic  sum  of  the  movements  due  to 
each  fork;  when  both  drive  the  air  one  way  they  will  rein- 
force one  another,  and  vice  versa;  the  result  will  be  the 
movement  represented  by  the  dotted  line,  which  is  still 
periodic,  repeating  itself  at  equal  intervals  of  time,  but  no 
longer  pendular,  since  it  is  not  alike  on  the  ascending  and 
descending  limbs  of  the  curves.  We  thus  get  at  the  fact 
that  non-pendular  vibrations  may  be  produced  by  the  fu- 
sion of  pendular,  or,  in  technical  phrase,  by  their  compo- 
sition. 

Suppose  several  musical  instruments,  as  those  of  an  or- 
chestra, to  be  sounded  together.  Each  produces  its  own 
effect  on  the  air-particles,  whose  movements,  being  the  alge- 
braical sum  of  those  due  to  all,  must  at  any  given  instant  be 
very  complex;  yet  the  ear  can  pick  out  at  will  and  follow  the 
tones  of  any  one  instrument.  From  the  complex  aerial 
movement  it  can  select  that  fraction  of  it  which  one  vibrat- 
ing body  produces.  The  air  in  the  external  auditory  meatus 
at  any  given  moment  can  only  be  in  one  state  of  rarefaction 
or  condensation  and  at  one  rate  and  in  one  direction  of  move- 
ment, this  being  the  resultant  of  all  the  forces  acting  upon 
it:  all  clashing,  and  some  pushing  one  way  and  others  an- 
other.    If  the  resultant  movement  be  not  periodic  it  will  be 


568  THE  HE  MAN  BODY. 

recognized  as  due  to  noises  or  to  several  simultaneous  in- 
harmonic musical  tones;  this  is  commonly  the  case  when 
musical  tones  are  not  united  designedly,  and  the  ear  thus 
gets  one  criterion  for  distinguishing  movements  of  the  air 
due  to  several  simultaneous  musical  tones.  Eowever,  a  com- 
posite set  of  tones  will  give  rise  to  periodic  vibrations  when 
all  are  due  to  vibrations  of  rates  which  are  multiples  of  the 
same  whole  number.  In  such  eases  the  movement  of  the 
air  in  the  auditory  meatus  has  no  property  except  vibrational 
form  by  which  the  ear  could  distinguish  it  from  a  simple 
tone;  when  the  two  tuning-forks  giving  the  forms  of  vibra- 
tion (with  rates  as  1  to  2),  represented  in  Fig.  Hi!)  by  con- 
tinuous lines,  are  sounded  together,  we  get  the  new  form  of 
vibration  represented  by  the  dotted  line,  and  this  has  the 
same  period  as  that  of  the  lower-pitched  fork;  yet  the  ear 
can  clearly  distinguish  the  resultant  sound  from  that  of  this 
fork  alone,  as  a  note  of  the  same  pitch  but  of  different 
timbre;  and  with  practice  can  recognize  exactly  what  simple 
vibrations  go  to  make  it  up. 

The  Analysis  of  Non-Pendular  Vibrations.  If  a  per- 
son with  a  trained  ear  listens  attentively  to  any  ordinary 
musical  tone,  such  as  that  of  a  piano,  he  hears,  not  only  the 
note  whose  vibrational  rate  determines  the  pitch  of  the  tone 
as  a  whole,  but  a  whole  series  of  higher  notes,  in  harmony 
with  the  general  or  fundamental  tone;  this  latter  is  the 
primary  partial  tone,  and  the  others  are  secondary  partial 
tones;  nearly  all  tones  used  in  music  contain  both.  If  the 
prime  tone  be  due  to  132  vibrations  a  second  (c),  its  first 
upper  partial  is  c'  (=  264  vibrations  per  second);  the  next  is 
the  fifth  of  this  octave  {</  =  396  =  132  X  3  vibrations  per  1'); 
the  next  is  the  second  octave,  c"  (132x4  =  528  vibrations  per 
1');  the  next  is  the  major  third  of  the  c"  (=  132  X  5  =  660 
vibrations  per  second  =  e"),  and  so  on.  The  only  form  of 
vibration  which  gives  no  upper  partial  tones  is  the  pendular; 
we  may  call  notes  due  to  such  vibrations  simple  tones;  and 
we,  consequently,  recognize  in  music  tones  which  are  simple 
(such  as  those  of  tuning-forks)  and  those  which  are  com- 
pound; these  latter  are  non-pendular  in  form. 

We  find,  then,  that  the  form  of  aerial  vibrations  deter- 
mines in  our  sensations  the  occurrence  or  non-occurrence  of 
upper  partial  tones.  It  also,  as  we  have  seen,  determines  the 
quality  or  timbre  of  the  tone,  since  vibrational  amplitude  and 


THE  EAR  AND  HEARING.  569 

rate  are  otherwise  accounted  for  iu  sensation  by  loudness 
and  pitch. 

It  can  be  proved,  by  the  employment  of  the  higher  mathe- 
matics, that  every  ])eriodic  non-pendular  movement  can  be 
analyzed  (as  the  dotted  curve  of  Fig.  169  may  be)  into  a 
.given  number  of  pendular  vibrations,  that  is,-every  compound 
vibration  into  a  set  of  simple  ones;  and  that  every  periodic 
non-pendular  vibration  can  be  made  by  the  combination  of 
pendular.  Moreover,  any  given  compound  vibration  can  be 
analyzed  into  but  one  set  of  simple  ones;  no  other  combina- 
tion will  produce  it.  Consequently  a  vibrational  movement 
of  the  air  in  the  external  auditory  passage,  producing  a  com- 
pound musical  tone  sensation,  can  be  exhibited  in  every  case, 
but  only  in  one  way,  as  the  sum  of  a  number  of  simple  vibra- 
tions, whose  rates  are  multiples  of  that  which  determines  the 
pitch  of  the  tone. 

Now  when  the  trained  ear  listens  to  a  tone  with  the  ob- 
ject of  detecting  upper  partials  if  present,  it  hears  them  only 
when  the  vibrations  producing  the  tone  are  non-pendular, 
i.e.  when  upper  partials,  tbeoretically,  might  be  expected; 
and  those  heard  are  exactly  those  demanded  by  theory;  by 
the  help  of  instruments  their  detection  is  made  easy  even  to 
untrained  ears.  In  ordinary  circumstances  we  do  not  heed 
secondary  partial  tones;  we  hear  a  note  of  the  pitch  of  the 
primary  partial  and  of  a  certahi  timbre;  and  whenever  the 
upper  partials  present  are  different,  or  of  different  relative 
intensities,  the  timl  re  of  the  note  varies.  Hence  it  becomes 
probable  that,  just  as  the  ear  can  at  will  follow  any  instru- 
ment in  an  orchestra,  analyzing  the  aerial  movement  so  as  to 
select  and  follow  the  fraction  of  the  whole  due  to  that  one, 
BO  it  can  and  does  analyze  compound  tones  when  proceeding 
from  one  instrument,  and  that  the  upper  partials,  not  rising 
into  consciousness  as  definite  tones,  but  present  as  subdued 
sensation 8,  give  its  character  to  the  whole  tone  and  determine 
its  timbre.  Tt  might  be,  however,  that  the  composition  of 
non-pendular  vibrations  from  pendular  was  a  mere  mathe- 
matical possibility,  having  no  real  existence  in  nature.  Be- 
fore we  can  accept  the  above  explanation  of  timbre,  we  must 
sec  if  there  is  any  evidence  that,  as  a  matter  of  fact,  non- 
pendnlar  vibrations,  not  only  may  be,  but  are,  made  up  by 
the  combination  of  pendular. 

Sympathetic  Resonance.     Imagine    slight    taps    to    be 


570  THE  llf MAN  BODY. 

given  to  a,  pendulum;  if  these  be  repeated  at  such  intervals 
of  time  as  to  always  help  the  swing  and  never  to  retard  it, 
the  pendulum  will  soon  be  set  in  powerful  movement.  If 
the  taps  are  irregular,  or  when  regular  come  at  such  inter- 
vals as  sometimes  to  promote  and  sometimes  retard  the  move- 
ment, no  great  swing  will  be  produced;  but  if  they  always 
push  the  pendulum  in  the  way  it  is  going  at  that  instant, 
they  need  not  come  every  swing  in  order  to  set  up  a  powerful 
vibration;  once  in  two,  three,  or  four  swings  will  do.  A 
stretched  string,  such  as  that  of  a  piano,  is  so  far  like  a 
pendulum  that  it  tends  to  vibrate  at  one  rate  and  no  other; 
if  aerial  waves  hit  it  at  exactly  the  right  times  they  soon  set 
it  in  sufficiently  powerful  vibrations  to  cause  it  to  emit  an 
audible  note.  By  using  such  strings  we  might  hope  to  de- 
tect the  separate  pendular  vibrations  in  any  non-pendular 
aerial  periodic  movement  if  such  really  existed;  certain 
strings  would  pick  out  the  pendular  component  agreeing 
in  rate  with  their  own  vibrational  period  and  be  soon  set 
in  powerful  movement;  while  those  not  vibrating  in  the 
same  period  as  any  of  the  pendular  components,  would 
remain  practically  at  rest,  like  the  pendulum  getting  taps 
which  sometimes  helped  and  sometimes  impeded  its  swing. 
If  the  dampers  of  a  piano  be  raised  and  a  note  be  sung 
loudly  to  it,  it  will  be  found  that  several  strings  are  set 
in  vibration,  such  vibrations  being  called  sympathetic.  The 
human  voice  emits  compound  tones  which  can  be  mathe- 
matically analyzed  into  simple  vibrations,  and  if  the  piano 
strings  set  in  movement  by  it  be  examined,  they  will  be 
found  to  be  exactly  those  which  answer  to  these  pendular 
vibrations  and  to  no  others.  We  thus  get  experimental 
grounds  for  believing  that  compound  tones  are  really  made 
up  of  a  number  of  simple  vibrations,  and  get  an  additional 
justification  for  the  supposition  that  in  the  ear  each  note  is 
analyzed  into  its  pendular  components;  and  that  the  differ- 
eiice  of  sensation  which  we  call  timbre  is  due  to  the  effect  of 
the  secondary  partial  tones  thus  perceived.  If  so.  the  ear 
must  have  in  it  an  apparatus  adapted  for  sympathetic  reso- 
nance. 

It  may  be  asked  why,  if  the  ear  analyzes  vibrations  in 
this  way,  do  we  not  commonly  perceive  it  ?  How  is  it  that 
what  we  ordinarilv  hear  is  the  timbre  of  a  given  tone  and  not 


THE  EAR  AND  HEARING.  571 

the  separate  upper  partials  which  give  it  this  character  ? 
The  explanation  is  more  psychological  than  physiological,  and 
belongs  to  the  same  category  as  the  reason  why  we  do  not 
ordinarily  notice  the  blind  spot  in  the  eye,  or  the  doubleness 
of  objects  out  of  the  horopter,  or  the  duplicity  of  stereoscopic 
images.  We  only  use  our  senses  in  daily  life  when  they  can 
tell  us  something  that  may  be  useful  to  us,  and  we  neglect  so 
habitually  all  sensations  which  would  be  useless  or  confusing, 
that  at  last  it  needs  special  attention  to  observe  them  at  all. 
The  way  in  which  tones  are  combined  to  give  timbre  to  a 
note  is  a  matter  of  no  importance  in  the  daily  use  of  them, 
and  so  we  fail  entirely  to  observe  the  components  and  note 
only  the  resultant,  until  we  make  a  careful  and  scientific 
examination  of  our  sensations. 

The  Functions  of  the  Tympanic  Membrane.  If  a 
stretched  membrane,  such  as  a  drum-head,  be  struck,  it  will 
be  thrown  into  periodic  vibration  and  emit  for  a  time  a  note 
of  a  determined  pitch.  The  smaller  the  membrane  and  the 
tighter  it  is  stretched  the  higher  the  pitch  of  its  note;  every 
stretched  membrane  thus  has  a  rate  of  its  own  at  which  it 
tends  to  vibrate,  just  as  a  piano  or  violin  string  has.  When 
a  note  is  sounded  in  the  air  near  such  a  membrane,  the  alter- 
nating waves  of  aerial  condensation  and  rarefaction  will 
move  it;  and  if  the  waves  succeed  at  the  vibrational  rate  of 
the  membrane  the  latter  will  be  set  in  powerful  sympathetic 
vibration;  if  they  do  not  push  the  membrane  at  the  proper 
times,  their  effects  will  neutralize  one  another:  hence  such 
membranes  respond  well  to  only  one  note.  The  tympanic 
membrane,  howover,  responds  equally  well  to  a  large  number 
of  notes;  at  the  least  for  those  due  to  aerial  vibrations  of  rates 
from  GO  to  4000  per  second,  running  over  eight  octaves  and 
constituting  those  commonly  used  in  music.  This  faculty 
depends  on  two  things:  (1)  the  membrane  is  comparatively 
loosely  and  not  uniformly  stretched;  (2)  it  is  loaded  by  the 
tympanic  bones. 

The  dram-membrane  is  ;i  shallow  funnel  with  its  sides  con- 
vex towards  the  external  auditory  meatus;  something  like  an 
umbrella  turned  inside  out;  in  Buch  a  membrane  the  tension  is 
not  uniform  but  increases  towards  the  centre,  and  ithasaccord- 
ingly  no  proper  note  of  its  own.  Further,  whatever  tendency 
such  a  membrane  may  have  to  vibrate  raiher  at  one  rate  than 


512  THE  III' MAX  BODY. 

another,  is  almosl  completely  removed  by  "damping"  it;  i.e. 
placing  in  contacl  with  ii  something  comparatively  heavj  and 
which  has  to  be  moved  when  the  membrane  vibrates.  This 
is  effected  by  the  tympanic  hones,  fixed  to  the  drum-membrane 
h\  the  handle  of  the  malleus.  Another  advantage  is  gained 
by  the  damping;  once  a  stretched  membrane  is  sel  vibrating  it 
c< mi ti niics  so  doing  for  some  time;  but  if  Loaded  its  movements 
ccisc  almost  as  soon  as  the  moving  impulses.  The  dampers 
of  a  piano  are  for  this  purpose;  and  violin-players  have  to 
••damp"  with  the  fingers  the  strings  they  have  used  when 
they  wish  the  note  to  case.  The  tympanic  bones  act  as 
dampers. 

Functions  of  the  Auditory  Ossicles.  AVlien  the  air  in 
the  external  auditory  meatus  is  condensed  it  pushes  in  the 
handle  of  the  malleus.  This  bone  then  slightly  rotates  on 
the  axial  ligament  and,  locking  into  the  incus  where  the 
two  hones  articulate,  causes  the  long  process  (Jl,  Fig.  163) 
of  the  latter  to  move  inwards.  The  incus  thus  pnshes-in  the 
stapes;  the  reverse  occurs  when  air  in  the  auditory  passage  is 
rarefied.  Aerial  vibrations  thus  set  the  chain  of  bones  swing- 
ing, and  push  in  and  pull  out  the  base  of  the  stapes,  which 
sets  up  waves  in  the  perilymph  of  the  labyrinth,  and  these 
are  transmitted  through  the  membranous  labyrinth  to  the 
endolymph.  These  liquids  being  chiefly  water,  and  practi- 
cally incompressihle,  the  end  of  the  stapes  could  not  work  in 
and  out  at  the  oval  foramen,  were  the  labyrinth  elsewhere 
completely  surrounded  by  bone:  but  the  membrane  covering 
the  round  foramen  bulges  out  when  the  base  of  the  stapes  is 
pushed  in,  and  vice  versa  :  and  so  allows  of  waves  being  set 
up  in  the  labyrinthic  liquids.  These  correspond  in  period 
and  form  to  those  in  the  auditory  meatus:  their  amplitude  is 
determined  by  the  extent  of  the  vibrations  of  the  drum  mem- 
brane. 

The  form  of  the  tympanic  membrane  causes  it  to  transmit 
to  its  centre,  where  the  malleus  is  attached,  vibrations  of  its 
lateral  parts  in  diminished  amplitude  hut  increased  power;  so 
that  the  tympanic  hones  are  pushed  only  a  little  way  but  with 
considerable  force.  Its  area,  too,  is  about  twenty  times  as 
great  as  that  of  the  oval  foramen,  so  that  force  collected  on 
the  large  area  is.  by  pushing  the  tympanic  bones,  all  concen- 
trated on  the  -mailer.  The  ossicles  also  form  a  bent  lever 
(Fig.  1G3)  of  which  the  fulcrum  is  at  the  axial  ligament  and 


THE  EAR  AND  UEAEING.  573 

the  effective  outer  arm  of  this  lever  is  about  half  as  long  again 
as  the  inner,  and  so  the  movements  transmitted  by  the  drum- 
membrane  to  the  handle  of  the  malleus  are  communicated 
with  diminished  range,  but  increased  power,  to  the  base  of 
the  stapes. 

Ordinarily,  sound-waves  reach  the  labyrinth  through  the 
tympanum,  but  they  may  also  be  transmitted  through  the 
bones  of  the  head;  if  the  handle  of  a  vibrating  tuning-fork 
be  placed  on  the  vertex,  the  sounds  heard  by  the  person  ex- 
perimented upon  seem  to  have  their  origin  inside  his  own 
cranium.  Similarly,  when  a  vibrating  body  is  held  between 
the  teeth,  sound  reaches  the  end  organs  of  the  auditory  nerve 
through  the  sknll-bones;  and  persons  who  are  deaf  from  dis- 
ease or  injury  of  the  tympanum  can  thus  be  made  to  hear,  as 
with  the  audiphone.  Of  course  if  deafness  be  due  to  disease 
of  the  proper  nervous  auditory  apparatus  no  device  can  make 
the  person  hear. 

Function  of  the  Cochlea.  We  have  already  seen  reason 
to  believe  that  in  the  ear  there  is  an  apparatus  adapted  for 
sympathetic  resonance,  by  which  we  recognize  different  musi- 
cal tone-colors;  the  minute  structure  of  the  membranous 
cochlea  is  such  as  to  lead  us  to  look  for  it  there.  An  old  view 
was  that  the  rods  of  Corti,  which  vary  in  length,  were  like  so 
many  piano-strings,  each  tending  to  vibrate  at  a  given  rate 
and  picking  out  and  responding  to  pendular  aerial  vibrations 
of  its  own  period,  and  exciting  a  nerve  which  gave  rise  to  a 
particular  tone  sensation.  When  the  labyrinthic  fluids  were 
set  in  non-pendular  vibrations,  the  rods  of  Corti  were  thought 
to  analyze  these  into  their  pendular  components,  all  rods  of 
the  vibrational  rate  of  these  being  set  in  sympathetic  move- 
ment, but  that  rod  most  whose  period  was  that  of  the  primary 
partial  tone;  this  rod  would  determine  the  pitch  of  the  note, 
and  the  less-marked  sensation  due  to  the  others  affected  would 
give  the  timbre.  The  rods,  however,  do  not  differ  in  size 
sufficiently  to  account  for  the  range  of  notes  which  we  hear; 
they  are  absenl  111  birds,  which  undoubtedly  distinguish  differ- 
ed musical  notes;  and  the  nerve-iibres  of  the  Cochlea  are  not 
connected  with  them  but  with  the  hair-cells. 

On  the  whole  it  seems  probable  that  the  basilar  membrane 
is  to  lie  looked  upon  ae  the  primary  arrangemenl  for  sympa- 
thetic resonance  in  the  ear.  It  increases  in  breadth  twelve 
tine-  from  the  base  ot  the  cochlea  to  its  tip  (the  Less  width  of 


574  THE  HUMAN  BODY. 

the  lamina  spiralis  at  the  apex  more  than  compensating  for 
the  less  size  of  the  bony  tube  there)  and  is  stretched  tight 
across,  but  loosely  in  the  other  direction.  A  membrane  so 
stretched  behaves  as  a  set  of  separate  strings  placed  side  by 
side,  somewhat  as  those  of  a  harp  but  much  closer  together; 
and  each  string  would  vibrate  at  its  own  period  without  in- 
fluencing much  those  on  each  side  of  it.  Probably,  then, 
each  transverse  band  vibrates  to  simple  tones  of  its  own 
period,  and  excites  the  hair-cells  which  lie  on  it,  and  through 
them  the  nerve-fibres.  Perhaps  the  rods  of  Corti,  being  stiff, 
and  carrying  the  reticular  membrane,  rub  that  against  the 
upper  ends  of  the  hair-cells  which  project  into  its  apertures 
and  so  help  in  a  subsiduary  way,  each  pair  of  rods  being 
especially  moved  when  the  band  of  basilar  membrane  carrying 
it  is  set  in  vibration.  The  tectorial  membrane  is  probably  a 
"damper;"  it  is  soft  and  inelastic,  and  suppresses  the  vibra- 
tions as  soon  as  the  moving  force  ceases. 

Function  of  the  Vestibule  and  Semicircular  Canals. 
Many  noises  are  merely  spoiled  music ;  they  are  due  to  tones  so 
combined  as  not  to  give  rise  to  periodic  vibrations;  these  are 
probably  heard  by  the  cochlea.  If  a  single  violent  air-wave 
ever  cause  a  sound  sensation  (which  is  doubtful,  since  any  vio- 
lent push  of  an  elastic  substance,  such  as  the  air,  will  cause  it 
to  make  several  rebounds  before  coming  to  rest)  we  perhaps 
hear  it  by  the  vestibule;  the  otoliths,  there  in  contact  with 
the  auditory  hairs,  are  imbedded  in  a  tenacious  gummy  mass 
quite  distinct  from  the  proper  endolymph,  and  are  not 
adapted  for  executing  regular  vibrations,  but  they  might 
yield  to  a  single  powerful  impulse  and  transmit  it  to  the  hair- 
cells,  and  through  them  stimulate  the  nerves.  There  is  reason 
to  believe  that  the  semicircular  canals  have  nothing  to  do 
with  hearing;  their  supposed  function  is  described  in  Chapter 
XXXVI. 

Auditory  Perceptions.  Sounds,  as  a  general  rule,  do  not 
seem  to  us  to  originate  within  the  auditory  apparatus;  we 
refer  them  to  an  external  source,  and  to  a  certain  extent  can 
judge  the  distance  and  direction  of  this.  As  already  men- 
tioned, the  extrinsic  reference  of  sounds  which  reach  the  laby- 
rinth through  the  general  skull-bones  instead  of  through  the 
tympanic  chain  is  imperfect  or  absent.  The  recognition  of 
the  distance  of  a  sounding  body  is  possible  only  when  the 
sound  is  well  known,  and  then  not  very  accurately;  from  its 


THE  EAR  AND  HEARING.  575 

faintness  or  loudness  we  may  make  in  some  cases  a  pretty- 
good  guess.  Judgments  as  to  the  direction  of  a  sound  are 
also  liable  to  be  grossly  wrong,  as  most  persons  have  experi- 
enced. However,  when  a  sound  is  heard  louder  by  the  left 
than  the  right  ear  we  can  recognize  that  its  source  is  on  the 
left;  when  equally  with  both  ears,  that  it  is  straight  in  front 
or  behind;  and  so  on.  The  concha  has  perhaps  something  to 
do  with  enabling  us  to  detect  whether  a  sound  originates  be- 
fore or  behind  the  ear,  since  it  collects,  and  turns  with  more 
intensity  into  the  external  auditory  meatus,  sound-waves 
coming  from  the  front.  By  turning  the  head  and  noting 
the  accompanying  changes  of  sensation  in  each  ear  we  can 
localize  sounds  better  than  if  the  head  be  kept  motionless. 
The  large  movable  concha  of  many  animals,  as  a  rabbit  or  a 
horse,  which  can  he  turned  in  several  directions,  is  probably 
an  important  aid  to  them  in  detecting  the  position  of  the 
source  of  a  sound.  That  the  recognition  of  the  direction  of 
sounds  is  not  a  true  sensation,  but  a  judgment,  founded  on 
experience,  is  illustrated  by  the  fact  that  we  can  estimate 
much  more  accurately  the  direction  of  the  human  voice, 
which  we  hear  and  heed  most,  than  that  of  any  other  sound. 


CHAPTER   XXXV. 

TOUCH.     TEMPERATURE     SENSATIONS.      PATN.      COMMON 
SENSATIONS.     SMELL.     TASTE.     Tl IK  MUSCULAR  SENSE. 

The  skin  is  very  abundantly  supplied  with  afferent  nerve- 
fibres,  and  from  it  we  get  several  very  distinct  kinds  of  sen- 
sations;  it  is  therefore  not  surprising  that  nerve-fibres  are 

found  to  end  in  it  in  different  ways,  hut  at  present  we  are  not 
able  to  associate  satisfactorily  any  one  particular  variety  of 
cutaneous  nerve-ending  with  the  origination  of  the  impulses 
which  lead  to  the  occurrence  of  any  one  kind  of  the  skin  sen- 
sations. 

Many  cutaneous  afferent  nerve-fibres  end  in  a  very  simple 
way:  they  form  plexuses  in  the  outermost  layer  of  the  dermis 
and  then,  losing  the  medullary  sheath,  the  axis  cylinders  enter 
the  epidermis  and  there  break  up  into  extremely  minute  fila- 
ments which  ramify  among  the  cells  of  the  Malpighian  layer 
and  terminate  there  without  any  special  end  organs.  Other 
fibres  have  special  terminal  apparatuses,  known  as  (1)  tactile 
cells;  (2)  end  bulbs;  (3)  tactile  corpuscles;  (4)  Pacinian 
bodies. 

The  Tactile  Cells  lie  usually  in  the  deepest  layer  of  the 
epidermis,  hut  sometimes  are  found  also  in  the  dermis.  They 
are  larger  and  more  granular  than  the  neighboring  epidermic 
cells,  more  oval,  and  stain  more  deeply  with  some  reagent-. 
especially  gold  chloride.  Minute  axis-cylinder  branches  can 
be  traced  into  close  relation  to  them,  and  according  to  Borne 
histologists  cud  in  Hat  expansions  closely  applied  to  the  tactile 
cells,  while  others  believe  the  nerve-filament  to  be  directly 
continuous  with  the  cell  substance.  These  cells  are  especially 
abundant  in  the  epidermis  lining  the  root-sheath-  of  such 
tactile  hairs  as  the  "whiskers"  of  a  cat,  but  they  exist  in 
many  if  not  most  regions  of  the  human  skin. 

The  End  Bulbs  lie  in  the  dermis  of  certain  regions  as  the 
lips,  but  they  are  mainly  confined  to  the  conjunctiva  and  to 
tin'  mucous  membrane  lining  the  mouth  and  that  of  the  lowest 

576 


TOUCH.     TEMPERATURE  SENSATIONS. 


mi 


part  of  the  rectum,  all  of  which  possess  tactile  sensibility. 
Very  similar  bodies  are  found  in  the  synovial  membranes  of 
some  joints.  In  man  they  are  spheroidal  and  vary  in  diameter 
from  .03  to  0.1  m.m.  (^o-2-i-o  inch).  Each  has  an  external 
capsule  of  connective  tissue  within  which  is  a  core  consisting 
of  polygonal  nucleated  ill-defined  cells.  The  nerve-fibre 
loses  its  medullary  sheath  close  to  the  end  bulb  and  the  axis 
cylinder  enters  the  core  and  there  usually  breaks  up  into  fila- 
ments which  ramify  between  the  cells  of  the  core  and  end  in 
little  knobs:  sometimes  the  axis  cylinder  does  not  branch. 
The  tactile  corpuscles  (Fig.  1T0)  are  found  especially  in  the 


Fig.  170.— Section  of  skin  showing-  two  papillae  of  the  dermis  and  some  of  the 
deeper  cells  or'  t  he  epidermia  ;  «.  papilla  containing  blood  vessels;  b.  papilla  con- 
taining a  tactile  corpuscle,  t ;  d,  medu Hated  nerve-fibres  going  to  the  corpuscle; 
at/,  optical  cross-sections  of  the  fibres  are  sen  as  they  wind  round  the  outside  of 
tin-  corpuscle;  the  general  transverse  direction  of  the  connective-tissue  bundles  of 
the  capsule  of  the  corpuscle  is  shown. 

skin  of  the  hands  and  feet,  but  also  on  the  inner  surface  of 
the  forearm,  on  the  nipple,  the  lips,  and  mucous  membrane 
of  the  tip  of  the  tongue.  They  lie  in  dermic  papillae  and  are 
oval  in  form,  measuring  about  0.8  m.m.  (^i-^  inch)  in  the  long 
and  0.3  m.m.  (¥(>7|-  inch)  in  the  transverse  diameter.  Each 
has  an  outer  capsule  of  connective  tissue  from  which  many 
transverse  or  oblique  dissepiments  enter  and  divide  the  in- 
terior into  many  small  chambers.  Two  or  three  medullated 
nerve-fihres  go  to  each  corpuscle,  and  alter  winding  around  ib 
obliquely  Beveral  times  penetrate  the  capsule  at  various  levels, 
at  the  same  time  becoming  uon-medullated.  The  axis  cylin- 
der- run  in  the  clefts  between  the  connective-tissue  dissepi- 
ments and  after  branching  many  times  end  in  pear-shaped  or 
spherical  enlargements,  which  are  always  placed  near  the  out- 
side of  the  corpuscle. 


578 


THE  HUMAN  BODY. 


The  Pacinian  Bodies  or  Corpuscles  (Fig.  171)  arc  found 
in  Large  numbers  in  the  subcutaneous  areolar  tissue  of  the  hand 
and  foot,  and  occasionally  in  other  regions  of  the  skin.  But 
they  are  also  found  in  internal  part.-,  as  on  the  nerves  of 
tendons  and  ligaments  and  on  sonic  brunches  of  the  solar 
plexus;  and  they  are  very  abundanl  and  easily  seen  in  the 
mesentery  of  the  cat,  so  that  though  almost  certainly  organs 
in  which  afferent  nerve  impulses  originate,  they  are  not  organs 
of  touch.  The  corpuscles  are  oval,  often  curved  on  the  long 
axis,  and  from  1.5  to  2.5  m.ni.  ( ,'.—  ,10  inch)  in  length. 
When  fresh  they  have  a  whitish  translucent  appearance  and 
are  somewhat  more  opaque  in  the  centre.  When  magnified 
each  Pacinian  body  is  seen  to  consist  of  an  almost  structure- 
less core  surrounded  by  many  concentric  capsules.  Each 
capsule  is  a  layer  of  imperfectly  developed  connective  tissue 
having  a  few  very  fine  fibres,  the  interstices  between  which 
are  filled  with  liquid:  each  surface  of  each  capsule  is  formed 
by  a  well-marked  layer  of  flat  nucleated  cells,  and  the  cell 
layer  on  the  inner  side  of  one  capsule  is  separated  from  the 
layer  on  the  outer  side  of  the  next  by  a  narrow  cleft,  which 

is  a  lymph  lacuna.  The  capsules 
are  usually  so  closely  applied  to 
one  another  that  the  lymph  spaces 
between  them  are  almost  oblit- 
erated. A  medullated  nerve- 
fibre  runs  to  one  pole  of  each  Pa- 
cinian body  and  the  axis  cylinder 
and  medullary  sheath  are  contin- 
ued through  the  capsules  to  the 
core;  the  medullary  sheath  be- 
coming thinner  on  the  way.  The 
axis  cylinder  enters  the  core  and 
runs  to  near  its  opposite  end, 
where  it  ends  in  a  rounded  en- 
largement or  sometimes  divides 
into  several  short  branches,  each 
with  a  knobbed  end. 

Touch,  or  the  Pressure 
Sense.  Through  the  skin  we 
get  several  kinds  of  sensation;  touch  proper,  heat  and  cold, 
and  pain;  and  we  can  with  more  or  less  accuracy  localize 
fchem  on  the  surface  of  the  Body.     The  interior  of  the  mouth 


FlO.   171.— A   Pacinian  corpuscle 
magnified. 


TOUCH      TEMPERATURE  SENSATIONS.  579 

possesses  also  these  sensibilities.  Through  touch  proper  we 
recognize  pressure  or  traction  exerted  on  the  skin,  and  the 
force  of  the  pressure;  the  softness  or  hardness,  roughness  or 
smoothness,  of  the  body  producing  it;  and  the  form  of  this, 
when  not  too  large  to  be  felt  all  over.  When  to  learn  the  form 
of  an  object  we  move  the  hand  over  it,  muscular  sensations 
are  combined  with  proper  tactile,  and  such  a  combination  of 
the  two  sensations  is  frequent;  moreover,  we  rarely  touch 
anything  without  at  the  same  time  getting  temperature  sen- 
sations ;  therefore  pure  tactile  feelings  are  rare. 

From  an  evolution  point  of  view,  touch  is  probably  the  first 
distinctly  differentiated  sensation,  and  this  primary  position 
it  still  largely  holds  in  our  mental  life;  we  mainly  think  of  the 
things  about  us  as  objects  which  would  give  us  certain  tactile 
sensations  if  we  were  in  contact  with  them.  Though  the  eye 
tells  us  much  quicker,  and  at  a  greater  range,  what  are  the 
shapes  of  objects  and  whether  they  are  smooth,  rough,  and  so 
on,  our  real  conceptions  of  round  and  square  and  rough 
bodies  are  derived  through  touch,  and  we  largely  translate 
unconsciously  the  teachings  of  the  eye  into  mental  terms  of 
the  tactile  sense. 

The  delicacy  of  tbe  pressure  sense  varies  on  different  parts 
of  the  skin;  it  is  greatest  on  the  forehead,  temples,  and  back 
of  the  forearm,  where  a  weight  of  2  milligr.  (.03  grain)  press- 
ing on  an  area  of  9  sq.  millim.  (.0139  sq.  inch)  can  be  felt. 
On  the  front  of  the  forearm  3  milligr.  (.030  grain)  can  be 
similarly  felt,  and  on  the  front  of  the  forefinger  5  to  15  milligr. 
(.07-0. 23  grain). 

In  order  that  the  sense  of  touch  may  be  excited  neighboring 
skin  areas  must  be  differently  pressed;  when  we  lay  the  hand 
on  a  table  this  is  secured  by  the  inequalities  of  the  skin,  which 
prevent  end  organs,  lying  near  together,  from  being  equally 
compressed.  When,  however,  the.  hand  is  immersed  in  a 
liquid,  as  mercury,  which  fits  into  all  its  inequalities  and 
presses  with  practically  the  same  weight  on  all  neighboring 
immersed  areas,  the  sense  of  pressure  is  only  felt  at  a  line  along 
the  surface,  where  the  immersed  and  non -immersed  parts  of 
the  -kin  meet. 

[t  was  in  connection  with  the  tactile  sense  thai  the  facts  on 
which  so-called  psycho-physical  law  (Chap.  XXXI.)  is  based, 
were  ftrsi  observed.  The.  smallest  perceptible  difference  of 
pressure  recognizable  when  touch  alone  is  used,  is  about  I; 


580  THE  LIU  MAN  BODY. 

i.e.,  we  can  just  tell  a  weight  of  20  grains  (310  grains)  from 
one  of  30  (465  grains)  or  of  40  grams  (620  grains)  from  one 
of  60  (930  grains) ;  the  change  which  can  just  be  recognized 
being  thus  the  same  fraction  of  that  already  acting  as  a  stimu- 
lus. The  ratio  only  holds  good,  however,  for  a  certain  mean 
range  of  pressures;  it  is  not  true  for  very  small  or  very  great 
pressures.  The  experimental  difficulties  in  determining  the 
question  are  considerable;  muscular  sensations  must  be  rigidly 
excluded;  the  time  elapsing  between  laving  the  different 
weights  on  the  skin  must  always  be  equal;  the  same  region 
and  area  of  the  skin  must  be  used;  the  weights  must  have 
the  same  temperature;  and  fatigue  of  the  organs  must  be 
eliminated.  Considerable  individual  variations  are  also  ob- 
served, the  least  perceptible  difference  not  being  the  same  in 
all  persons. 

The  Localizing  Power  of  the  Skin.  When  the  eyes  are 
closed  and  a  point  of  the  skin  is  touched  we  can  with  some 
accuracy  indicate  the  region  stimulated ;  although  tactile 
feelings  are  in  general  characters  alike,  they  differ  in  some- 
thing (local  sign)  besides  intensity  by  which  we  can  distin- 
guish them ;  some  sensation  quality  must  be  present  enabling 
us  to  tell  from  one  another  two  precisely  similar  contacts  of 
an  external  object  when  applied,  say,  to  the  tips  of  the  fore 
and  ring  fingers  respectively.  The  accuracy  of  the  localizing 
power  is  not  nearly  so  great  as  in  the  retina  and  varies  widely 
in  different  skin  regions;  it  may  be  measured  by  observing 
the  least  distance  which  must  separate  two  objects  (as  the 
blunted  points  of  a  pair  of  compasses)  in  order  that  they  may 
be  felt  as  two.  The  following  table  illustrates  some  of  the 
differences  observed — • 

Tongue-tip 1.1  mm.     (.04  inch) 

Palm  side  of  last  phalanx  of  finger 2.2  mm.     (.08  inch) 

Red  part  of  lips 4.4  mm.     (.16  inch) 

Tip  of  nose 6.6  mm.     (.24  inch) 

Back  of  second  phalanx  of  finger 11.0  mm.     (.44  inch) 

Heel 22.0  mm.     (.88  inch) 

Back  of  hand 30.8  mm.   (1.23  inches) 

Forearm 39.6  mm.  (1.58  inches) 

Sternum 44.0  mm.  (1.76  inches) 

Back  of  neck 52.8  mm.  (2.11  inches) 

Middle  of  back 66.0  mm.  (2.64  inches) 

The  localizing  power  is  a  little  more  acute  across  the  long 


TOUCH.     TEMPERATURE  SENSATIONS.  581 

axis  of  a  limb,  and  is  better  wben  the  pressure  is  only  strong 
enough  to  just  cause  a  distinct  tactile  ^-r-n-+^ 

sensation,  than  when  it  is  more  power-     a  —  "iWrWA Wpfts. 
ful ;  it  is  also  very  readily  and  rapidly  wJfyJ^^ 

improvable  by  practice.  mn  (TT^Wl  u  1 1\ 

It  might  be  thought  that  this  local-  \CwlU\vX^xv\ 

izing  power  depended  directly  on  nerve  '""'/'YnnTTumYiT 
distribution;  that  each  touch  nerve  had         ,'    lujXlX^ 
connection  with  a  special  brain-centre        •     YTinn  fUL^ 
at  one  end    (the   excitation   of   which    .     \     y^JJjgP^  ' 
caused  a  sensation  with  a  char*»overistic  ;  j 

local  sign),  and  at  tbe  other  end  was  !  / 

distributed  over  a  certain  skin  area,  and  V  /' 

that   the  larger  this   area   the  farther  " -' 

apart  might  two  points  be  and  still  give  FlG-  17~- 

rise  to  only  one  sensation.  If  this  were  so,  however,  the 
peripheral  tactile  areas  (each  being  determined  by  the  ana- 
tomical distribution  of  a  nerve-fibre)  must  have  definite  un- 
changeable limits,  which  experiment  shows  that  they  do  not 
possess.  Suppose  the  small  areas  in  Fig.  172  to  each  repre- 
sent a  peripheral  area  of  nerve  distribution.  If  any  two 
points  in  c  were  touched  we  would  according  to  the  theory 
get  but  a  single  sensation ;  but  if,  while  the  compass  points 
remained  the  same  distance  apart,  or  were  even  approximated, 
one  were  placed  in  c  and  the  other  on  a  contiguous  area,  two 
fibres  would  be  stimulated  and  we  ought  to  get  two  sensa- 
tions; but  such  is  not  the  case;  on  the  same  skin  region  the 
points  must  be  always  the  same  distance  apart,  no  matter  how 
they  be  shifted,  in  order  to  give  rise  to  two  just  distinguish 
able  sensations. 

It  is  probable  that  the  nerve  areas  are  much  smaller  than 
the  tactile;  and  that  several  unstimulated  must  intervene  be- 
tween the  excited,  in  order  to  produce  sensations  which  shall 
be  distinct.  If  we  suppose  twelve  unexcited  nerve  areas 
must  intervene,  then,  in  Fig.  172,  a  and  h  will  be  just  on  the 
Limits  at  a  single  tactile  area;  and  no  matter  how  the  points 
are  moved,  so  Long  as  eleven,  or  fewer,  unexcited  areas  come 
between,  we  would  get  a  single  tactile  sensation;  in  this  way 
we  can  explain  the  fact  that  tactile  areas  have  no  fixed  boun- 
daries in  the  skin,  although  the  nerve  distribution  id  any  part 
iini-t  be  constant.  We  also  see  why  the  back-  of  a  knife  laid 
on  the  .surface  causes  a  continuous  linear  sensation,  although 


582  THE  II UMAX  BODY. 

it  touches  many  distind  nerve  areas;  if  we  could  discriminate 
the  excitations  of  each   of  these  from  that  of  its  immediate 

neighbors  we  would  get  the  sensation  of  ;i  series  of  points 
touching  us,  one  for  each  nerve  region  excited;  bu1  in  the 
absence  of  intervening  nnexcited  nen  e  areas  the  Bensations  are 
fused  together. 

The  ultimate  differentiation  of  tactile  areas  take-  place  in 
the  central  organs,  as  will  be  more  fully  pointed  out  in  the 
nexl  chapter.  Afferent  nerve  impulses  reaching  the  spinal 
cord  from  a  finger-tip  enter  the  gray  matter  and  tend  to 
spread  or  radiate  in  it;  from  the  gray  region  through  which 
they  spread,  impulses  are  sent  on  to  perceptive  tactile  centres 
in  the  brain;  if  two  skin-points  are  so  close  that  their  regions 
of  irradiation  in  the  cord  overlap,  then  the  two  points  touched 
cannot  be  discriminated  in  consciousness,  since  the  brain  region 
excited  is  in  part  common  to  both.  The  more  powerful  the 
stimulus  the  wider  the  irradiation  in  the  cord,  and  hence  the 
less  accurate  the  discriminating  power.  The  more  often  an 
impulse  has  travelled,  the  more  does  it  tend  to  keep  its  own 
proper  tract  through  the  gray  matter  of  the  cord,  and  get 
on  to  its  own  proper  brain-centre  alone;  hence  the  increase 
of  tactile  discrimination  with  practice,  for  we  cannot  suppose 
it  to  be  due  to  a  growth  of  more  nerve-fibres  down  to  the 
skin,  and  a  rearrangement  of  the  old,  with  smaller  areas  of 
anatomical  distribution.  As  a  general  rule,  more  movable 
parts  have  smaller  tactile  areas;  this  probably  depends  on 
practice,  since  they  are  the  parts  which  get  the  greatest 
number  of  different  tactile  stimulations. 

The  Temperature  Sense.  By  this  Ave  mean  our  faculty 
of  perceiving  cold  and  warmth ;  and,  with  the  help  of  these 
sensations,  of  perceiving  temperature  ditferences  in  external 
objects.  Its  organ  is  the  whole  skin,  the  mucous  membrane 
of  mouth  and  fauces,  pharynx  and  upper  part  of  gullet,  and 
the  entry  of  the  nares.  Direct  heating  or  cooling  of  a  sensory 
nerve  may  stimulate  it  and  cause  pain,  but  not  a  true  tem- 
perature sensation;  and  the  amount  of  heat  or  cold  requisite 
is  much  greater  than  that  necessary  when  a  temperature- 
perceiving  surface  is  acted  upon;  hence  Ave  must  assume  the 
presence  of  temperature  end  organs. 

In  a  comfortable  room  we  feel  at  no  part  of  the  Body 
either  heat  or  cold,  although  different  parts  of  its  surface  are 


TOUCH.     TEMPERATURE  SENSATIONS.  583 

at  different  temperatures;  the  fingers  and  nose  being  cooler 
than  the  trunk  which  is  covered  by  clothes,  and  this,  in  turn, 
cooler  than  the  interior  of  the  mouth.  The  temperature 
which  a  given  region  of  the  temperature  organ  has  (as 
measured  by  a  thermometer)  when  it  feels  neither  hot  nor 
cold  is  its  temperature- sensation  zero  for  that  time,  and  is 
not  associated  with  any  one  objective  temperature;  for  not 
only,  as  we  have  just  seen,  does  it  vary  in  different  parts  of 
the  organ,  hut  also  on  the  same  part  from  time  to  time. 
Whenever  a  skin  region  passes  with  a  certain  rapidity  to  a 
temperature  above  its  sensation  zero  we  feel  warmth;  and 
vice  versa :  the  sensation  is  more  marked  the  greater  the  dif- 
ference, and  the  more  suddenly  it  is  produced;  touching  a 
metallic  body,  which  conducts  heat  rapidly  to  or  from  the 
skin,  causes  a  more  marked  hot  or  cold  sensation  than  touching 
a  worse  conductor,  as  a  piece  of  wood,  of  the  same  temperature. 

The  change  of  temperature  in  the  organ  may  be  brought 
about  by  changes  in  the  circulatory  apj)aratus  (more  blood 
flowing  tli rough  the  skin  warms  it  and  less  leads  to  its  cool- 
ing), or  by  temperature  changes  in  gases,  liquids,  or  solids  in 
contact  with  it.  Sometimes  we  fail  to  distinguish  clearly 
whether  the  cause  is  external  or  internal;  a  person  coming  in 
from  a  windy  walk  often  feels  a  room  uncomfortably  warm 
which  is  not  really  so ;  the  exercise  has  accelerated  his  circu- 
lation and  tended  to  warm  his  skin,  but  the  moving  outer 
air  1ms  rapidly  conducted  off  the  extra  heat;  on  entering  the 
house  the  stationary  air  there  does  this  less  quickly,  the  skin 
becomes  hotter,  and  the  cause  is  supposed  to  be  oppressive 
heat  of  the  room.  Hence,  frequently,  opening  of  windows 
ami  sitting  in  a  draught,  with  its  concomitant  risks;  whereas 
keeping  quiet  for  five  or  ten  minutes,  until  the  circulation 
had  returned  to  its  normal  rate,  would  attain  the  same  end 
without  danger. 

The  acutenese  of  the  temperature  sense  is  greatest  at  tem- 
peratures within  a  few  degrees  of  30°  0.  (86°  P.);  at,  these, 
differences  of  less  than  .1°  C.  can  be  discriminated.  Asa, 
means  of  measuring  absolute  temperatures,  however,  the  skin 
i-  verv  unreliable,  on  account  of  the  changeability  of  its  sen- 
sation zero.  We  can  localize  temperature  sensations  much 
as  tactile,  imt  not  bo  accurately. 

Arc  Touch  and  Temperature  Sensations  of  Different 
Modality  P     Tactile  and  temperature  reelings  are  ordinarily  SO 


584  the  in  max  nonr. 

\ci\  different  thai  we  can  no  more  compare  them  than  lumi- 
nous and  auditory;  and  if  we  accepl  the  modern  modified 
form  of  the  doctrine  of  specific  nerve  energies  (Chap.  Mil), 
in  accordance  wit  1 1  which  the  same  sensory  fibre  when  ex- 
cited always  arouses  a  sensation  of  the  same  quality,  if  any, 
because  it  excites  the  same  brain-cent  re,  it  is  hard  to  conceive 
how  the  same  fibre  could  at  one  time  arouse  a  tactile,  and  at 
another  a  temperature  sensation.  It  has,  however,  been 
maintained  thai  touch  and  temperature  feelings  sometimes 
pass  into  one  another  insensibly.  If  a  half  dollar  cooled  to  5° 
('.  (41°  F.)  he  placed  on  a  person's  brow,  and  then  two  (one 
on  the  other)  warmed  to  37°  ('.  (98.5°  F.),  he  commonly 
thinks  the  weighl  in  the  two  cases  is  equal;  i.e.,  the  tempera- 
ture difference  leads  to  errors  in  his  pressure  judgments.  But 
this  docs  not  prove  an  identity  in  the  sensations;  the  cold 
half-dollar  may  produce  contraction  of  the  cutaneous  tissues, 
leading  to  compression  of  the  tactile  end  organs,  which  is 
mistaken,  in  mental  interpretation,  for  a  heavier  pressure. 
When  sensations  are  combined  in  other  cases,  as  red  and  blue- 
green  to  produce  white,  or  partial  tones  to  form  a  compound, 
we  either  cannot,  or  can  hut  with  difficulty,  recognize  the 
components;  in  this  case  the  person  feels  both  the  cold  and 
pressure  distinctly  when  the  half-dollar  is  laid  on  him. 

In  certain  cases  a  person  mistakes  the  contact  of  a  piece  of 
raw  cotton  with  his  skin,  for  the  approach  of  a  warm  object  ; 
this  has  been  taken  to  prove  that  touch  and  temperature  feel- 
ings graduate  into  one  another.  However,  the  feeble  touch 
of  the  raw  cotton  might  well  be  less  felt  than  the  increased 
temperature  of  the  skin,  due  to  diminished  radiation  when  it 
was  covered  by  this  non-conducting  substance;  and  the  con- 
stancy with  which,  in  the  ordinary  circumstances  of  life,  we 
feel  and  discriminate  clearly,  on  the  same  skin  region  at  the 
same  time,  both  temperature  and  touch  sensations,  is  a  strong 
argument  against  any  transition  of  one  into  the  other. 

Moreover,  there  is  direct  evidence  that  three  different  ap- 
paratuses in  the  skin  or  at  least  differently  located  apparatuses, 
are  concerned  in  arousing  touch,  beat  and  cold  sensations.  If 
a  metal  point,  lightly  weighted,  lie  slowly  and  evenly  moved 
along  the  skin  by  clockwork,  it  gives  rise  to  sensations  of 
touch  at  some  places  and  if  hotter  or  cooler  than  the  skin 
to  sensations  of  temperature  at  others;  but  never  when  in 
contact  with  one  point   to  more  than  one  sensation.     If  the 


PAIN.     COMMON  SENSATIONS.  585 

points  at  which  the  observed  person  says  I  feel  touch  or  I 
feel  cold  or  I  feel  heat,  be  carefully  marked  on  the  skin  and 
the  experiment  repeated  on  one  or  more  subsequent  days 
the  contact  points  for  the  three  sensations  are  found  to  be 
unchanged.  In  certain  cases  of  spinal-cord  disease,  moreover, 
it  has  been  noticed  that  tactile  sensibility  may  be  lost  while 
temperature  sensibility  remains;  and  in  others  that  the  capac- 
ity of  feeling  warmth  may  be  nearly  or  completely  lost  while 
cold  sensation  remains  normal.  Excluding  pain  ("  abnormal 
sensation"),  we  must  conclude  that  there  are  in  the  skin 
three  distinct  sets  of  nerve-fibres : — One,  when  excited,  arouses 
"  touch  "  sensation;  a  second,  "  warm  "  sensation ;  the  third, 
"  cold  "  sensation. 

Fain  and  Common  Sensibility.  When  the  skin  is  power- 
fully stimulated  by  heat,  cold  or  pressure,  or  is  inflamed,  we 
get  a  new  sensation  which  we  call  pain.  This  is  something 
quite  different  from  the  unpleasantness  caused  by  a  dazzling 
light  or  a  musical  discord  or  a  disagreeable  odor  or  taste. 
We  recognize  these  as  being  still  sight  or  sound  or  smell  or 
taste  sensations.  Pain  on  the  one  hand  is  different  from  any 
of  the  normal  skin  sensations  and,  on  the  other,  is  recog- 
nized in  consciousness  as  often  proceeding  from  diseased  in- 
ternal organs  from  which  normally  we  get  no  noticeable  sen- 
sations. An  exposed  healthy  tendon  is  quite  insensible  to 
touch,  but  if  it  be  inflamed  the  slightest  pressure  may  give 
rise  to  nerve  impulses  causing  very  acute  pain,  and  pain  which 
to  the  consciousness  is  similar  to  cutaneous  pains  or  pains  of 
other  organs.  Since  direct  stimulation  of  the  sensory  nerves 
proceeding  from  the  skin  in  any  way  except  through  their 
end  organs  gives  rise  to  feelings  of  pain  rather  than  to  the 
special  skin  sensations,  and  pressure  and  temperature  feelings 
do  insensibly  give  way  to  pain  feelings  when  the  stimuli  ap- 
plied to  the  skin  are  gradually  increased,  it  has  been  supposed 
that  pain  is  not  due  to  excitation  of  a  special  nerve  apparatus 
of  it-  own,  but  to  over-excitation  of  the  tactile  apparatus. 
On  this  theory  it  would  be  hard  to  account  for  the  fact  that 
skin  pain  is  so  very  different  in  modality  from  a,  touch  or  tem- 
perature feeling,  and  to  understand  why  it  gives  rise  in  con- 
sciousness to  conceptions  concerning  a  condition  of  the  Body 
and  not  of  some  external  object :  it,  is  not  extrinsically  referred 
by  the  mind  to  a  quality  of  anything  bu1  the  painful  part  itself, 
;i    a  dazzling  light  sensation  or  a  fetid  odor  is.     There  is  also 


586  THE  HUMAN  BODT. 

experimental  and  pathological  evidence  that  the  paths  taken  in 

bhe  spinal  cord  by  nerve  impulses  causing  pain  are  different 
from  those  leading  to  a  consciousness  of  touch.     If  certain 

[•acts  of  the  cord  are  cut  in  the  thoracic  region  of  a  rabbit, 
gentle  touches  on  the  hind  limb  appear  to  be  felt;  the  animal 
erects  its  ears  or  moves  its  head:  but  powerful  stimulation  of 
the  sciatic  nerve  causes  no  signs  of  pain,  while  if  the  posterior 
white  columns  he  cut  the  animal  still  can  feel  stimuli  applied 
to  the  hind  limb  and  sufficient  to  cause  pain  under  normal 
conditions,  but  it  appears  insensible  to  gentle  pressure  on  the 
skin.  In  human  beings  very  similar  phenomena  have  been 
observed  in  cases  of  spinal-cord  disease :  and  in  a  certain  stage 
of  chloroform  or  ether  narcosis  the  patient  feels  the  surgeon's 
hand  or  his  knife  where  it  touches  the  skin,  but  he  experiences 
no  pain  when  deeper  parts  are  cut. 

Such  considerations  seem  to  lead  to  the  conclusion  that  the 
nerve-fibres  and  sense  apparatuses  concerned  with  painful 
sensations  are  quite  distinct  from  those  of  all  the  special  senses. 
If  that  be  so  we  must  also  assume  that  there  are  "  pain  " 
fibres  very  widely  distributed  over  the  skin  and  through  most 
other  parts  of  the  Body,  and  usually  not  so  stimulated  as  to 
cause  sensations  which  are  present  in  consciousness.  In  acci- 
dent or  disease  the  afferent  impulses  become  powerful  enough 
to  arouse  perception  and  imperiously  call  attention  to  danger. 
The  nerve-fibres  concerned  may  be  named  "  fibres  of  common 
sensibility,"  and  there  is  reason  to  believe  that,  normally, 
feeble  afferent  impulses  travel  along  them  from  nearly  all 
organs  to  the  fore  brain;  but  so  weak  and  so  uniform  as  not 
to  excite  a  perceived  feeling:  these  impulses  would  thus  form 
a  great  background  of  subconscious  feeling,  on  which  special 
points  from  time  to  time  become  conspicuous  as  one  or  other 
nerve  of  special  sense  is  stimulated  or  some  fibre  of  common 
sensibility  is  abnormally  excited.  So  far  as  the  epidermis  is 
concerned,  the  axis-cylinder  branches,  which  end  in  it  with- 
out any  special  terminal  apparatus,  may  be  specially  fibres  of 
common  sensibility. 

Pains  can  be  localized  though  but  only  imperfectly,  and 
the  less  perfectly  the  more  severe  they  are.  The  exact  place 
of  a  needle  prick  after  removal  of  the  needle  (so  that  there  is 
no  guiding  concomitant  touch  sensation)  cannot  be  recognized 
as  we'd  as  pin  touch  on  the  same  region  of  the  skin,  but  still 
fairly  well;  while  the  acute  pain  caused  by  a  small  abscess 


PAIN.     COMMON  SENSATIONS.  587 

(bone  felon)  under  the  periosteum  of  a  finger  bone  is  often 
felt  all  over  the  forearm;  and  a  single  diseased  tooth  may 
cause  pain  felt  over  the  whole  of  that  side  of  the  face.  This 
is  probably  due  to  imperfection  in  brain  and  spinal  cord  of  the 
isolation  of  the  paths  of  conduction  of  the  nervous  impulses 
concerned. 

Common  Sensations.  These  agree  with  pain  sensations  in 
calling  attention  to  conditions  of  our  Bodies  and  not  of  outer 
things.  Some  of  them,  as  general  malaise  and ' '  feeling  well, ' ' 
are  probably  due  to  modifications  of  the  general  inflow  of  im- 
pulses through  the  apparatus  of  common  sensibility,  not  suffi- 
cient to  cause  a  feeling  of  definite  pain  or  pleasure.  Others, 
as  hunger,  thirst  and  nausea,  may  have  similar  origin,  but  in 
a  more  localized  region. 

Hunger  and  Thirst.  These  sensations,  which  regulate 
the  taking  of  food,  are  peripherally  localized  in  consciousness, 
the  former  in  the  stomach  and  the  latter  in  the  throat,  and 
local  conditions  no  doubt  play  a  part  in  their  production; 
though  general  states  of  the  Body  are  also  concerned. 

Hunger  in  its  first  stages  is  probably  due  to  a  condition  of 
the  gastric  mucous  membrane  which  comes  on  when  the  stom- 
ach has  been  empty  some  time,  and  it  may  be  temporarily 
stilled  by  filling  the  organ  with  indigestible  substances.  But 
soon  the  feeling  comes  back  intensified  and  can  only  be  allayed 
bv  the  ingestion  of  nutritive  substances;  provided  these  are 
absorbed  and  reach  the  blood,  their  mode  of  entry  is  unessen- 
tial ;  the  hunger  may  be  stayed  by  injections  of  food  into  the 
rectum  as  well  as  by  putting  it  into  the  stomach. 

Similarly,  thirst  may  be  temporarily  relieved  by  moisten- 
ing the  throat  without  swallowing,  but  then  soon  returns; 
while  it  may  be  permanently  relieved  by  water  injections  into 
the  veins,  without  wetting  the  throat. 

While  both  sensations  depend  in  part  on  local  peripheral 
conditions,  they  may  also  be,  and  more  powerfully,  excited  by 
poverty  of  the  blood  in  foods  and  water;  such  deficiency 
probably  directly  stimulates  hunger  and  thirst  brain-centres. 

Smell.  The  region  of  the  nostril  nearest  its  outer  end 
possesses  the  sense  of  touch,  and  most  of  its  lining  mucous 
membrane  baa  common  sensibility,  which  can  be  aroused  by 
such  Bubstancefl  as  ammonia  vapor:  the  nerve-fibres  concerned 
in  these  feelinge  are  derived  from  the  superior  maxillary  branch 
of  the  fifth  nerve. 


688 


THE  HUMAN  BODY. 


The  olfactory  organ  proper  consists  of  the  upper  porl ions  of 
the  two  aasal  ca\  ities,  m  er  which  the  endings  of  the  olfactory 
nerves  are  spread  and  where  the  mucous  membrane  lias  a 
brownish-yellow  color.  This  region  (regio  olfactoria)  covers 
the  upper  anl  lower  turbinate  hones,  which  are  expansions  of 
the  ethmoid  on  the  outer  wall  of  the  nostril  chamber,  the 
opposite  part  of  the  partition  between  the  nares,  and  the  part 
of  the  root'  of  the  nose  separating  it  from  the  cranial  cavity. 
The  epithelium  covering  the  mucous  membrane  contains  three 
varieties  of  cells  (2,  Fig.  IT:!).  The  cells  of  one  set  are  much 
like  ordinary  columnar  epithelium,  but  with  long  branched 
processes  attached  to  their  deeper  ends;  mixed  with  these  are 

peculiar  cells,  each  of  which  has 
a  large  nucleus  surrounded  by  a 
little  protoplasm;  a  slender  ex- 
ternal process  reaching  to  the  sur- 
face ;  and  a  very  slender  deep  one. 
The  latter  cells  have  been  sup- 
posed to  be  the  proper  olfactory 
end  organs,  and  to  be  connected 
with  the  fibres  of  the  olfactory 
nerve,  which  enter  the  deeper 
strata  of  the  epithelium  and  there 
divide.  In  Amphibia  the  corre- 
sponding cells  have  fine  filaments 
on  their  free  ends.  The  cells  of 
the  third  kind  are  irregular  in 
form  and  lie  in  several  rows  in  the 
deeper  parts  of  the  epithelium. 
It  may  be  that  the  cylindrical  cells 
if  not  (as  is  possible)  directly  con- 
>m  the  olfactory  cerned  in  olfaction,  have  import- 
iti5  ant  functions  in  regard  to  the 
>p  process    .,  so-   nourishment  of  the  olfactory  cells 

cell:  C,   il^  narrow  J 


Fig.  173. 
epithelium,    l,   from    the  froj; 
from   man:  a,  columnar  cell,  with 
iis   branched   de^i 

called    olfacti      . 

outer  processed,  its  slender  central  which   thev  surround;    they  may 
process.    3,  gray  nerve-fibres  of  the  •  i  .,? 

olfactory  nerve,  seen  dividing  into    for    example     supply     them     with 
fine  peripheral  branches  at  a.  ,,    ,  ,      .    ,  ,n 

needtul  material,  as  the  pigment- 

in  the  formation  of  visual 


cells  of  the  retina  are  concerned 
purple  in  the  rods. 

Odorous  substances,  the  stimuli  of  the  olfactory  apparatus, 
are  always  gaseous  and  frequently  act  powerfully  when  present 
in  very  small  amount.     We  cannot,  however,  classify  them  by 


TASTE.  589 

the  sensations  they  arouse,  or  arrange  them  in  series;  and 
smells  are  but  minor  sensory  factors  in  our  mental  life,  al- 
though very  powerful  associations  of  memory  are  often  aroused 
by  odors.  AVe  commonly  refer  them  to  external  objects,  since 
we  find  that  the  sensation  is  intensified  by  "  sniffing  "  air  into 
the  nose,  and  ceases  when  the  nostrils  are  closed.  Their 
peripheral  localization  is,  however,  imperfect,  for  we  confound 
many  smells  with  tastes  (see  below) ;  nor  can  we  well  judge  of 
the  direction  of  an  odorous  body  through  the  olfactory  sen- 
sations which  it  arouses. 

Taste.  The  organ  of  taste  is  the  mucous  membrane  on 
the  dorsum  of  the  tongue  and,  in  some  persons,  of  the  soft 
palate  and  fauces.  The  nerves  concerned  are  the  glosso- 
pharyngeal, distributed  over  the  hind  part  of  the  tongue, 
and  the  Ungual  branches  of  the  inferior  maxillary  division  of 
the  trigeminals  on  its  anterior  two  thirds. 

On  the  tongue  most  of  the  sensory  nerves  run  to  papilla? ; 
the  circumvallate  have  the  richest  supply,  and  on  these  are 
peculiar  end  organs   (Fig.   174)   known  as  taste  buds;   they 


Fig.  174.— Taste-buds. 

are  oval  and  imbedded  in  the  epidermis  covering  the  side 
of  the  papilla.  Each  consists,  externally,  of  a  number  of  flat, 
fusiform,  nucleated  cells  and,  internally,  of  six  or  eight  so- 
called  taste-cells.  The  latter  are  much  like  the  olfactory  cells 
of  the  nose,  and  are  probably  connected  with  nerve-fibres  at 
their  deeper  cw]*.  The  capsule  formed  by  the  enveloping 
cells  has  a  small  openini:  on  the  surface;  each  taste-cell  termi- 
nates in  a  very  fine  thread  vrhich  there  protrudes.  Taste- 
buds  are  also  found  on  some  of  the  fungiform  papillae,  and 
it  is  possible  that  Bimpler  structures,  not  yet  recognized,  and 
consisting  of  single   taste-cells  are   widely  spread  over  the 


590  THE  HUMAN  BODY. 

tongue,  since  the  sense  of  taste  exists  where  no  taste-buds  can 
be  found.     The  filiform  papillae  are  probably  tactile. 

That  substances  be  tasted  they  must  be  in  solution:  wipe 
the  tongue  dry  and  put  a  crystal  of  sugar  on  it;  no  taste 
will  be  felt  until  exuding  moisture  lias  dissolved  some  of  the 
crystal.  Excluding  the  feelings  aroused  by  acid  substances, 
tastes  proper  may  be  divided  into  sweet,  bitter,  acid,  and 
saline.  Although  contributing  much  to  the  pleasures  of 
life,  they  are  intellectually,  like  smells,  of  small  value;  tbe 
perceptions  we  attain  through  them  as  to  qualities  of  external 
objects  being  of  little  use,  except  as  aiding  in  the  selection  of 
food,  and  for  that  purpose  they  are  not  safe  guides  at  all 
times. 

Many  so-called  tastes  (flavors)  are  really  smells ;  odoriferous 
particles  of  substances  which  are  being  eaten  reach  the  olfac- 
tory region  through  the  posterior  nares  and  arouse  sensations 
which,  since  they  accompany  the  presence  of  objects  in  the 
mouth,  we  take  for  tastes.  Such  is  the  case,  e.g.,  with  most 
spices;  when  the  nasal  chambers  are  blocked  or  inflamed  by 
a  cold  in  the  head,  or  closed  by  compressing  the  nose,  the  so- 
called  taste  of  spices  is  not  perceived  when  they  are  eaten ;  all 
that  is  felt,  when  cinnamon,  e.g.,  is  chewed  under  such  cir- 
cumstances is  a  certain  pungency  due  to  its  stimulating  nerves 
of  common  sensation  in  the  tongue.  This  fact  is  sometimes 
taken  advantage  of  in  the  practice  of  domestic  medicine  when 
a  nauseous  dose,  as  rhubarb,  is  to  be  given  to  a  child.  Tactile 
sensations  play  also  a  part  in  many  so-called  tastes. 

As  the  tongue,  in  addition  to  taste  functions,  possesses 
tactile,  temperature,  and  general  sensibility,  its  nerve  ap- 
paratus must  be  complex;  and  there  is  even  reason  to  be- 
lieve that  different  nerve-fibres  with  presumably  different  end 
organs  are  concerned  in  the  different  true  tastes.  Mist 
persons  taste  bitter  things  better  with  the  back  part  of  the 
tongue  and  sweet  things  with  the  tip,  and  in  some  persons 
the  separation  of  function  is  quite  complete.  Chemical  com- 
pounds are  known  which  in  such  persons  cause  a  pure  sweet 
sensation  if  placed  on  the  tongue  tip  and  a  pure  bitter  sensa- 
tion if  placed  in  the  region  of  the  circumvallate  papillae; 
these  facts  seem  to  show  that  the  fibres  concerned  in  bitter 
and  sweet  sensation  are  distinct.  Again,  if  leaves  of  a  certain 
plant  {Oymnema  sylvestre)  be  chewed,  the  capacity  to  taste 
sweet  or  bitter  things  is  lost  for  some  time,  but  salts  and  acids 


THE  MUSCULAR  SENSE.  591 

are  tasted  as  well  as  usual;  aud  most  persons  taste  salines 
better  at  the  sides  of  the  tongue  than  elsewhere;  so  that  the 
salt  and  acid  sensations  seem  to  have  a  different  apparatus,  not 
only  from  the  sweet  and  bitter,  but  from  one  another 

The  Muscular  Sense.  The  muscles  are  endowed  with  com- 
mon sensibility,  as  proved  by  the  pains  of  cramp  and  fatigue, 
but  in  connection  with  them  we  have  other  sensations  of  great 
importance,  although  they  do  not  often  become  so  obtrusive 
in  consciousness  as  to  arouse  separate  attention.  Certain  of 
these  feelings  {muscle  sensations  proper)  are  due  to  the  ex- 
citation of  sensory  nerves  ending  within  the  muscles  them- 
selves: others  {innervation  sensations)  have  possibly  a  central 
origin  and  accompany  the  starting  of  volitional  impulses  from 
brain-cells;  they  are  only  felt  in  connection  with  the  voluntary 
skeletal  muscles. 

We  have  at  any  moment  a  fairly  accurate  knowledge  of  the 
position  of  various  parts  of  our  Bodies,  even  when  we  do  not 
see  them ;  and  we  can  also  judge  fairly  accurately  the  extent 
of  a  movement  made  with  the  eyes  shut.  The  afferent  nerve 
impulses  concerned  in  the  development  of  such  judgments  may 
be  various;  different  parts  of  the  skin  are  pressed  or  creased; 
different  joints  are  subjected  to  pressure;  different  tendons 
are  put  on  the  stretch  and  different  muscles  are  in  different 
states  of  contraction,  and  it  is  by  no  means  easy  to  determine 
the  part  played  in  each  case  by  the  sensory  nerves  of  the 
different  organs.  Moreover,  when  we  push  against  an  object, 
or  lift  it,  we  are  able  to  form  a  judgment  as  to  the  amount  of 
effort  exerted ;  but  here  again  pressure  on  skin  and  joints  and 
tension  of  tendons  come  in.  Although  under  normal  circum- 
stances the  skin  sensations  are  undoubtedly  of  importance,  they 
are  not  necessary:  persons  with  cutaneous  paralysis  can,  apart 
from  sight,  judge  truly  the  position  of  a  limb  and  the  extent 
of  movement  made  by  it ;  and  in  many  movements  change  in 
joint  pressure  must  be  very  little  if  any.  We  have  then  to 
look  to  muscles  and  tendons  themselves  for  an  important 
part  of  the  sensations,  and  in  both  muscles  and  tendons  there 
are  organ  i  in  connection  with  nerve-fibres  which  are  certainly 
sensory  in  nature:  moreover,  muscle  sensory  nerves,  whether 
through  the  organs  of  Golgi  or  some  other  end  organ,  appear 
to  he  excited  by  mere  passive  change  of  form  in  the  muscle: 
with  the  eves  closed  each  of  us  can  tell  how  much  another 
person  has  lifted  one  of  our  arms. 


592  THE  HUMAN  BODY. 

Whether,  in  addition  to  the  true  muscle  sense,  dependent 
on  afferent  impulses  sent  to  the  brain  from  the  contracted 
muscle  or  its  tendons,  we  have  a  more  direct  consciousness  of 
the  amount  of  will  exerted  to  produce  a  given  muscular  con- 
traction, and  can  form  thereby  a  judgment  as  to  the  extent 
of  the  movement  or  effort,  is  a  question  .-till  in  dispute.  A 
main  argument  in  favor  of  the  existence  of  such  centrally  origi- 
nating "  innervation  sensations"  is  based  on  phenomena  ob- 
served in  persons  afflicted  with  paresis.  They  frequently 
judge  erroneously  for  a  time  as  to  the  extent  of  movements  made 
by  them,  thinking  that  the  movement  is  greater  than  it  really 
is.  It  is  argued  that  in  such  cases  the  error  cannot  lie  based 
on  peripheral  sensations,  but  must  be  due  to  the  fact  that 
the  person  judges  by  the  amount  of  volitional  effort  he  has 
made,  which  was  such  as  in  his  previous  condition  of  health 
would  have  produced  a  greater  muscular  contraction  than  it 
now  does  in  his  paretic  condition.  It  is  especially  in  connec- 
tion with  eve  muscles  that  such  errors  have  been  noticed. 
When  we  follow  a  moving  object  with  the  eyes  we  judge  of 
the  rate  of  movement  by  the  degree  of  contraction  of  the  ocular 
muscles  needed  to  keep  its  image  on  the  two  foveas:  if  the  eye 
muscles  become  suddenly  enfeebled  the  person  at  first  thinks 
he  turns  his  eyeballs  faster  than  he  really  does  and  hence  that 
the  object  is  moving  faster  than  it  actually  does:  or  he  may  not 
move  his  eye  at  all  when  he  has  willed  to  do  so,  and  hence 
conclude  that  stationary  objects  are  in  motion  because  their 
images  are  still  formed  on  the  same  region  of  the  retina,  which 
could  not  be  the  case  with  stationary  objects  if  the  position  of 
the  eyes  were  changed. 

Whether  the  sensations  by  which  we  judge  the  extent  of  a 
muscular  movement  be  entirely  peripheral  or  in  part  central, 
they  enable  us  to  determine  very  minute  differences  of  con- 
traction: the  ocular  determination  of  the  distance  of  an  object 
not  too  far  off  to  have  its  absolute  distance  determined  with 
considerable  accuracy,  dejiends  almost  entirely  upon  judg- 
ments based  upon  very  small  changes  in  the  degree  of  con- 
t  faction  of  the  internal  and  external  straight  (recti)  muscles, 
converging  or  diverging  the  eyeballs;  and  of  the  ciliary  muscle 
determining  the  necessary  accommodation  of  the  lens.  A 
singer,  too,  must  be  able  to  judge  with  great  minuteness  the 
degree  of  contraction  of  the  small  muscles  of  the  larynx  nec- 
essary  to  produce  a  certain  tension  of  the  vocal  cords.     It  may 


THE  MUSCULAR  SENSE.  593 

be  well  to  point  out  that  we  do  not  refer  a  muscular  sensation 
to  any  given  muscle  or  muscles:  it  is  merely  associated  with  a 
certain  movement  or  position,  and  a  person  who  knows  noth- 
ing about  his  ocular  muscles  can  judge  distance  through  sen- 
sations derived  from  them,  quite  as  well  as  any  anatomist. 
This  fact  is  of  course  correlated  with  the  fact  that  in  voluntary 
movement  we  do  not  make  a  conscious  effort  to  contract  any 
particular  muscles:  the  higher  nerve  centres  are  merely  con- 
cerned with  the  initiation  of  a  given  movement  of  a  given  ex- 
tent, and  all  the  details  are  carried  out  by  lower  co-ordinating 
centres.  In  ordinary  daily  life  in  fact  we  have  no  interest 
whatever  in  a  muscular  contraction  per  se ;  all  we  are  con- 
cerned with  is  the  result,  and  consciousness  has  never  had  need 
to  trouble  itself,  if  it  could,  with  associating  a  particular  feel- 
ing or  a  particular  movement  with  any  individual  muscle. 

Muscular  feelings  are,  as  already  pointed  out,  frequently 
and  closely  combined  not  only  with  visual  but  also  with  tactile, 
in  providing  sensations  on  which  to  base  judgments:  in  the 
dark,  when  an  object  is  of  such  size  and  form  that  it  cannot 
be  felt  all  over  by  any  one  region  of  the  skin,  we  deduce  its 
shape  and  extent  by  combining  the  tactile  feelings  it  gives  rise 
to,  with  the  muscular  feelings  accompanying  the  movements 
of  the  hands  over  it.  Even  when  the  eyes  are  used  the  sen- 
sations attained  through  them  mainly  serve  as  short-cuts  which 
we  have  learned  by  experience  to  interpret,  as  telling  us  what 
tactile  and  muscular  feelings  the  object  seen  would  give  us  if 
felt;  and,  in  regard  to  distant  points,  although  we  have  learnt 
to  apply  arbitrarily  selected  standards  of  measurement,  it  is 
probable  that  distance,  in  relation  to  perception,  is  primarily 
a  judgment  as  to  how  much  muscular  effort  would  be  needed 
to  come  into  contact  with  the  thing  looked  at. 

When  we  wish  to  estimate  the  weight  of  an  object  we  al- 
ways, when  possible,  lift  it,  and  so  combine  muscular  with 
tactile  sensations.  By  this  means  we  can  form  much  better 
judgments.  While  with  touch  alone  just  perceptibly  differ- 
ent pressures  have  the  ratio  1  :  3,  with  the  muscular  sense 
added  differences  of  -fa  can  be  perceived. 


CHAPTER   XXXVI. 
THE   SPINAL  CORD  AND  KEFLEX  ACTIONS. 

The  Special  Physiology  of  Nerve-Centres.  We  have  al- 
ready studied  the  general  physiological  properties  of  nerves 
and  nerve-centres  (Chap.  X11I)  and  found  that  while  the 
former  are  mere  transmitters  of  nervous  impulses,  the  latter 
do  much  more  than  merely  conduct.  In  sonic  cases  the  centres- 
are  automatic;  they  originate  nerve  impulses,  as  illustrated  by 
the  rhythmic  impulses  starting  from  the  respiratory  centre. 
In  other  cases  a  feeble  impulse  reaching  the  centre  gives  rise  to 
a  great  discharge  of  energy  from  it  (as  when  an  unexpected 
noise  produces  a  violent  start,  due  to  many  impulses  sent  out 
from  the  excited  centre  to  numerous  muscles),  so  that  certain 
centres  are  irritable,.  Such  nerve-centres  contain  a  store  of 
energy-liberating  material  which  only  needs  a  slight  disturb- 
ance to  upset  its  equilibrium  and  initiate  powerful  efferent 
impulses  as  the  result  of  one  feeble  afferent.  Further,  the  im- 
pulses thus  liberated  are  co-ordinated.  When  mucus  in  the 
larynx  tickles  the  throat  and  excites  afferent  nerve  impulses, 
these,  reaching  a  centre,  cause  discharges  along  many  efferent 
fibres,  so  combined  in  secpience  and  power  as  to  produce,  not 
a  mere  aimless  spasm,  but  a  cough  which  clears  the  passage. 
In  very  many  cases  the  excitation  of  centres,  with  or  without 
at  the  same  time  some  of  the  above  phenomena,  is  associated 
with  sensations  or  other  states  of  consciousness. 

We  have  now  to  study  which  of  these  powers  is  manifested 
by  different  parts  of  the  central  cerebro-spinal  nervous  system, 
and  under  what  circumstances  and  in  what  degree:  and  also 
some  of  the  phenomena  of  conduction  in  spinal  cord  and 
brain. 

Conduction  in  the  Spinal  Cord.  The  spinal  cord,  form- 
ing (except  the  slender  sympathetic)  the  only  direct  com- 
munication between  the  brain  and  most  of  the  nerves  of  the 
Body,  was  considered  by  the  older  physiologists  as  merely  a 
huge  nerve-trunk,  into  which  the  various  spinal  nerves  were 

594 


THE  SPINAL    CORD  AND  REFLEX  ACTIONS.       595 

collected  on  their  way  to  the  encephalon.  It  does,  it  is  true, 
contain  the  paths  for  the  conduction  of  all  those  impulses 
which,  originating  in  the  cerebrum,  give  rise  to  voluntary 
movements  of  the  trunk  and  limbs;  also  for  all  the  centrally 
travelling  impulses  which  give  rise  to  sensations  ascribed  to 
those  parts;  and  it  is  also  the  path  for  certain  impulses  giving 
rise  to  involuntary  movements  as,  for  example,  those  which, 
originating  in  the  respiratory  centre,  travel  to  the  phrenic  and 
intercostal  nerves. 

If,  however,  the  cord  were  merely  collected  and  continued 
nerve-roots  it  ought  to  increase  considerably  in  bulk  as  it  ap- 
proached the  skull,  and  this  it  does  not  do  in  anything  like 
the  required  proportion;  a  histological  enumeration  also  shows 
that  the  total  number  of  fibres  cut  across  in  a  transverse  sec- 
tion of  the  cord  in  the  upper  cervical  region  is  far  less  than 
the  total  number  of  fibres  in  all  the  spinal  nerve-roots.  Most 
of  the  root-fibres,  in  fact,  pass  at  once  into  the  central  gray 
mass  and  their  axis  cylinders  end  in  its  cells,  or  lose  their  in- 
dividuality by  joining  its  network  of  cell  branches  and  fine 
non-medullated  fibres.  Most  of  the  fibres  of  the  anterior  root 
end  in  nerve-cells  near  the  level  at  which  they  join  the  cord, 
especially  in  the  cells  of  the  anterior  horns :  many  of  the  fibres 
of  the  posterior  roots  also  join  the  gray  network,  either  at  or 
a  little  above  or  a  little  below  the  level  at  which  they  reach 
the  cord,  but  some  appear  to  run  on  to  the  brain  without  en- 
tering the  gray  core.  Those  which  do  pass  into  it  probably 
break  up  in  its  network  and  are  not  directly  continued  into 
a  cell,  but  this  is  still  uncertain.  In  correspondence  with  the 
fact  that  most  of  the  spinal  nerve-fibres  have  their  primary  ter- 
mination in  it  near  their  point  of  entry,  is  the  fact  that  the 
amount  of  gray  matter  at  any  level  is  greater  or  less  accord- 
ing as  the  nerve-roots  at  that  level  are  large  or  small:  the 
cervical  and  lumbar  enlargements  for  example  are  almost 
entirely  due  to  increase  of  gray  matter  in  those  regions. 
When  wo  make  a  voluntary  movement  of  a  limb  the  impulse 
orginating  in  the  brain  does  not  pass  directly  to  the  motor 
aervee  of  the  muscles  concerned,  but  to  a  mechanism  in  the 
gray  matter  of  the  cord,  which  is  in  connection  with  those 
muscles;  and  when  we  feci  an  object  touching  the  finger,  the 
afferenl  impulses  probably,  though  not  so  certainly,  firs!  enter 

the   gray  '-ore  of   the   cord    and    thence   make  a   fresh  start  to 

the  brain.     When  the  blood-vessels  constrict  on  painful  stimu- 


596  TEE  HUMAN  BODY. 

latinii  of  the  sciatic  nerve,  impulses  must  travel  from  the 
lumbar  enlargement  of  the  cord  to  the  vaso-constrictor  centre 
in  the  medulla  and  reflex  afferent  impulses  from  it  down  the 
cord  to  the  region  of  the  gray  matter  from  which  the  anterior 
roots  conveying  motor  fibres  for  the  blood-vessels  pass  out 
Although  part  of  the  whole  course  of  such  impulses  lies 
iu  the  gray  core,  yet  most  of  it,  in  the  normal  physiological 
working  of  the  Body  lies,  so  far  as  the  cord  is  concerned, 
in  its  white  columns,  ami  we  have  now  to  try  and  track 
these  paths:  as  also  paths  of  special  conduction  between 
different  regions  of  the  spinal  gray  matter  themselves.  The 
gray  matter  of  the  cord  being  directly  continuous  with  the 
gray  matter  of  the  medulla  oblongata  and  through  it  with  that 
of  some  other  parts  of  the  brain  can  transmit  impulses  after 
all  the  white  columns  of  the  cord  have  been  divided,  hut  with 
such  conduction  we  are  not  for  the  present  concerned. 

To  determine  the  special  paths  in  the  white  substance  of 
the  cord  from  and  to  the  brain  several  methods  have  been 
employed.  Experiment  on  animals  as  to  loss  of  sensation  or 
the  power  of  voluntary  movement  in  parts  supplied  by  nerves 
arising  from  the  cord  posterior  to  a  partial  transverse  section 
give  on  the  whole  unsatisfactory  results:  partly  because  of  the 
difficulty  in  exactly  limiting  the  section  and  partly  because 
of  the  general  shock  to  the  nervous  system  resulting  from  the 
operation.  Still  something  has  been  learned  in  that  way, 
and  something  also  from  observations  on  persons  suffering 
from  more  or  less  localized  diseases  of  the  spinal  cord.  Direct 
stimulation  of  parts  of  the  cord  exposed  by  transverse  section 
have  also  given  some  results;  but  more  satisfactory  evidence 
as  to  tracts  of  conduction  between  the  brain  and  cord  have 
been  obtained  by  the  Wallerian  method  and  by  the  study 
of  development.  Removal  or  disease  of  certain  parts  of 
the  brain  and  partial  cross-sections  of  portions  of  it  or  of 
the  cord  itself,  give  rise  to  degeneration  of  localized  groups  of 
fibres  in  parts  of  the  cord  posterior  to  the  disease  or  injury, 
these  are  tracts  of  descending  degeneration.  Partial  cross- 
section  of  other  parts  of  the  cord  or  of  the  posterior  spinal 
roots  lead  to  degenerations  above  the  injury  or  ascending  de- 
generations:  and  in  general  all  the  fibres  which  degenerate  as 
a  result  of  a  given  injury  acquire  in  embryonic  development 
their  medullary  sheaths  at  the  same  time,  which  is  different 
from  the  period  at  which  other  groups  acquire  theirs.     Finally, 


THE  SPINAL   CORD  AND  REFLEX  ACTIONS.        597 


some  regions  of  the  white  substance  of  the  cord  undergo  no 
degeneration  as  the  result  of  injuries  above  or  below  them. 

The  details  as  to  the  result  of  sections  or  injuries  at  various 
levels  differ  considerably,  but  their  broad  features  are  indi- 
cated in  Fig.  175,  in  which  tracts  of  descending  degeneration 


deal 


Fig.  175.— Diagram  of  a  cross-section  of  the  spinal  cord  near  the  upper  part  of 
the  cervical  enlargement  to  indicate  the  main  tracts  of  ascending  and  descending 
degenerations.  The  gray  matter  is  in  solid  black;  tracts  of  descending  degenera- 
tion are  shaded  in  vertical  and  of  ascending  in  horizontal  lines;  pt,  pyramidal  or 
crossed  pyramidal  tract;  dpi,  direct  pyramidal  tract;  dc-al,  descending  antero- 
lateral tract ;  ct,  comma  tract;  cbt.  cerebellar  tract;  ac-al.  ascending  antero-lateral 
tract;  s,  e,  f,  c,  posterior  median  tract:  It,  tract  of  Lissauei-,  epc,  external  posterior 
column;  {.«{ internal  antero-lateral  column. 

are  shaded  in  vertical  lines,  and  of  ascending  degeneration  in 
black.  It  represents  a  cross-section  of  the  cord  at  about  the 
level  of  the  fifth  cervical  nerve.  The  descending  area  of  de- 
generation, pt,  is  the  pyramidal  tract  or  crossed  pyramidal 
tract;  its  fibres  degenerate  posterior  to  any  hemisection  of  the 
cord  on  the  same  side,  and  to  section  of  the  anterior  pyramid 
of  the  medulla  oblongata,  or  of  the  crus  cerebri  on  the 
opposite  side,  or  as  a  result  of  disease  or  lesions  of  certain 
parts  of  the  convolutions  of  the  cerebral  hemisphere  of  the 
opposite  side.  This  tract  is  large  in  the  upper  part  of  the 
cord  and  becomes  smaller  the  further  down  it  is  examined, 
because  fibres  arc  all  the  way  separating  from  it  to  end  in  the 
gray  matter  of  the  cord,  where  they  join  the  mechanisms  from 
which  the  motor  fibres  of  the  anterior  spinal  roots  arise.  The 
fibres  of  the  pyramidal  tract  originate  and  have  their  mitri- 
centres  in  what  is  known  as  the  motor  area  of  the 
cerebral  cortex:  from  there  they  converge  and  arc  collected 
into  the  ventral  portion  of  the  crus  cerebri  and  pass  through 
it  and  the  pons  Varolii  to  the  ventral  median  portion  (an- 
terior pyramid)  of  the  medulla  oblongata,  and  there  cross  the 


598  THE  HUMAN  BODY. 

middle  line  ;d  what  is  known  as  the  decussation  of  the  pyr- 
amids and  enter  the  spinal  cord  on  the  opposite  Bide.  The 
area  of  descending  degeneration,  dpt,  lying  close  to  the  an- 
terior fissure  is  the  direct  pyramidal  tract.  Its  fibres  arise  in 
the  same  cerebral  region  as  those  of  pyramidal  tract,  and  have  ;i 
similar  course  and  ending,  except  thai  they  do  no!  cross  to  the 
other  side  in  the  medulla  oblongata,  hut  gradually  pass  over 
in  the  spinal  cord  itself,  to  end  in  the  gray  matter  connected 
with  the  origin  of  the  anterior  spinal  roots:  the  direct  pyram- 
idal tract  does  not  extend  so  Ear  down  the  cord  as  the  crossed 
tract,  ///.  Another  traci  of  descending  degeneration  i.-  deal, 
the  descending  an  ten  (-lateral:  it  represents  rather  an  area  over 
which  are  to  lie  found  some  degenerated  fibres  scattered  among 
many  undegenerated,  than  a  distinct  group  of  fibres.  The 
same  may  be  said  of  ct,  tin  comma  tract:  it  only  extends 
a  short  way  down  in  the  external  posterior  column  of  the  cord 
after  a  hemisection  has  been  made  on  the  same  side.  It-  fibres 
are  posterior  root-fibres  running  back  in  the  white  matter  a 
little  distance  before  entering  the  gray  core. 

A  conspicuous  tract  of  ascending  degeneration  is  the  cere- 
bellar tract  cb.t.  It  lies  on  the  outer  side  of  the  pyramidal 
tract  and  can  be  traced  along  the  dorsal  side  of  the  medulla 
oblongata  to  the  cerebellum.  It  commences  in  the  lumbar 
region  of  the  cord,  and  seems  to  contain  two  sets  of  fibres; 
some  originating  in  the  gray  matter  and  passing  on  to  re-enter 
it  at  a  higher  level  of  the  cord;  and  others  continued  to  the 
cerebellum.  The  nerve-fibres  of  this  tract  are  very  large. 
Another  important  ascending  tract,  s,e,  t,  c,  lies  in  the  median 
portion  of  the  posterior  white  column  and  is  named  the  median 
posterior  tract.  It  commences  in  the  lower  portion  of  the 
cord  and  gradually  increases  in  size  upwards.  Its  degenera- 
tion follows  not  only  section  of  the  posterior  column,  but 
section  of  the  dorsal  roots  of  the  spinal  nerves:  sections  of 
these  roots  in  the  sacral,  lumbar,  thoracic,  and  cervical  nerves 
cause  degenerations  in  the  areas  marked  respectively  s,  e,  t,  -■; 
hemisection  of  the  cord  is  followed  above  the  section  by  de- 
generations in  this  tract  corresponding  to  all  fcixe  spinal  nerves 
which  join  the  cord  below  the  section.  The  posterior  median 
tract  is  lost  as  a  distinct  tract  in  the  medulla  oblongata:  its 
fibres  are  nearly  all  small.  The  ascending  antero-lateral  tract, 
ac.al,  contains  many  fibres  which  undergo  degeneration  after 
section  of  the  cord  on  the  same  side,  mixed  with  many  fibres 


THE  SPINAL   CORD   AND  REFLEX  ACTIONS.        599 

which  do  not  degenerate.  It  resembles  the  cerebellar  tract 
and  differs  from  the  median  posterior  in  only  undergoing  de- 
generation after  section  of  the  cord  itself,  and  not  of  the  pos- 
terior roots  also.  The  upward  ending  of  its  fibres  is  still 
uncertain,  but  is  probably  in  the  cerebellum:  the  origin  of  the 
fibres  is  in  the  gray  matter  of  the  cord. 

Allowing  for  all  the  tracts  of  degeneration  above  described 
it  will  be  seen  that  considerable  portions  of  the  white  col- 
umns of  the  cord  (left  unshaded  in  Fig.  175)  are  un- 
affected: at  the  most,  trifling  degenerations  extending  a  little 
way  above  or  below  the  point  of  cross-section  are  found. 
Some  of  these  are  due  to  bundles  of  posterior  root-fibres  which 
run  for  a  short  distance  in  the  external  posterior  column,  epc, 
before  separating  into  two  sets,  one  entering  the  gray  matter 
of  the  posterior  cornu,  the  other  passing  into  the  internal 
median  tract.  Another  bundle  of  posterior  root-fibres  runs 
up  in  the  cord  a  short  way  in  what  is  named  the  column  of 
Lissauer,  It,  and  gives  rise  to  an  ascending  degeneration  ex- 
tending a  short  way  above  any  hemisection.  The  main  bulk 
of  the  unshaded  parts  of  the  figure,  however,  represents  longi- 
tudinal fibres  which  do  not  degenerate  up  or  down  after  section 
of  the  cord :  they  appear,  therefore,  to  have  nutritive  centres 
at  each  end;  and  probably  are  fibres  uniting  different  levels 
of  the  gray  matter.  In  addition  to  the  longitudinal  are  of 
course  some  horizontal:  these  are  partly  fibres  of  spinal  roots 
passing  into  the  gray  core,  partly  medullated  fibres  crossing 
the  middle  line  in  the  anterior  white  commissure;  and  in 
addition  to  fibres  of  the  gray  matter  proper  uniting  its  halves 
across  the  middle  line  are  many  fine  medullated  fibres  which 
run  dorso-ventrally  and  from  side  to  side  in  it. 

As  regards  longitudinal  conduction  in  the  Avhite  columns  of 
the  cord,  we  may  sum  up  the  main  facts  as  follows:  The  py- 
ramidal and  direct  pyramidal  tracts  consist  of  efferent  fibres 
uniting  the  cerebral  cortex  with  various  levels  of  the  gray 
matter  of  the  cord  from  which  motor  fibres  for  the  voluntary 
muscles  pass  out.  The  descending  antero-lateral  trad  prob- 
ably also  contains  efferent  fibres  uniting  tin-  brain  with 
different  parts  of  the  gray  matter  of  the  cord.  The  cerebellar 
and  ascending  lateral  tracts  convey  afferent  impulses  from  the 
gray  matter  of  the  cord  to  the  brain,  bnt  are  only  indirectly 
connected  with  the  fibres  of  the  dorsal  spinal  roots.  The 
median  posterior  tract  is  afferent  and  mainly  made  of  fibres 


600  THE  HUMAN  BODY. 

which  pass  directly  from  the  dorsal  spinal  roots  to  the  brain 
without  intervention  of  the  gray  matter  of  the  cord;  but  some 
of  its  fibres  pass  into  the  gray  matter  of  the  cord  before 
reaching  the  medulla  oblongata.  Finally,  the  tracts  which 
show  no  special  ascending  or  descending  degenerations  are 
mainly  made  of  longitudinal  commissural  fibres  uniting  differ- 
ent regions  of  the  gray  matter  of  the  cord. 

The  Spinal  Cord  as  a  Reflex  Centre.  Jn  order  to  explain 
physiological  facts  we  must  assume  in  addition  to  the  sjiecial 
paths  of  union  between  parts  of  the  gray  matter  of  the  cord 
afforded  by  certain  fibres  of  the  white  columns,  first,  that  a 
nervous  impulse  entering  the  gray  network  at  any  point  may, 
under  certain  conditions,  travel  all  through  it,  and  give  rise 
to  efferent  impulses  emerging  at  any  level;  and,  on  the  other 
hand,  that  there  are  certain  lines  or  paths  of  easiest  propa- 
gation between  different  points  in  this  network,  which  the 
impulses  keep  to  under  ordinary  conditions. 

When  a  frog  is  decapitated  it  lies  down  squat  on  its  belly 
instead  of  assuming  the  more  erect  position  of  the  uninjured 
animal;  its  respiratory  movements  cease  (their  centre  being 
removed  with  the  medulla) ;  the  hind  legs  at  first  remain 
sprawled  out  in  any  position  into  which  they  may  happen  to 
fall,  but  after  a  time  are  drawn  up  into  their  usual  position, 
with  the  hip  and  knee-joints  flexed :  having  made  this  move- 
ment the  animal,  if  protected  from  external  stimuli,  makes  no 
other  by  its  skeletal  muscles ;  it  has  lost  all  spontaneity,  and 
only  stirs  under  the  influence  of  immediate  excitation.  Nev- 
ertheless the  heart  goes  on  beating  for  hours;  the  muscles 
and  nerves,  when  examined,  are  found  to  still  have  all  their 
usual  physiological  properties;  and,  by  suitable  irritation,  the 
animal  can  be  made  to  execute  a  great  variety  of  complex 
movements.  But  it  is  no  longer  a  creature  with  a  will,  doing 
things  which  ve  cannot  predict;  it  is  an  instrument  which 
can  be  played  upon,  giving  different  responses  to  different 
stimuli  (as  different  notes  are  produced  when  different  ke}rs 
of  a  piano  are  struck),  and  always  the  same  reaction  to  the 
same  stimulus;  so  that  we  can  say  beforehand  what  will  hap- 
pen when  we  touch  it.  Such  actions  are  called  reflex  or  excito- 
motor  and  fall  into  two  groups:  (1)  orderly  or  purpose-like 
reflexes,  which  are  correlated  to  the  stimulus  and  are  often 
defensive,  tending,  for  instance,  to  remove  an  irritated  part 
from  the  irritating  object ;  (2)  disorderly  or  convulsive  reflexes, 


THE  SPINAL   CORD  AND  REFLEX  ACTIONS.        601 

not  tending  to  produce  any  definite  result,  and  affecting  either 
a  limited  region  or  all  the  muscles  of  the  body. 

In  higher  animals  similar  phenomena  may  be  observed.  If 
a  rabbit's  spinal  cord  be  divided  at  the  bottom  of  the  neck  the 
animal  is  at  first  thrown  into  a  flaccid  limp  condition  like  the 
frog,  but  it  soon  recovers.  Voluntary  movements  in  muscles 
supplied  from  the  spinal  cord  behind  the  section  are  never  seen 
again;  but  on  pinching  the  hind  foot  it  is  forcibly  withdrawn. 
Men,  whose  spinal  cord  has  been  divided  by  stabs  or  disease 
below  the  level  of  the  fifth  cervical  spinal  roots  (above  which 
the  fibres  of  the  phrenic  nerve,  which  are  necessary  for  breath- 
ing, pass  out) ,  sometimes  live  for  a  time,  but  can  no  longer  move 
their  legs  by  any  effort  of  the  will,  nor  do  they  feel  touches, 
pinches,  or  hot  things  applied  to  them ;  if,  however,  the  soles 
of  the  feet  be  tickled  the  legs  are  thrown  into  vigorous  move- 
ment. As  a  rule,  however,  orderly  reflexes  are  less  marked 
and  less  numeroiis  in  the  higher  animals;  in  them  the  organ- 
ization is  less  machine-like,  the  spinal  cord  being  more  the 
servant  of  the  larger  brain,  and  less  capable  of  working  with- 
out directions.  Such  animals,  when  intact,  can  to  a  greater 
extent  control  the  muscular  responses  which  shall  be  made  to 
stimuli  under  various  conditions;  they  have  less  automatic 
protection  in  the  ordinary  risks  of  life,  but  a  greater  range  of 
possible  protection.  The  human  spinal  cord,  controlled  by 
the  brain,  can  adapt  the  reactions  of  the  Body,  with  great 
nicety,  to  a  vast  variety  of  conditions;  the  frog's  cord  by  itself 
does  this  for  a  smaller  number  of  possible  emergencies  without 
troubling  at  all  such  brain  as  the  animal  has,  but  is  less  com- 
pletely under  the  control  of  the  higher  centres  for  adaptation 
to  other  and  more  complex  conditions.  The  difference  being, 
however,  but  one  of  degree  and  not  of  kind,  it  is  best  to 
approach  the  study  of  the  reflex  actions  of  the  human  spinal 
cord  through  an  examination  of  those  exhibited  by  the  frog. 

The  Ordinary  Reflex  Movements  of  a  Decapitated. 
Prog.  por  the  occurrence  of  these  the  following  parts  must  he 
intact:  (a)  the  end  organs  of  sensory  nerve-fibres ;  (#)  afferent 
fibres  from  these  to  the  cord;  (c)  efferent  fibres  from  the 
Cord  to  the  muscles;  (d)  the  part  of  the  spin ;il  cord  between 
the  afferent  and  efferent  fibres ;  (<>)  the  mnscles  concerned  in 
the  movement.  If  the  decapitatc<l  animal  be  suspended  ver- 
tically after  the  Bhock  of  the  operation  is  over,  it  makes  a  few 
attempts  to  hold  its  hind  legs  in  their  usual  flexed  position; 


602  THE  HUMAN  BODY. 

these  soon  cease,  the  legs  hang  down,  and  the  creature  comes 
to  rest.  If  one  Hank  be  now  gently  scratched  with  the  point 
of  a  pencil  a  reflex  movement  occurs,  limited  to  the  muscles 
of  that  region;  they  twitch,  somewhat  as  a  horse's  neck  when 
tickled  by  flies.  If  a  pinch  with  small  forceps  be  given 
at  the  same  spot,  more  muscles  on  the  same  side  come 
into  play;  a  harder  pinch  causes  also  the  hind  leg  of  that 
side  to  he  raised  to  push  away  the  offending  object  ;  more 
violent  and  prolonged  irritation  causes  all  the  muscles  of 
the  body  to  contract,  and  the  animal  is  convulsed.  Here 
then  we  see  that  a  feeble  stimulation  causes  a  limited  and 
purpose-like  response;  stronger  causes  a  wider  radiation  of 
efferent  impulses  from  the  cord  and  the  contraction  of 
more  muscles,  but  still  the  movements  are  co-ordinated  to 
an  end;  while  abnormally  powerful  stimulation  of  the  sen- 
sory nerves  throws  all  the  motor  fibres  arising  from  the 
cord  into  activity,  and  calls  forth  inco-ordinate  spasmodic 
action.  The  orderly  movements  are  very  uniform  for  a  given 
stimulation;  if  the  anal  region  be  pinched,  both  hind  legs  are 
raised  to  push  away  the  forceps;  if  a  tiny  bit  of  bibulous  paper 
moistened  with  dilute  vinegar  be  put  on  the  thigh,  the  lower 
part  of  that  leg  is  raised  to  wipe  it  off;  if  on  the  middle  of  the 
back  near  the  head,  both  feet  are  wiped  over  the  spot;  if  on 
one  flank,  the  leg  and  foot  of  that  side  are  used,  and  so  on; 
in  fact,  by  careful  working,  the  frog's  skin  can  be  mapped 
into  many  regions,  the  application  of  acidulated  water  to  each 
causing  one  particular  movement,  due  to  the  co-ordinated 
contractions  of  muscles  in  different  combinations,  and  never, 
under  ordinary  circumstances,  any  but  that  one  movement. 
The  above  purpose-like  reflex  movements  may  all  be  charac- 
terized as  defensive,  but  all  orderly  reflexes  are  not  so.  For 
example,  in  the  breeding  season  the  male  frog  clasps  the  female 
for  several  days  with  his  fore  limbs.  If  a  male  at  this  season 
be  decapitated  and  left  to  recover  from  the  shock,  it  will  be 
found  that  gently  rubbing  his  sternal  region  with  the  finger 
causes  him  to  clasp  it  vigorously. 

Disorderly  Reflexes  or  Reflex  Convulsions.  These  come 
on  when  an  afferent  nerve-trunk  is  stimulated  instead  of  the 
tactile  end  organs  in  the  skin ;  or  when  the  skin  is  very  power- 
fully excited ;  or,  with  feeble  stimuli,  in  certain  diseased  states 
{pathological  tetanus),  and  under  the  influence  of  certain 
poisons,  especially  strychnine.     If  a  frog  or  a  warm-blooded 


THE  SPINAL   CORD  AND  REFLEX  ACTIONS.        603 

animal  be  given  a  dose  of  the  latter  drug,  a  stimulus,  such  as 
normally  would  excite  only  limited  orderly  reflexes,  will  excite 
the  whole  cord,  and  lead  to  discharges  along  all  the  efferent 
fibres  so  that  general  convulsions  result.  It  has  been  clearly 
proved  that,  in  such  cases,  not  the  skin,  or  afferent  or  efferent 
nerves,  or  the  muscles,  but  the  spinal  cord  itself  is  affected  by 
the  poison  (at  least  primarily),  unless  unnecessarily  large  doses 
have  been  given. 

The  Least-Resistance  Hypothesis.  In  order  to  compre- 
hend reflex  acts  we  mast  assume  a  manifold  union  of  afferent 
with  efferent  nerve-fibres;  this  is  anatomically  afforded  by  the 
minute  plexus  of  the  gray  network,  which  is  continuous  through 
the  whole  cord,  and  in  which  many  fibres  of  the  anterior  and 
posterior  nerve-roots  directly  or  indirectly  end.  The  contin- 
uity of  this  network  serves  to  explain  general  reflex  convul- 
sions, and  the  spread  of  an  afferent  impulse,  or  its  results, 
through  the  whole  cord,  with  the  consequent  emission  of  effe- 
rent impulses  through  many  or  all  the  anterior  roots;  but,  on 
the  other  hand,  it  renders  it  difficult  to  understand  limited 
and  orderly  reflexes,  in  which  only  a  few  efferent  fibres  are 
stimulated.  To  explain  them  we  have  to  assume  a  great  re- 
sistance to  conduction  in  the  gray  network,  so  that  a  nerve 
impulse  entering  it  is  soon  blocked  and  transmuted  into  some 
other  form  of  energy;  hence  it  only  reaches  efferent  fibres 
originating  near  the  point  at  which  it  enters,  or  fibres  placed 
in  specially  easy  communication  with  that.  When  the  frog's 
flank  is  tickled,  only  muscles  innervated  from  anterior  roots 
on  the  same  side  of  the  body,  and  springing  from  the  same 
level  of  the  cord,  are  made  to  contract ;  when  the  stimulus  is 
more  powerful,  the  stronger  afferent  impulse  radiates  farther, 
but  mainly  in  directions  determined  by  lines  of  conductivity 
in  the  cord;  e.g.,  to  the  origin  of  the  efferent  fibres  which 
f;n i<e  lifting  of  the  hind  leg  to  the  irritated  spot.  These 
paths  of  easiest  conduction,  or  of  least  resistance,  in  some 
cases  lie  in  the  gray  matter  itself,  in  others  in  the  inter-central 
or  commissural  fibres  of  the  highly  conductive  medullated 
kind,  which,  [Kissing  out  of  the  gray  substance  at  one  level, 
ran  in  the  white  columns  to  it  at  another,  where  the  efferent 
fibres  of  the  muscles  called  into  play  originate.  A  still  stronger 
afferenl  impulse  radiates  wider  still,  and,  liberating  energy 
from  all  the  nerve-cells  in  the  gray  matter,  produces  a  useless 
general  convulsion.     Under  the  influence  of  strychnine  and 


004  THE  HUMAN  BODY. 

in  pathological  tetanus  (as  observed,  for  example,  in  hydro- 
phobia), the  conductivity  of  the  whole  gray  matter  i.-  bo  in- 
creased that  all  paths  through  it  are  easy,  and  so  a  feeble 
afferenl  impulse  spreads  in  all  directions. 

To  account  for  the  phenomena  of  localized  skin  sensations 
and  of  limited  voluntary  movements  we  must  make  a  similar 
hypothesis.      If  the  nervous  impulses  entering  the  gray  net- 
work of  the  cord  or,  through  fibres  of  the  posterior  median 
tract,  the  gray   matter  of   the   medulla  oblongata,   when   the 
tip  of  a  finger  is  touched  spread  all   through    it   irregularly, 
we  could  not  tell  what  region  of  the  skin  had  been  stimu- 
lated, for  the  central  results  of  stimulating  the  most  varied 
peripheral  parts  would  be  the  same.     From  each  region  of  the 
gray   network    where  a  sensory  skin-nerve  enters  there  must, 
therefore,  be  a  special  path  of  conduction  to  an  anterior  brain 
region,    producing  results  which  differ  recognizably  in  con- 
sciousness from  those  following  the  stimulation  of  a  different 
skin  region.     Possibly  for  true  touch  and  temperature  sensa- 
tions these  paths  are  in  the  post-median  tract.    The  acuteness  of 
the  localizing  power  will  largely  depend  on  the  defmiteness 
of  the  path  of  least  resistance  in  the  gray  matter,  since  while 
traveling  in  a  medullated   nerve-fibre   from   the  skin  to  the 
cord,  or  (in  the  white  columns)  from  the  gray  matter  of  the 
latter  to  the  brain,  the  nervous  impulse  is  confined  to  a  definite 
track.     Hence  anything  tending  to  let  the  afferent  impulse 
radiate  when  it  enters  the  gray  network  will  diminish  the  ac- 
curacy with  which  its  peripheral  origin  can  be  located.     This 
we  see  in  violent  pains;  a  whitlow  on  the  finger  affects  only 
a  few  nerve-fibres,  but  gives  rise  to  so  powerful  nerve  impulses 
that  when  they  reach  the  cord  they  spread  widely  and,  break- 
ing out  of   the  usual  track  of  propagation   to  the  brain,  give 
rise  to  ill-localized  feelings  of  pain  often   referred  all  the  way 
up  the  arm  to  the  elbow.      Such  cases  are  comparable  to  the 
transformation  of  an  orderly  reflex  into  a  general  convulsion 
when  the  stimulus  increases. 

As  animals  exhibit  no,  or  at  most  limited,  spontaneous  move- 
ments when  their  whole  cerebral  hemispheres  are  removed,  we 
conclude  that  the  nerve  impulses  giving  rise  to  such  movements 
normally  start  in  those  parts  of  the  brain.  Thence  they  travel 
down  the  pyramidal  tracts  of  the  cord  to  its  gray  matter,  which* 
they  enter  at  different  levels,  each  in  the  neighborhood  of  a 
centre  for  producing  a  given  movement.     If  they  there  radiated 


TEE  SPINAL   CORD   AND  REFLEX  ACTIONS.        605 

far  and  wide  no  definite  movement  could  result,  for  all  the 
muscles  supplied  from  the  cord  would  be  made  to  contract, 
and  not  morely  those  necessary  to  bend  the  index  finger,  for 
example.  We  must  here  again,  therefore,  assume  a  path  of 
least  resistance  for  the  propagation  of  nerve  impulses  from  a 
given  fibre  coming  down  from  the  brain,  to  the  efferent  fibres 
going  to  a  certain  muscle  or  group  of  muscles.  The  path 
between  the  two  is  almost  certainly  not  direct ;  a  co-ordinating 
spinal  centre  intervenes,  and  all  that  the  brain  has  to  do  is 
to  excite  this  centre,  which  then  secures  the  proper  muscular 
co-ordination.  If  the  hand  be  laid  flat  on  the  table  and  its 
palm  be  rolled  over,  many  muscles,  including  thousands  of 
muscular  fibres,  have  to  contract  in  definite  order  and  sequence. 
Persons  who  have  not  studied  anatomy  and  who  are  quite 
ignorant  of  the  muscles  to  be  used  can  perform  the  movement 
perfectly;  and  even  a  skilled  anatomist  and  physiologist,  if 
he  knew  them  all  and  their  actions,  could  not  by  conscious 
effort  combine  them  so  well  as  the  cord  does  without  such 
direct  interference.  We  have  then  to  look  on  the  cord  as 
containing  a  host  of  co-ordinating  centres  for  different  muscles. 
These  centres  are  put  in  nervous  connection,  on  the  one  hand, 
with  certain  regions  of  the  skin,  and,  on  the  other,  with  regions 
of  the  brain,  and  may  be  excited  from  either;  in  the  former 
case  the  movement  is  called  reflex ;  in  the  latter  it  may  be 
reflex,  or  may  be  accompanied  with  a  feeling  of  "  will  "  and 
is  then  called  voluntary.  The  more  accurately  the  required 
centre,  and  no  other,  is  excited,  the  more  definite  and  precise 
the  movement. 

The  Education  of  the  Cord.  Much  of  what  is  called  edu- 
cating our  touch  or  our  muscles  is  really  education  of  the 
spinal  cord.  A  person  who  begins  to  play  the  piano  finds  at 
first  much  difficulty  in  moving  his  fingers  independently;  the 
nervous  impulses  from  the  brain  to  the  cord  radiate  from  the 
spina]  centres  of  the  muscle  which  it  is  desired  to  move,  to 
•  -tin]-.  But  with  practice  the  independent  movements  be- 
come  easy.  So,  loo,  flu- localizing  power  of  the  skin  can  be 
L'n-atly  increased  by  exercise  as  one  observes  in  blind  per- 
-OU-,  who  often  can  distinguish  two  stimuli  on  parts  of  the 
skin  which  are  so  near  together  as  bo  give  only  one  sensation 
to  other  people.  Such  phenomena  depend  on  the  fact  that 
the  more  often  a  nervous  impulse  has  traveled  along  a  given 
road   in  the  gray  matter,  the  easier  does  its  path  become,  and 


606  THE  HUMAN  BODY. 

the  less  does  it  tend  to  wander  from  it  into  others.  We 
may  compare  the  gray  matter  to  a  thicket;  persons  seeking  to 
beat  a  road  through  from  one  point  to  another  would  keep  the 
same  general  direction,  determined  by  the  larger  obstacles  in 
the  way,  but  all  would  diverge  more  or  less  from  the  straight 
path  on  account  of  undergrowth,  tree  trunks,  etc.,  and  would 
meet  with  considerable  difficulty  in  their  progress.  After 
some  hundreds  had  passed,  however,  a  tolerably  beaten  track 
would  he  marked  out,  along  which  travel  was  easy  and  all 
after-comers  would  take  it.  If  instead  of  one  entry  and  one 
exit  we  imagine  thousands  of  each,  and  that  the  paths  between 
certain  have  been  often  traveled,  others  less,  and  some  hardly 
at  all,  we  get  a  pretty  good  mental  picture  of  what  happens  in 
the  passage  of  nervous  impulses  through  the  gray  matter  of 
the  cord;  the  clearing  of  the  more  trodden  paths  answering 
to  the  effects  of  use  and  practice.  The  human  cord  and  that 
•of  the  frog  must  not,  however,  be  looked  upon  as  pathless 
thickets  at  the  commencement ;  each  individual  inherits  cer- 
tain paths  of  least  resistance  determined  by  the  stricture  of 
the  cord,  which  is  the  transmitted  material  result  of  the  life 
experiences  of  a  long  line  of  ancestors. 

The  Inhibition  of  Reflexes.  Since  it  is  possible,  as  by 
strychnine,  to  diminish  the  resistance  in  the  gray  matter,  it 
is  conceivably  also  possible  to  increase  it,  and  diminish  or 
prevent  reflexes.  Such  is  found  to  be  actually  the  case.  "We 
can  to  a  great  extent  control  reflexes  by  the  will ;  for  example, 
the  jerking  of  the  muscles  which  tends  to  follow  tickling:  and 
it  is  found  that  after  a  frog's  brain  is  removed  it  is  much 
easier  to  get  reflex  actions  out  of  the  spinal  cord.  Certain 
drugs,  as  bromide  of  potassium,  also  diminish  reflex  excit- 
ability. •  If  a  frog's  brain  be  removed  and  the  animal's  toe  be 
dipped  into  very  dilute  acid,  it  will  be  removed  after  a  few 
seconds;  the  time  elapsing  between  the  immersion  and  the 
lifting  of  the  foot  is  known  as  the  reflex  time;  anything 
diminishing  reflex  excitability  increases  this,  as  the  stimulus 
(which  has  a  cumulative  effect  on  the  centre)  has  to  act  longer 
before  it  arouses  the  cord  to  the  discharging  point.  If  the 
sciatic  nerve  of  the  other  leg  be  stimulated  while  the  toe  is  in 
the  acid  the  reflex  time  is  increased,  or  the  reflex  may  fail 
entirely  to  appear.  This  is  one  case  of  a  general  law,  that 
any  powerful  stimulation  of  one  sensory  nerve  tends  to  in- 
hibit orderlv  reflexes  due  to  the  excitation  of  another.     A 


THE  SPINAL   CORD  AND   REFLEX  ACTIONS.        607 

common  example  is  the  well-known  trick  of  pinching  the 
nose  or  upper  lip  to  prevent  a  sneeeze.  The  whole  question 
of  reflex  inhibition  is  at  present  very  obscure.  It  may  be  due 
to  the  excitation  of  special  fibres  which  inhibit  reflex  centres, 
as  the  fibres  of  the  depressor  nerve  do  the  activity  of  the  vaso- 
constrictor centre;  or  to  the  fact  that  one  nerve  impulse  in 
the  cord  in  some  cases  blocks  or  interferes  with  another;  or 
partly  to  both. 

Psychical  Activities  of  the  Cord.  Since  we  can  get  quite 
marked  reflex  movements  in  the  lower  part  of  the  Body  of  a 
man  whose  cord  is  divided  and  who  cannot  voluntarily  move 
his  lower  limbs,  and  on  questioning  him  find  that  he  feels 
nothing  and  is  quite  ignorant  of  his  movements  unless  he  sees 
his  legs,  it  is  most  probable  that  the  spinal  cord  in  all  cases  is 
devoid  of  centres  of  consciousness  and  volition:  this  is  not 
certain,  however;  for  there  might  well  be  a  less  division  of 
physiological  labor  between  the  cord  and  brain  of  a  frog,  than 
between  those  of  a  man.  Still  we  are  entitled  to  good  evi- 
dence before  we  admit  that  things  so  similar  as  the  human 
cord  and  that  of  the  frog  possesses  different  properties.  Co- 
ordinated movements  following  a  given  stimulus,  or  cries 
emitted  by  an  animal,  will  not  suffice  to  prove  that  it  is  con- 
scious, since  we  know  these  may  occur  entirely  unconsciously 
in  men,  who  alone  can  tell  us  of  their  feelings.  We  must 
look  for  something  that  resembles  actions  only  done  by  men 
consciously.  In  the  frog  it  has  been  maintained  that  we  have 
evidence  of  such.  If  a  bit  of  acidulated  paper  be  put  on  the 
thigh  of  a  decapitated  frog,  the  animal  will  bend  its  knee  and 
use  its  leg  to  brush  off  the  irritant;  always  using  this  same 
leg  if  the  stimulus  be  not  so  strong  as  to  produce  disorderly 
reflexes.  If  now  the  foot  be  tied  down  so  that  the  frog  can- 
not  raise  it,  after  a  few  ineffectual  efforts  it  will  move  the 
other  leg,  and  may  wipe  the  paper  off  with  it.  This  it  has 
been  said  shows  a  true  psychical  activity  in  the  cord;  a  con- 
scious and  voluntary  employment  of  new  procedures  under 
unusual  circumstances.  But  a  close  observation  of  the  phe- 
nomenon shows  that  it  ./ill  hardly  bear  this  interpretation; 
the  movements  of  the  other  leg  are  very  irregular  and  inco- 
ordinate, and  much  resemble  reflex  convulsions  stirred  up  by 
the  prolonged  action  of  the  acid,  which  goes  on  stimulating 
the  .-kin  nerve-  more,  and  more  powerfully.  Kven  if  new 
musclee  came,  in  an  orderly  way,  into  play  under  bhe  stronger 


608  THE  HUMAN  BODY. 

stimulus,  that  would    nol    prove   a  volitional    conscious    H8G  of 

them;  we  see  quite  similar  phenomenon  when  there  is  nothing 
purpose-like  in  the  movement.  Many  dogs  reflexly  kick  vio- 
lently the  hind  leg  of  the  same  side  when  one  flank  is  tickled. 
If  this  leg  he  held  and  the  tickling  continued,  very  frequently 
the  opposite  hind  leg  will  take  on  the  movements,  which  it 
never  does  in  ordinary  circumstances.  This  is  quite  compar- 
able to  the  frog's  use  of  its  other  leg  under  the  circumstances 
above  described,  hut  here  it  would  he  obviously  absurd  to  talk 
of  a  volitional  source  for  such  a  senseless  movement. 

Reflex  Time.  This  is  the  time  elapsing  between  the  stimu- 
lation of  a  sensory  surface  and  the  resulting  reflex  contraction 
of  a  muscle.  It  contains,  of  course,  several  elements — the 
time  taken  in  the  origination  and  afferent  course  of  the  nerve 
impulse,  the  time  occupied  in  the  centre,  and  that  in  the 
efferent  nerve-fihres,  and  the  period  of  latent  excitement  of 
the  muscles.  Since  the  rate  of  travel  of  nerve  impulses  and 
the  time  of  latent  excitement  are  known  with  tolerable  ac- 
curacy they  can  be  estimated ;  and  their  .sum  subtracted  from 
the  whole  time  gives  the  time  taken  up  in  the  central  organ. 
This,  as  might  be  expected,  when  we  consider  the  highly 
complex  nature  of  the  processes  required  to  produce  a  co- 
ordinated reflex  movement,  is  very  much  greater  than  the 
time  occupied  in  traversing  an  equal  length  of  nerve  trunk. 
An  electric  shock  given  to  one  eyelid  causes  a  reflex  wink  of 
both,  and  by  suitable  apparatus  the  time  lapsing  between 
stimulation  of  one  eyelid  and  movement  of  the  other  can  be 
measured.  It  is  about  .0000  sec. ;  the  calculated  time  for  the 
passage  of  the  afferent  impulse  to  the  centre  in  the  gray  matter 
of  the  fourth  ventricle  and  of  the  efferent  to  the  orbicularis 
muscle  of  the  other  eyelid,  or  the  period  of  latent  excitation, 
is  about  .0100  sec,  leaving  .0500  sec.  for  the  central  processes. 
Reflex  time  varied  considerably.  It  is  longer  for  more  com- 
plicated reflex  movements;  also  the  strength  of  the  stimulus 
has  an  influence;  if  one  toe  of  a  decapitated  frog  be  immersed 
in  very  dilute  acid  the  time  which  elapses  before  it  is  with- 
drawn is  greater  than  when  the  acid  is  a  little  stronger. 


CHAPTER  XXXVII. 

THE   PHYSIOLOGY  OF  THE   BKAIN. 

The  Functions  of  the  Brain  in  General.  The  brain,  at 
least  in  man  and  the  higher  animals,  is  the  seat  of  conscious- 
ness and  intelligence ;  these  disappear  when  its  blood-supply 
is  cut  off,  as  in  fainting ;  pressure  on  parts  of  it,  as  by  a  tumor 
or  by  aD  effusion  of  blood  in  apoj)lexy,  has  the  same  result ; 
inflammation  of  it  causes  delirium;  and  when  the  cerebral 
hemispheres  are  unusually  small  idiotcy  is  observed.  The 
brain  has,  however,  many  other  important  functions ;  it  is  the 
seat  of  many  reflex,  automatic,  and  co-ordinating  centres, 
which  can  act  as  entirely  apart  from  consciousness  as  those 
of  the  spinal  cord .  It  is  also  traversed  by  many  paths  of  con- 
duction, some  uniting  it  with  the  spinal  cord  and  numerous 
others  putting  its  own  parts  in  anatomical  connection. 

The  psychical  activities,  at  least  in  man,  seem  to  be  depend- 
ent on  the  forebrain,  the  rest  of  the  complex  mass  having 
other  non-mental  functions  or  at  most  being  only  concerned 
in  very  simple  mental  states.  After  the  cerebral  hemispheres 
have  been  removed  from  a  frog  it  is  still  able  to  perform  every 
movement  as  before,  but  it  no  longer  performs  any  spontane- 
ously. Suitably  stimulated  it  will  leap,  swim,  crawl,  climb, 
turn  off  its  back  to  its  normal  position ;  and  if  the  optic  thalami 
have  not  been  injured  will  in  leaping  forward  avoid  an  ob- 
stacle placed  between  it  and  the  light.  Its  whole  essential 
mechanism  of  movement  is  clearly  intact,  and  can  be  thrown 
into  action  and  to  a  certain  extent  be  guided  by  afferent 
nervous  impulses.  Quite  similar  phenomena  may  be  observed 
in  pigeons;  they  not  only  can  Btand,  but  walk,  fly  if  thrown 
into  the  air,  and  preen  their  feathers,  after  removal  of  the 
cerebral  hemispheres;  and  if  carefully  tended  will  live  for 
months.  Mammals  bear  badly  extensive  operations  on  the 
forebrain  and  usually  die  before  fully  recovering  from  the 
shock    of    the    operation;    but.    rats    survive    some    hours,   and 

then  exhibit  very  similar  phenomena.     However  it  has  been 

609 


610  THE  11C MAX  BODY. 

possible  by  repeated  operations,  taking  away  only  a  part  at 
a  time,  to  successfully  remove  almost  all  the  surface  graj  matter 
of  the  cerebral  hemispheres  from  dogs;  and  the  animals  have 
recovered  so  as  to  perform  many  ordinary  movements  so  well 
that  a  person  observing  them  only  for  a  short  time  would 
notice  nothing  abnormal.  But  in  such  cases  not  only  some 
cerebral  cortex  has  been  left  but  also  the  deeper lying  corpora 
striata  and  optic  thalami:  when  these  gray  masses  and  all 
the  cerebral  cortex  are  removed,  as  is  possible  in  frogs  and 
birds,  the  animal  does  not  move  unless  directly  stimulated, 
or  SO  rarely  that  movements  which  appear  due  to  a  spontane- 
ous volition  are  probably  due  to  some  unobserved  irritation  or 
stimulation.  In  addition  to  loss  of  willed  movements  there 
is  loss  or  nearly  complete  loss  of  perception,  that  is,  of  the 
power  of  mentally  interpreting  and  giving  a  meaning  to  in- 
coming nervous  impulses.  The  pigeon  or  rat  will  start  at  a 
loud  noise,  but  makes  no  attempt  to  escape,  as  if  it  conceived 
danger;  it  will  follow  a  light  with  the  eyes  but  make  no  at- 
tempt to  escape  from  a  hand  stretched  out  to  seize  it;  it  can 
and  does  swallow  food  placed  in  its  mouth,  but  will  starve  if 
left  alone  with  plenty  of  it,  the  sight  of  edible  things  seeming 
to  arouse  no  idea  or  conception.  It  lias  been  doubted  whether 
the  animals  have  any  true  sensations;  they  start  at  sounds, 
avoid  opaque  objects  in  their  road,  and  cry  when  pinched  :  but 
all  these  may  be  unconscious  reflex  acts :  on  the  whole  it  seems 
more  probable,  however,  that  they  have  sensation-  but  not 
perceptions;  they  feel  redness  and  blueness,  hardness  and  soft- 
ness, and  so  on;  but  sensations,  as  already  pointed  out,  tell 
in  themselves  nothing;  they  are  but  signs  which  have  to  be 
mentally  interpreted  as  indications  of  external  objects  or  of 
conditions  of  the  Body:  it  is  this  interpreting  power  winch 
seems  deficient  in  the  animal  deprived  of  its  forebrain.  In 
some  cases  a  like  state  appears  to  occur  in  man  in  connection 
with  abnormal  states  of  parts  of  the  cerebral  hemispheres. 
The  patient  may  have  eye,  retina,  optic  nerves  and  all  the 
endings  of  these  in  the  optic  thalami  and  corpora  quadrigemina 
intact,  and  his  pupils  react  to  light,  and  the  eyes  follow  a 
bright  object,  yet  the  object  arouses  in  the  patient  no  idea  as 
to  its  nature:  apparently  he  sees  it,  but  he  is  mind  blind. 

The  Medulla  Oblongata.  Lying  on  the  ventral  aspect  of 
this  (Chap.  XII)  on  the  sides  of  the  continuation  of  the  anterior 
fissure  of  the  cord  are  the  two  masses  of  nerve-fibres  known 


THE  PHYSIOLOGY  OF  THE  BRAIN.  611 

as  the  anterior  pyramids :  most  of  the  fibres  of  these  are  con- 
tinuations of  the  pyramidal  tracts  of  the  cord  and  here  cross 
the  middle  line,  forming  thus  the  decussation  of  the  pyramids. 
The  fibres  of  the  direct  pyramidal  tract  pass  on  in  the  pyramid 
of  the  same  side,  only  crossing  in  the  cord.  The  pyramidal 
fibres  pass  on  through  the  pons  Varolii  and  along  the  ventral 
or  basal  side  of  the  crura  cerebri  (Fig.  17G),  and  enter  the 
cerebral  hemispheres.  In  the  medulla  are  a  number  of  masses 
of  gray  matter  (often  named  nuclei)  which  have  the  same 
relation  to  the  motor  fibres  of  cranial  nerves  as  areas  of  gray 
matter  in  the  cord  have  to  the  motor  fibres  of  the  spinal  roots, 
and  from  these  motor  nuclei  medullated  fibres  join  the  pyr- 
amids and  go  with  them  into  the  forebrain.  .Such  fibres  of 
ascending  degeneration  in  the  cerebellar  tract  of  the  cord  and 
of  the  ascending  antero-lateral  tract  as  extend  above  the  cord 
run  on  the  dorsal  side  of  the  medulla  oblongata  as  the  restiform 
bodies;  they  diverge  in  front  so  as  to  lie  on  the  sides  of  the 
fourth  ventricle  and  enter  the  cerebellum.  The  fibres  of  the 
posterior  median  column  terminate  in  a  mass  of  gray  matter 
in  the  medulla  known  as  the  nucleus  gracilis:  those  of  the 
exterior  median  column  in  a  similar  nucleus  cuneatus.  These 
nuclei  in  turn  give  origin  to  many  fibres,  a  large  number 
crossing  the  middle  line,  and  some  of  these  are  then  continued 
as  the  fillet  along  the  dorsal  side  of  the  eras  cerebri  to  the  fore 
brain;  others  join  the  restiform  body  and  through  it  the 
opposite  side  of  the  cerebellum:  these  crossings  constitute 
the  sensory  decussation,  as  distinguished  from  the  pyramidal 
or  motor.  The  fibres  of  the  antero-lateral  descending  tract 
which  do  not  undergo  descending  degeneration  probably  join 
the  pyramids;  all  their  fibres  entering  the  medulla  from  the 
cord  end  in  gray  matter  of  the  medulla.  By  the  word  "  end- 
ing" is  meant,  of  course,  only  that  they  cannot  be  further 
t laced  as  individual  fibres,  not  that  no  physiological  represen- 
tatives of  them  arise  in  the  gray  matter  of  the  medulla  and 
pass  to  other  parts  of  the  brain. 

The  central  canal  of  the  spinal  cord  passes  (Chap.  XII) 
into  the  medulla  oblongata,  in  the  anterior  portion  of  which  it 
expands  to  form  the  fourth  vent  fide.  The  gray  matter  of  the 
cord  is  continued  around  the  canal  and  on  the  floor  and  sides  of 
the  vent  ride;  and  in  connection  wit  h  it  are  special  thickenings, 
rich  in  nerve-cells  forming  the  nuclei  or  deep  origins  of  most 

of   the   cranial    nerves:    some   Of   these    nerves   arise  from  more 


612  THE  HUMAN  BODY. 

than  one  nucleus  and  some  of  t he  nuclei  are  separated  from 
the  gray  matter  around  the  central  cavity,  hut  a  minute  ana- 
tomical description  would  be  here  out  of  place.  The  olivary 
capsules,  however,  placed  in  the  olivary  bodies  which  lie  on 
the  outer  side  of  each  anterior  pyramid  may,  however,  be 
mentioned.  The  nerves  having  their  nuclei  in  the  medulla 
oblongata  are  the  hypoglossal  (xn),  the  spinal  accessory  (xi) 
except  its  spinal  portion,  the  vagus  (x),  the  glossopharyngeal 
(ix)  (some  fibres  of  which  perhaps  come  from  the  cord),  the 
auditory  (vill)  (by  two  distinct  bundles  of  fibres,  cochlear 
and  vestibular,  connected  with  distinct  nuclei),  the  facial 
(vn),  the  patheticus  (vi),  part  of  the  trigeminal.  Some  of 
the  trigeminal  arises  from  gray  matter  in  the  corpora  quad- 
rigemina.  The  nucleus  of  the  abducens  (iv)  lies  just  under 
the  floor  of  the  aqueduct  of  Sylvius  (Fig.  176),  opposite  the 
posterior  border  of  the  anterior  corpora  quadrigemina.  The 
oculo-motor  (in)  arises  from  gray  matter  under  the  front 
of  the  aqueduct  and  from  the  posterior  part  of  the  third  ven- 
tricle. All  the  fibres  of  the  above  ten  nerves  arise,  then, 
from  gray  matter  around  the  cerebral  continuation  of  the  gray 
matter  of  the  cord,  and  most  of  them  behind  the  midbrain. 

Besides  its  functions  as  affording  paths  between  the  cord 
and  the  rest  of  the  brain  and  as  the  seat  of  many  relay  and 
junction  centres  the  medulla  has  important  reflex  and  auto- 
matic activities.  As  in  the  case  of  the  cord,  its  motor  centres 
may  be  thrown  into  reflex  activity  by  afferent  impulses  from 
below,  as  well  as  by  efferent  travelling  down  from  cerebrum 
or  cerebellum.  It  is  especially  concerned  with  nervous  con- 
trol of  the  organs  more  immediately  connected  with  circu- 
lation, respiration,  and  mastication.  The  physiological  action 
of  most  of  the  medullary  centres  has  already  been  described; 
the  more  important  are — 1.  The  respiratory  centre.  2.  The 
cardio-inhibitory  centre ;  the  centre  of  the  accelerator  heart- 
fibres  lies  in  the  medulla.  3.  The  vaso-motor  centres.  4. 
The  centre  for  the  dilator  muscle-fibres  of  the  pupil.  5. 
The  centre  for  the  muscles  of  chewing  and  swallowing,  which 
are  commonly  thrown  into  action  reflexly,  though  they  may 
be  made  to  contract  voluntarily.  6.  The  convulsive  centre 
7.  The  diabetic  centre.  8.  The  centre  reflexly  exciting  activ- 
ity in  the  salivary  glands,  when  sensory  nerves  in  the  mouth 
are  stimulated.  9.  Certain  centres  for  complex  bodily  move- 
ments; an    animal  with    its    medulla  oblongata   can   execute 


THE  PHYSIOLOGY  OF  THE  BRAIN.  613 

much  more  complicated  reflex  acts  than  one  with  its  spinal 
cord  alone. 

The  Cerebellum  and  Pons  Varolii.  (Figs.  74,  75).  The 
anterior  part  of  the  medulla  oblongata  is  covered  above  by 
the  cerebellum  and  below  by  the  pons,  the  latter  of  which  is 
mainly  a  transverse  commissure  uniting  the  hemispheres  of 
the  cerebellum,  though  the  pyramidal  and  other  longitudinal 
commissural  fibres  run  through  it;  and  in  it  are  many  gray 
nuclei.  The  halves  of  the  cerebellum  are  also  united  with 
one  another  by  transverse  fibres  of  its  middle  lobe ;  and,  be- 
hind, by.  the  posterior  peduncles  witli  the  restiform  bodies 
and  the  medulla,  and,  in  front,  by  the  anterior  peduncles, 
with  the  cerebrum.  Besides  its  gray  surface  with  small  nerve- 
cells  and  the  cells  of  Purkinje  (Fig.  82)  it  contains  other 
more  central  gray  matter.  The  most  striking  anatomical  fact 
in  relation  to  the  cerebellum  is  its  close  connection  with  the 
afferent  tracts  of  the  spinal  cord,  nearly  all  of  which  except 
the  fibres  of  the  fillet  are  only  connected  with  the  cerebrum 
through  the  intervention  of  the  cerebellum.  The  same  is  true 
of  the  vestibular  portion  of  the  auditory  nerve  and  probably 
also  of  most  of  the  afferent  fibres  of  all  the  posterior  cranial 
nerves.  The  cerebellum  is  thus  subjected  to  influences  from 
many  regions  of  the  Body;  the  skin,  the  muscles,  the  ears,  and 
probably  also  the  eyes  are  sources  of  impulses  streaming  into 
it  all  the  time,  and  modifying  the  conditions  of  its  gray  matter 
and  the  nature  of  the  impulses  in  turn  issued  from  that.  The 
most  marked  result  of  extensive  injury  of  the  cerebellum  is 
muscular  inco-ordination ;  it  seems  to  be  a  chief  organ  of  what 
we  may  call  personally  acquired  reflexes,  as  distinguished  from 
inherited. 

Every  one  has  to  learn  to  stand,  walk,  run,  and  so  on;  at 
first  all  are  difficult,  but  after  a  time  become  easy  and  are 
performed  unconsciously.  In  standing  or  walking  very  many 
muscles  are  concerned,  and  if  the  mind  had  all  the  time  to 
look  directly  after  them  we  could  do  nothing  else  at  the  same 
time;  we  have  forgotten  how  we  learnt  to  walk,  but  in  ac- 
quiring a  new  mode  of  progression  in  later  years,  as  skating, 
we  find  that  at  first  it  needs  all  our  attention,  but  when  once 
learnt  we  have;  only  to  start  the  series  of  movements  and  they 
are  almost  unconsciously  carried  on  for  us.  At  first  we  had 
to  learn  to  contract  certain  muscle  groups  when  we  got  par- 
ticular sensations,  either  tactile,  from  the  soles,  or  muscular, 


614  THE  HUMAN  BODY. 

from  the  general  position  of  the  limbs,  or  visual,  or  others 
(equilibrium  sensations,  see  below)  from  the  semicircular 
canals.  But  the  oftener  a  given  group  of  sensations  has  been 
followed  by  a  given  muscular  contraction  the  more  close  be- 
comes the  association  of  the  two;  the  path  of  connection  be- 
tween the  afferent  and  efferent  fibres  becomes  easier  the  more 
it  is  travelled,  and  ;ii  last  the  afferenl  impulses  arouse  the 
proper  movement  without  volitional  interference  at  all,  and 
while  hardly  exciting  any  consciousness;  we  can  then  walk  or 
skate  without  thinking  about  it.  The  will,  which  had  at  first 
to  excite  the  proper  muscular  nerve-centres  in  accordance  with 
the  felt  directing  sensations,  now  has  no  more  trouble  in  the 
matter;  the  afferent  impulses  stimulate  the  proper  motor 
centres  in  an  unconscious  and  unheeded  way.  Injury  or  dis- 
ease of  the  cerebellum  produces  great  disturbances  of  locomo- 
tion and  insecurity  in  maintaining  various  postures.  After  a 
time  the  animals  (birds,  which  bear  the  operation  best)  can 
walk  again,  and  fly,  but  they  soon  become  fatigued,  probably 
because  the  movements  require  close  mental  attention  and 
direction  all  the  time. 

Sensations  of  Equilibrium.  _  In  order  to  make  proper 
movements  of  balancing  or  locomotion  we  need  a  knowledge 
of  the  space  relations  of  the  Body  to  its  surroundings.  When 
eyes,  muscles,  and  skin  send  in  concordant  afferent  impulses, 
movements  are  precise;  if  sensations  of  any  one  of  these  groups 
are  wanting  (excluding  blind  persons  who  have  learned  to  do 
without  some  of  them)  or  abnormal  the  whole  mechanism  is 
thrown  out  of  gear.  Persons  who  have  lost  muscular  or  tactile 
sensibility  stand  and  walk  with  difficulty;  those  who  have 
nystagmus  (jerking  unconscious  movements  of  the  eyeballs 
which  cause  the  visual  field  to  seem  to  move  in  space)  do  the 
same  and  feel  giddy;  and,  if  a  person  be  rapidly  rotated  with 
his  eyes  open  he  soon  becomes  giddy;  the  succession  of  retinal 
images  suggests  that  he  is  moving  in  space,  but  the  muscular 
and  tactile  afferent  impulses  are  in  conflict  with  that;  and 
though  this  discordance  hardly  comes  into  direct  consciousness 
as  a  definite  contradiction  between  sensations,  the  want  of 
harmony  in  the  afferent  impulses  throws  co-ordinating  motor 
mechanisms  out  of  gear,  with  resulting  uncertainty  in  loco- 
motion. An  important  group  of  afferent  impulses  concerned 
with  the  maintenance  of  bodily  equilibrium  in  addition  to 
those  above  referred  to  is  probably  derived  through  the  semi- 


THE  PHYSIOLOGY  OF  THE  BRAIN.  615 

circular  canals  of  the  ear,  which  are  supplied  by  the  vestibular 
portion  of  the  auditor)-  nerve;  and  it  has,  as  we  have  seen,  a 
special  cerebellar  connection.  An  old  view  was  that,  lying 
in  three  planes  at  right  angles  to  one  another,  they  served  to 
distinguish  the  direction  of  sound-waves  reaching  the  ear;  but 
as  the  direction  of  oscillation  of  the  tympanic  ossicles  is  the 
same,  no  matter  what  that  of  the  sound-waves  entering  the 
external  auditory  meatus  may  be,  such  an  hypothesis  has  no 
foundation.  The  cochlea  sufficiently  accounts  for  the  appre- 
ciation of  notes,  and  such  noises  as  are  due  to  inharmonically 
combined  tones;  while  the  sacculus  will  suffice  for  other 
noises:  and  it  is  found  that  disease  of  the  semicircular  canals 
does  not  interfere  with  hearing,  but  often  causes  uncertainty 
of  movements  and  feelings  of  giddiness. 

Experiment  shows  that  cutting  a  semicircular  canal  is  fol- 
lowed by  violent  movements  of  the  head  in  the  plane  of  the 
canal  divided;  the  animal  staggers,  also,  if  made  to  walk; 
and,  if  a  pigeon  and  thrown  into  the  air,  cannot  fly.  All  its 
muscles  can  contract  as  before,  but  they  are  no  longer  so  co- 
ordinated as  to  enable  the  animal  to  maintain  or  regain  a 
position  of  equilibrium.  It  is  like  a  creature  suffering  from 
giddiness;  and  similar  phenomena  follow,  in  man,  electrical 
stimulation  of  the  regions  of  the  skull  in  which  the  semicir- 
cular canals  lie. 

If,  moreover,  a  person  lie  perfectly  quiet  witli  closed  eyes 
on  a  table  which  can  be  rotated,  he  is  able  to  tell  when  the 
table  is  turned  and  in  which  direction,  and  often  with  con- 
siderable accuracy  through  what  angle.  If  the  rotation  be 
continued  for  a  time  the  feeling  of  it  is  lost,  and  then  when 
the  movement  ceases  there  is  a  sense  of  rotation  in  the  oppo- 
site direction.  In  such  case  neither  tactile,  muscular,  nor 
visual  sensations  can  help,  and  in  the  semicircular  canals  we 
seem  to  have  a  mechanism  through  which  rotation  of  the  head 
could  give  origin  to  afferent  impulses,  whether  the  head  be 
passively  moved  with  the  rest  of  the  Body  or  independently 
by  its  own  muscles.  Movements  of  endolymph  in  relation 
to  the  walls  of  tin-  canals  may  ac1  ;i-  stimuli  by  causing  a 
swaying  of  the  projecting  hairs  of  the  ampullae  (Pig.  167). 
Place  a  few  small  bits  of  cork  in  a  tumbler  of  water,  and  rotate 
tin'  tumbler;  at  first  the  water  does  not  move  with  it;   then  it 

begins  to  go  in  the  same  direction,  hut   more  slowly;  and, 

finally,  move-    at    the  same    angular  velocity   as   the  tumbler. 


61 G  THE  HUMAN  BODY. 

Then  stop  the  tumbler,  and  the  water  will  go  on  rotating  for 
some  time.  Now  if  the  head  be  turned  or  rotated  in  a  hori- 
zontal plane  similar  phenomena  will  occur  in  the  endolympb 
of  the  horizontal  canal ;  if  it  be  bent  sidewise  in  the  vertical 
plane,  in  the  anterior  vertical  canal;  and  if  nodded,  in  the 
posterior  vertical;  the  hairs  moving  with  the  canal  would 
meet  the  more  stationary  water  and  be  pushed  and  so,  possibly, 
excite  the  nerves  at  the  deep  ends  of  the  cells  which  bear 
them,  and  generate  afferent  impulses  which  will  cause  the 
general  nerve-centres  of  bodily  equilibration  to  be  differently 
acted  upon  in  each  case.  Under  ordinary  circumstances  the 
results  of  these  impulses  do  not  become  prominent  in  con- 
sciousness as  definite  sensations;  but  they  are  probably  always 
present.  If  one  spins  round  for  a  time,  the  endolympb  takes 
up  the  movement  of  the  canals,  as  the  water  in  the  tumbler 
does  that  of  the  glass ;  on  stopping,  the  liquid  still  goes  on 
moving  and  stimulates  the  hairs  which  are  now  stationary; 
and  we  feel  giddy,  from  the  ears  telling  us  we  are  rotating 
and  the  eyes  that  we  are  not;  hence  difficulty  in  standing 
erect  or  walking  straight.  A  common  trick  illustrates  this 
very  well:  make  a  person  place  his  forehead  on  the  handle 
of  an  umbrella,  the  other  end  of  which  is  on  the  floor,  and 
then  walk  three  or  four  times  round  it,  rise,  and  try  to  go  out 
of  a  door;  he  will  nearly  always  fail,  being  unable  to  combine 
his  muscles  properly  on  account  of  the  conflicting  afferent 
impulses.  This  and  the  feeling  of  rotation  in  the  contrary 
direction  when  a  previous  rotation  ceases  become  readily  intel- 
ligible if  we  suppose  feelings  to  be  excited  by  relative  move- 
ments of  the  endolympb  and  the  canals  inclosing  it. 

The  Midbrain.  The  general  arrangement  of  these  parts 
has  been  already  described  (Fig.  82).  Cross-sections  show 
(Fig.  176)  the  aqueduct  of  Sylvius,  S,  traversing  the  mid- 
brain near  its  upper  part  and  surrounded  by  a  thin  layer  of 
gray  matter,  in  close  connection  with  which  are  the  origins 
of  the  third  and  fourth  cranial  nerves,  IV,  and  of  part  of  the 
fifth.  The  crura  cerebri  form  the  main  mass  of  the  midbrain. 
Each  is  divided  by  gray  matter  (locus  niger,  Lri)  into  a  ven- 
tral portion  (pes  or  crusta,  P),  which  forms  the  semicylin- 
drical  portion  of  the  crus  seen  on  the  base  of  the  brain  and  a 
dorsal  portion,  the  tegmentum,  Tg.  The  pes  consists  mainly 
of  the  fibres  of  the  pyramidal  tract,  Py,  but  some  fibres  of  the 
fillet,/,  also  run  forward  in  it,  as  do  fibres,  fr  and  oc,  connecting 


THE  PHYSIOLOGY  OF  THE  BRAIN.  617 

it  with  the  frontal  and  occipital  lohes  of  the  cerebral  hemisphere. 
The  tegmentum  contains  gray  masses  and  many  transverse 
and  longitudinal  fibres.  Many  of  the  fibres,  cp,  come  from  the 
anterior  peduncle  of  the  cerebellum ;  these  cross  in  the  pos- 
terior part  of  the  tegmentum;  most  of  them  end  in  a  large 
mass  of  gray  matter  in  the  front  of  the  tegmentum  named  the 
red  nucleus  ;  others  run  forward  to  the  optic  thalamus  direct. 
Other  longitudinal  fibres  are  continued  from  the  fillet  some  of 


Fig.  176.— Diagram  of  cross-sectiou  of  midbrain  in  region  of  posterior  corpora 
quadrigemina:  P,  pes;  Ln,  locus  niger;  Tij,  tegmentum;  .S\  aqueduct  of  Sylvius; 
cq,  light  corpus  quadrigeminum.  In  the  pes  rtu  Wr  side  are  indicated  by  fr,  fibres 
from  frontal  lobe  of  cerebral  hemisphere;  /,  fibres  of  fillet;  py,  fibres  of  pyramidal 
tract;  oc,  fibres  from  occipital  lobe  of  cerebral  hemisphere.  In  tegmental  region. 
/',  fillet  fibres  to  anterior  corpus  quadrigeminum;  /".  fillet  fibres  for  posterior 
corpus  quad  rigeminu  ni ;  cp.  fibres  from  cerebellar  peduncle.  Parts  containing  gray 
nerve  matter  are  shaded  in  horizontal  lines. 

these,/",  end  in  the  posterior  corpora  quadrigemina;  others, 
/',  in  the  superior  corpora  quadrigemina  and  the  occipital  re- 
gion of  the  cerebral  hemispheres.  The  corpora  quadrigemina 
are  covered  by  a  layer  of  medullated  fibres,  but  their  main 
mass  is  gray  matter.  The  anterior  pair  are  closely  connected 
with  the  optic  tracts,  and  therefore  with  the  optic  nerves  and 
the  retinas:  to  their  outer  sides  and  in  front  are  the  external 
corpora  geniculates,  gray  masses  closely  associated  with  the 
optic  trad  -. 

The  structure  of  the  midbrain  shows  that  it  is  in  great  part 
merely  a  commissure  between  the  parts  in  front  of  and  behind 
it:  but  its  connect  ion  with  fibres  of  the  optic  tract  shows 
that  it  has  a  close  relation  to  visual  sensations;  and  the  origin 
in  it  of  the  oculo-motor  ami  abducens  nerves,  that  from  it  the 
eye  muscles,  and  the  iris  and  ciliary  muscle  are  innervated. 


618 


THE   III  MA. \   BODY. 


The  Brain  Regions  in  Front  of  the  Midbrain.  It  would 
be  quite  a  hopeless  tast  to  attempt  ina  few  pages  any  detailed 
account  of  the  topograph}  of  these,  bul  in  addition  to  the 
facts  already  stated  a  Hew  points  of  special  physiological  signifi- 
cance may  be  indicated.  These  portions  of  the  brain  may 
be  in  general  described  as  consisting  of  three  masses  of  gray 
matter  on  each  side;  optic  thalamus,  corpus  striatum,  cerebral 


Fig.  177.— Diagram  to  illustrate  cerebral  distribution  of  fibres  proceeding  from 
the  pes  of  the  orn<  cerebri.     For  description  see  texi 

cortex.  They  are  united  in  manifold  ways  by  the  transverse, 
longitudinal,  and  oblique  fibres  of  the  white  substance  of  the 
cerebral  hemisphere.  Their  more  fundamental  relations  to 
the  midbrain  and  to  one  another  are  shown  in  a  very  sim- 
plified and  diagrammatic  manner  in  Figs.  ITT  and  178.  The 
iter,  or  aqueduct  of  Sylvius,  i,  is  seen  passing  into  the  pos- 
terior end  of  the  third  ventricle,  3,  which  is  separated  by 
only  a  very  thin  layer  of  white  matter  from  the  large  ovoid 
gray  mass  ot,  which  is  the  optic  thalamus.     Connected  by  the 


THE  PHYSIOLOGY  OF  THE  BRAIN.  619 

foramen  of  Monro,  fM,  with  the  third  ventricle  is  the  left 
lateral  ventricle,  2,  bounded  on  the  inner  side  by  the  thin 
septum  lucidum.  Between  the  septa  lncida  is  the  fifth  ven- 
tricle, 5.  The  gray  mass,  Nc,  to  the  side  of  the  lateral  ven- 
tricle is  the  caudate  nucleus  and  the  mass  Ln  the  lenticular 
nucleus  of  the  corpus  striatum.  In  front  and  at  a  level  dif- 
ferent from  that  of  the  diagram  the  two  are  continuous. 

The  band  of  white  fibres,  ic,  lying  here  between  the 
lenticular  nucleus  on  the  outer  side  and  the  caudate  nucleus 
and  optic  thalamus  on  the  inner  side  is  the  internal  capsule  : 
■ec  is  the  external  capsule.  Fl  is  the  cortical  gray  matter  of 
the  frontal  lobe  of  the  cerebrum ;  PI,  of  the  parietal  lobe ; 
Oc.l.  of  the  occipital  lobe:  Ro,  the  fissure  of  Rolando;  Po,  the 
parieto-occipital  fissure.  The  course  of  many  fibres  in  the 
forebrain  is  still  uncertain,  but  some  important  paths  have 
been  traced  by  anatomical  and  microscopic  work,  and  still 
more  by  following  tracts  of  degeneration  resulting  from  cer- 
tain lesions,  as  in  the  case  of  the  spinal  cord ;  and  also  by 
noticing  the  results  of  stimulation  or  removal  of  definite  areas. 

Taking  first  the  pes  of  the  crus  cerebri  (Fig.  176),  which 
consists  entirely  of  longitudinal  fibres,  we  find  that  the  py- 
ramidal tract,  py,  Fig.  177,  is  continued  through  the  internal 
capsule  and  radiates  beyond  it,  to  end  in  the  cortex  of  the 
frontal  and  parietal  lobes  in  the  region  of  the  fissure  of  Rolando. 
These  fibres  are  all  efferent  and  degenerate  to  their  endings  in 
the  gray  matter  of  the  cord  or  the  motor  nuclei  of  cranial  nerves 
when  the  cortex  in  the  Rolandic  region  is  removed.  A  second 
collection  of  fibres  in  the  pes  is  the  frontal,  and  its  fibres,  fr, 
can  be  traced  to  the  frontal  region  of  the  cortex;  when  that 
is  removed  the  fibres  degenerate  as  far  as  the  gray  matter  of 
the  pons,  from  which  they  are  probably  connected  by  other 
fibres  with  the  opposite  side  of  the  cerebellum.  A  third  set 
of  fibres  in  the  pes  is  the  temporo-occipital,  oc:  they  also  pass 
through  the  internal  capsule  to  the  corresponding  region  of 
the  cortex:  they  can  be  traced  as  far  as  the  gray  matter  of 
the  pons,  and  appear  to  be  fibres  of  descending  degeneration. 
Another  set  of  fibres,  ca,  of  descending  degeneration  in  the 
internal  capsule  lias  no  immediate  connection  with  the  cortex: 
it  arises  in  the  caudate  nucleus;  the  course  of  the  fibres  be- 
yond the  pons  is  not  known.  The  lenticular  nucleus  also 
gives  off  fibres,  g,  to  the  internal  capsule,  which  probably  con- 
nect the  corpus  striatum  through  the  pes  with  the  pons  and 


620  ri HE  HUMAN  BODY. 

medulla  oblongata,  but  they  are  so  mingled  with  other  fibies 
that  they  have  not  been  satisfactorily  traced. 

Passing  now  to  the  tegmentum,  it  is  first  to  be  borne  in 
mind  thai  many  fibres  (including  mosl  of  those  of  the  anterior 
cerebellar  peduncle)  entering  it  from  behind,  end  in  the  large 
red  nucleus  lying  in  its  anterior  portion  and  in  its  other  gray 
masses.  From  these  (Fig.  178)  numerous  fibres,  rw,  pass  to 
the  optic  thalamus:  so  that  the  majority  of  the  tegmental 
fibres  differ  from  the  pedal  in  that  they  only  have  indirect 
connection  with  the  cortex  through  the  thalamal  and  other 
gray  matter.  The  thalamus  is  united  with  nearly  all  regions 
of  the  cortex  by  fibres,  af,  passing  from  its  outer  side  into 
the  internal  capsule,  and  distributed  in  special  abundance  to 
the  occipital  lobe.  Since  the  thalamus  receives  fibres  through 
the  tegmentum  from  the  anterior  quadrigemina  and  the 
lateral  geniculata  (which  Ave  have  seen  to  have  close  connec- 
tion with  the  optic  nerves),  and  there  is  independent  reason 
for  believing  parts  of  the  occipital  lobe  to  be  closely  associated 
with  visual  perceptions,  the  close  anatomical  association  of 
that  lobe  with  the  thalamus  is  significant.  Another  group  of 
fibres,  d,  connects  the  thalamus  with  the  temporal  and  occip- 
ital cortex,  but  does  not  take  its  path  through  the  internal 
capsule.  Some  fibres  of  the  tegmentum  reach  the  cortex 
without  primary  connection  with  the  thalamus:  of  these  is  a 
set,  e,  which  passes  through  the  lenticular  nucleus  (but  with- 
out any  communication  with  its  gray  substance)  on  its  way 
to  the  frontal  and  parietal  lobes.  At  ce  is  indicated  a  set  of 
fibres  of  the  tegmentum  which  there  is  some  reason  to  believe 
run  to  the  fore  part  of  the  cortex  direct,  having  no  connection 
with  the  thalamus  and  passing  ventral  to  the  internal  capsule. 

Most  of  the  fibres  of  the  fillet,  we  have  seen,  end  in  the 
red  nucleus  or  corpora  quadrigemina:  fibres  arising  in  these 
gray  masses  connect  them  with  the  thalamus  and  through  it 
with  the  cortex. 

Besides  fibres  connecting  the  cortex  with  other  parts  are 
many  which  unite  different  cortical  areas  directly.  A  vast 
number  (Ca,  Fig.  177)  cross  the  middle  line  in  the  corpus 
callosum  and  are  believed  to  join  corresponding  parts  of  the 
two  hemispheres.  Others  pass  over  in  the  small  white  an- 
terior commissure  and  unite  the  two  olfactory  lobes  and 
portions  of  the  temporal  lobes.  The  posterior  commissure 
unites  mainly  the  optic  thalami  and    the  front  ends  of  the 


THE  PHYSIOLOGY  OF  THE  BRAIN. 


621 


tegmenta.  The  soft  commissure  is  mainly  gray  matter. 
Finally  a  large  number  of  associational  fibres,  as,  unite 
different  parts  of  the  cortical  substance  of  the  same  hemi- 
sphere. 

The  different  gray  masses  on  the  same  side  of  the  forebrain 
are  also  united  by  fibres.  They  are  either  so  scattered  among 
others  that  they  cannot  be  tracked  out  along  special  tracts  of 
degeneration;  or,  as  is  possible,  resemble  some  of  the  com- 


Fig  178.— Diagram  to  illustrate  cerebral   distribution  of  the  fibres  proceeding 
from  the  tegmentum.    For  description  see  text. 

missural  fibres  uniting  upper  and  lower  regions  of  gray  matter 
in  the  spinal  cord  in  having  nutritive  centres  at  each  end, 
and  therefore  not  degenerating  on  either  side  of  a  section.  In 
any  case  very  little  is  known  as  to  their  numbers  or  paths: 
their  existence  is  indicated  I »y  the  dotted  lines  in  Figs.  177, 
1 78. 

Omitting  the  associational  anil  the  cross  commissure  fibres 
and    those   uniting  tbe  corpora   striata   and    optic    fchalami, 


622  TEE  HUMAN  BODY. 

it  maybe  said  in  general  that  the  systems  of  fibres  represented 
in  Fig.  177  are  all  almost  certainly  concerned  in  conveying 
impulses  from  the  cortex,  and  those  in  Fig-  178  in  the  trans- 
mission of  afferent  impulses.  It  will  be  noted  thai  both  affer- 
ent and  efferent  fibres  are  abundant  in  the  internal  capsule ; 
and  that  the  corpus  striatum  and  pes  are  more  especially  con- 
nected with  efferent  and  the  tegmentum  and  thalamus  with 
afferent  impulses.  It  can  hardly  be  necessary  to  add  that 
each  line  in  the  diagrams  represents  hundreds  of  thousands 
of  nerve-fibres. 

The  Functions  of  the  Cerebral  Cortex.  That  this  part  of 
the  nervous  system  is  in  close  association  with  the  intellect  and 
with  the  initiation  of  voluntary  movements  seems  beyond 
doubt:  but  it  may  have  other  functions  quite  apart  from  any 
states  of  consciousness;  and  intelligence  and  every  volition  may 
not  entirely  depend  on  it.  The  experiments  made  in  recent 
years  on  the  lower  animals  tend  to  the  conclusion  that  some- 
will  and  some  intellect  may  remain  in  animals  all  or  almost  all 
of  whose  gray  cerebral  surfaces  have  been  removed;  the  more 
complete  loss  of  those  powers  described  by  earlier  workers 
being  due  to  the  fact  that  the  animals  were  not  kept  alive  long 
enough  after  the  operation.  It  has  been  observed  that  a  dog 
whose  cerebral  cortex  (as  verified  by  subsequent  post-mortem 
examination)  had  been  nearly  completely  removed  did  learn 
after  some  months  to  walk  about  to  all  appearance  voluntarily, 
and  to  find  and  eat  his  food ;  he  even  learned  not  to  take  the 
food  of  other  dogs  after  he  had  been  severely  bitten  several 
times  for  so  doing.  But  more  complex  perceptions  were  lost : 
before  the  operation,  for  example,  hi'  was  greatly  terrified  by 
seeing  a  man  fantastically  dressed,  but  afterwards  no  such 
appearance  aroused  in  him  so  complex  a  conception  as  that  of 
a  strange  or  dangerous  object.  He  also  never  recovered  the 
trick  of  "giving  paw,1'  which  had  previously  been  taught 
him.  But  on  the  whole  a  person  casually  observing  him  would 
not  have  thought  him  very  different  Erom  any  other  dog,  ex- 
cept perhaps  that  he  was  rather  stupid:  put  into  a  low  open 
box,  for  example,  he  would  not  jump  out  of  it  when  called, 
though  he  easily  could  do  so  and  clearly  desired  to.  Such 
simple  and  fundamental  perceptions  and  volitions  as  remained 
in  this  and  some  similar  cases  probably  have  their  seats  in  the 
optic  thalami  and  corpora  striata,  and  indeed  embryology  shows 
that  the  corpora  striatum   is  morphologically  a  part  of  the 


THE  PHYSIOLOGY  OF  THE  BRAIN.  623 

cerebral  cortex:  it  is  therefore  probable  that  in  man  some  of 
the  lower  and  simpler  mental  faculties  are  associated  also  with 
those  parts.  There  are,  however,  great  and  obvious  chances 
of  error  in  arguing  from  the  actions  of  the  lower  animals  as  to 
their  mental  state:  and  these  are  increased  by  the  compara- 
tively small  proportion  the  cerebral  cortex  bears  to  the  whole 
cerebro-spinal  centre  in  these  animals  when  compared  with  its 
ratio  in  man,  showing  its  less  importance  in  the  management 
of  their  actions.  Hence  the  most  useful  observations  are 
those  made  of  late  years  on  apes  and  monkeys  and  on  men 
suffering  from  local  brain  disease.  By  utilizing  these  it  has 
been  possible  to  map  out  certain  areas  of  the  brain  surface 
as  having  special,  though  possibly  not  absolutely  unshared 
association,  with  volitional  movement  and  with  groups  of 
sensations  and  sensory  interpretations.  In  addition  to  facts 
obtained  by  removal  or  local  disease  of  parts  of  the  brain  we 
have  others  obtained  by  electrical  stimulation  of  certain 
parts  of  the  cortex,  which  although  quite  insensible  to  cut- 
ting or  mechanical  irritation  does  in  some  places  respond  to 
application  of  the  interrupted  or  constant  electric  current. 
The  more  important  results  obtained  are  indicated  in  a 
general  way  in  Figs.  179  and  180,  representing  respectively  the 
outer  and  inner  surfaces  of  the  right  cerebral  hemisphere; 
these  diagrams  should  be  compared  with  the  more  detailed 
figures  in  Chapter  XI. 

The  shaded  area  beginning  on  the  top  of  the  brain  and 
extending  down  the  sides  of  the  fissure  of  Rolando  or  central 
fissure,  Ro,  and  beyond  its  ventral  end  is  the  motor  area  of 
the  cortex.  It  also  extends  to  the  inner  side  of  the  hemi- 
sphere, as  shown  in  Fig.  179.  Electric  stimulation  of  dif- 
ferent parts  of  this  area  causes  movements  of  leg,  arm,  or 
face  as  indicated.  Removal  of  the  region  marked  "arm"  in 
the  monkey  causes  motor  paralysis  and  some  loss  of  sensi- 
bility in  the  arm  on  the  opposite  side  of  the  body.  It  is 
also  followed  by  degenerations  extending  from  the  re- 
moved region  of  cortex  through  the  internal  capsule  to 
gome  pyramidal  fibres  in  the  pea  and  thence  back  through 
the  pyramids  to  the  crossed  pyramidal  and  direct  pyram- 
idal tracts  in  the  cord  as  far  as  the  cervical  enlarge- 
ment. Localized  disease  of  this  area  in  man  is  followed 
by  paralysis  of  voluntary  movements  of  the  opposite  arm 
and  by  similar  degenerations.     Similar  statements  arc  true 


624 


THE   HUMAN  BODY. 


for  the  areas  marked  leg,  foot,  and  face,  except  that  the  re- 
sulting degeneration  would  extend  in  the  one  case  to  the 
lumbar  enlargement  of  the  cord,  in  the  other  to  the  nucleus 
of  the  vii  nerve  in  the  medulla.  Moreover,  each  of  these 
areas  can  be  mapped  out  into  smaller  ones,  giving  origin 
to  a  more  limited  movement  when  .stimulated  and  a  more 
limited  paralysis  ami  tract  of  degeneration  when  removed. 
Thus  areas  especially  associated  with  the  eyelids,  with  the 
muscles  of  the  angle  of  the  mouth,  with  the  flexor  muscles  of 


Fig.  179.—  Diagram  of  outer  surface  of  left  cerebral  hemisphere  to  illustrate  the 
localization  of  functions.  The  motor  area  is  shaded  in  vertical  and  transverse 
lines:  Syy  Assure  of  Sylvius;  an.  ansrnlar  gyrus  or  convolution;  Ro,  fissure  of 
Rolando;  Fv,  frontal  lobe;  Pa,  parietal  lobe;  Te,  temporal  lobe.  Only  a  very  few 
of  the  more  important  fissures  are  indicated.     Compare  with  Fig.  180. 

the  wrist,  all  have  their  definite  places  in  the  general  face  or 
arm  region.  So  definite  are  the  positions  of  these  areas  that 
in  cases  of  localized  paralysis,  diagnosed  as  due  to  lesions  of 
the  cerebral  cortex,  surgeons  now  have  no  hesitation  in  open- 
ing the  skull  in  order  if  possible  to  remove  the  cause  of 
trouble,  as  a  small  tumor:  they  know  precisely  in  what  spot 
they  will  find  it.  Although  the  localization  is  therefore 
tolerably  precise,  yet  the  limits  of  neighboring  areas  are  not 
as  sharp  cut  as  the  boundaries  of  neighboring  countries  on  a 
map:  as  shown  in  Fig.  179,  the  arm  area  in  its  lower  part 
overlaps  part  of  the  face  area;  and  the  minor  areas  within 
the  main  ones  also  overlap  one  another  at  their  margins. 

The  general  interpretation  put  upon  the  above  facts,  and 
one  which  seems  justified,  is  that  in  making  definitely  willed 


THE  PHYSIOLOGY  OF  THE  BRAIN.  625 

movements  the  cortical  area  connected  through  the  pyram- 
idal tract  with  the  muscles  concerned  is  the  place  from 
which  efferent  impulses  start  throwing  into  action  lower 
centres  which  more  immediately  co-ordinate  the  muscles: 
these  lower  centres  in  midbrain,  cerebellum,  medulla  or  cord 
may  of  course  be  thrown  into  reflex  action  by  afferent  im- 
pulses having  no  connection  with  the  cortex,  and  to  the  eye 
the  resulting  movement  would  be  exactly  the  same  as  a 
willed  one.  In  another  person,  and  still  more  in  a  dog  or 
monkey,  we  must  often  be  in  doubt  whether  an  action  is  or 
is  not  intentional;  and  as  already  pointed  out,  many  move- 
ments of  our  own  which  were  at  one  time  even  painfully  in- 
tentional become  quite  unconscious  after  practice  and  are 
carried  out  by  lower  centres.  It  is  also  to  be  borne  in  mind 
that  the  cortical  area  from  which  the  efferent  processes  of  a 
willed  movement  make  their  start  is  in  connection  by  as- 
sociational  and  other  commissural  fibres  with  many  other 
regions  of  the  cortex,  and  with  fibres  from  the  optic  thalamus 
which  may  bring  nerve  impulses  exciting  it,  and  it  is  also  in 
connection  with  the  whole  gray  cortical  network,  so  that  the 
brain  antecedents  or  excitants  leading  to  a  given  movement, 
either  alone  or  in  combination  with  others,  may  be  very 
different,  and  may  be  associated  or  not  with  concomitant 
sensations  or  emotions. 

Take  such  a  movement  as  clenching  the  fist.  On  a  corj^se 
this  might  be  brought  about  by  pulling  on  the  flexor  tendons 
of  the  digits,  but  in  an  imperfect  way;  or,  again  in  a  very 
imperfect  manner  by  stimulation  of  the  motor  nerves  of  the 
flexor  muscles  in  the  arm  of  a  living  person.  If,  however,  we 
knew  exactly  the  proper  sensory  fibres  in  spinal  nerve-roots 
to  stimulate  and  could  thus  act  on  the  centre  co-ordinatinc 
the  proper  muscles,  there  is  no  doubt  we  could  bring  about 
reflexly,  and  apart  from  all  consciousness,  a  quite  normal 
clenching  movement.  Next  suppose  a  person  struggling  for 
breath:  as  his  extraordinary  muscles  of  respiration  come  into 
play  his  fists  are  clenched;  here  impulses  from  the  medulla 
oblongata  travel  down  the  cord  and  throw  the  "  clenching  " 
spinal  centre  into  activity  along  with  many  other  muscles, 
and  co-ordinating  them  all  so  as  to  give  as  good  a  pull  as  pos- 
sible to  all  muscles  which  can  help  an  inspiration.  In  a 
higher  but  still  not  volitional  stage,  more  groups  of  muscles 
are  concerned,  and  centres  of  co-ordination  in  the  pons  and 


626  THE  HI  MAX  BODY. 

cerebellum  come  into  action  also;  take  a  man  preparing  for  a 
high  jump:  as  lie  crouches  and  puts  himself  in  balance  for 
t be  spring  he  clenches  his  fists,  quite  unconsciously  of  course. 
Bere  the  immediate  clenching  centre  is  thrown  into  activity 
along  with  the  muscles  of  breathing,  and  of  all  parts  of  the 
trunk  and  limbs.  Each  subsidiary  peripheral  centre  play.-  its 
part  and  the  instreaming  afferent  impulses  from  t  be  skin  of  the 
feet,  from  the  fihres  of  the  muscular  sense,  from  the  semicir- 
cular canals,  from  the  eyes,  are  all  concerned  (without  the 
person's  perception  of  them)  in  throwing  the  motor  mechanisms 
of  midbrain,  cerebellum,  medulla,  and  cord  into  harmonious 
activity,  so  that  when  the  jump  is  actually  willed  it  shall  be 
accomplished.  But  that  in  this  case  the  volition  plays  a  very 
secondary  part  is  obvious;  it  merely  acts  on  an  apparatus  all 
ready  to  discharge  in  a  given  way  when  a  suitable  additional 
nerve  impulse  reaches  it.  A  runner  all  tense  for  the  start  of 
a  hundred-yard  race  can  hardly  be  said  to  start  voluntarily 
when  he  hears  the  signal;  the  case  is  comparable  more  to  the 
self-balancing  of  a  pigeon  deprived  of  its  cerebral  hemi- 
spheres, when  its  perch  is  tilted.  Next,  suppose  I  clench  my 
fist  "  involuntarily,"  as  we  commonly  say,  when  I  see  some- 
thing that  arouses  my  indignation;  here  clearly  a  mental 
element  is  in  play,  but  not  a  volitional  one,  and  so  far  as 
the  movement  is  concerned  probably  the  motor  area  of  the 
cortex  has  little  or  nothing  to  do  with  it:  it  is  more  in  accord 
with  what  is  seen  on  animals  to  suppose  that  such  simple 
emotions  and  their  characteristic  movements  may  be  carried 
out  by  nerve  apparatuses  lying  no  higher  than  the  thalami 
and  corpora  striata.  If,  however,  I  strike  a  man  with  the 
intention  to  punish  him,  there  can  be  little  doubt  that  the 
"clenching"  centre  is  excited  by  fibres  from  the  cortex  and 
passing  down  in  the  pyramidal  tract.  But  this  cortical  area 
may  in  turn  be  thrown  into  activity  and  may  have  its  ten- 
dency to  discharge  modified  in  many  ways.  My  anger  may 
be  the  culminating  result  of  many  long  past  received  and 
interpreted  and  remembered  sensations,  and  whether  I  shall 
give  the  blow  or  restrain  myself  also  be  dependent  on  many 
antecedents  of  experience.  Again,  I  clench  my  hand  to 
knock  down  a  madman,  as  the  only  immediate  method  of 
preventing  him  from  committing  a  murder:  here  the  same 
motor  cortical  area  no  doubt  would  he  thrown  in  action  as 
when  the  blow  was  struck   in  anger,  but  it  is  clear  that  the 


THE  PHYSIOLOGY  OF  THE  BRAIN.  627 

antecedent  nerve  processes  arousing  its  activity  would  be 
quite  different  in  the  two  cases;  and  they  would  yet  again  be 
different  if  I  clenched  the  fist  in  order  to  explain  to  a  child 
the  meaning  of  the  word  clench.  We  see  then  that  the  im- 
mediate motor  centres  may  be'excited  iu  various  ways  and  in 
various  combinations  quite  apart  from  the  cortex  of  the  cere- 
brum arid  by  fibres  not  connected  with  the  pyramidal  tracts; 
and  that  when  excited  from  the  cortical  area  of  the  cere- 
brum through  fibres  of  the  pyramidal  tract,  that  area  itself 
may  be  excited  or  controlled  in  its  activity  by  a  vast  number 
of  other  parts  of  the  cortex,  and  by  non-cortical  parts  of  the 
nervous  system.  The  motor  area  cannot  properly  be  spoken 
of  as  the  seat  of  volition :  an  act  of  willing  is  the  final  out- 
come of  changes  in  other  and  often  numerous  other  regions 
of  the  cortex,  the  resultant  of  whose  material  processes  is  a 
discharge  of  efferent  impulses  from  some  region  of  the  motor 
area. 

The  permanent  effects  of  local  lesions  of  the  Rolandic 
region  differ  with  the  development  of  the  brain.  In  dogs 
removal  of  the  left  brain  region  connected  with  the  fore 
paw  causes  only  temporary  motor  paralysis  of  the  limb  on 
the  other  side;  after  a  time  the  animal  learns  to  walk  again 
as  well  as  before:  then  removal  of  the  corresponding  area  on 
the  right  side  of  the  brain  is  followed  by  paralysis  of  both  fore 
limbs.  This  has  been  supposed  to  show  that  the  centre  on 
the  right  side  had  taken  up  the  duty  of  control  for  both  sides 
after  that  on  the  left  had  been  removed.  However  that  may 
be,  the  second  paralysis  is  also  only  temporary,  disappearing 
in  some  weeks  or  months;  and  as  has  been  already  stated, 
even  after  removal  of  all  the  motor  area  the  animal  occasion- 
ally learns  in  the  course  of  time  to  walk  nearly  as  well  as 
ever.  This  must  be  due  to  lower  centres  (corpora  striata?), 
and  the  question  is  whether  the  movements  in  such  cases  are 
truly  volitional,  for  definite  acts  of  willing  a  movement  prob- 
ably play  a  very  small  part  in  a  dog's  life:  most  of  its  move- 
ment-, are  the  immediate  efferent  expression  of  afferent  im- 
pulses and  true  volitions  have  but  a  small  part  in  them.  In 
the  lower  monkey-'  definite  motor  effects  of  removal  of  part 
of  the  cortical  motor  area  are  also  temporary,  but  last  Longer 
than  in  dogs;  ami  in  the  anthropoid  apes  the  same  is  the  case 
in    a   greater    degree,  ami    according   to    some    experimenters 

certain  delicate  combined   movements  are  permanently  lost 


628  TUB  HUMAN  BODY. 

after  destruction  of  the  motor  area.  These  facts  are  correl- 
ated with  the  relatively  larger  size  of  the  cortical  motor  area 
and  of  the  pyramidal  tracts  iu  monkeys  as  compared  with  dogs, 
and  the  anthropoid  apes  as  compared  with  other  monkeys. 
The  larger  and  more  highly  organized  the  brain  area  the 
greater  the  part  it  plays  in  the  life-work  of  the  animal  and 
more  noticeable  are  the  results  of  its  absence.  In  man  local 
paralysis  due  to  local  cortical  lesion  is  often  only  temporary: 
this  may  be  due  to  disappearance  of  the  disease;  or  to  the 
primary  paralysis  being  only  a  "  shock  "  effect,  and  not  due  to 
actual  disease  of  the  motor  centre,  for  it  is  well  known  that 
in  animals  injury  to  one  region  of  the  brain  will  often  for  a 
considerable  time  inhibit  the  activity  of  other  parts:  or  it  may 
be  due  to  the  hemisphere  of  the  opposite  side  assuming  con- 
trol. Different  observers  attribute  very  various  values  to  these 
three  possible  factors.  In  this  connection  reference  may  be 
made  to  cases  of  what  is  called  aphasia,  which  in  its  fully  de- 
veloped state  is  a  loss  of  the  power  to  apply  words  to  express 
ideas.  The  power  of  speech  may,  of  course,  be  lost  through 
disease  of  the  larynx  or  paralysis  of  the  nerves  or  muscles  of  the 
voice  organs,  but  such  a  condition  is  not  true  aphasia:  the 
aphasic  person  can  often  articulate  perfectly  well,  but  he  can- 
not attach  a  meaning  to  his  spoken  word:  in  some  cases  he 
can  write  words  with  meaning,  though  he  cannot  say  them;  in 
other  cases  (agraphia)  the  power  of  using  written  words  to 
express  ideas  is  also  lost,  though  the  person  can  write,  and  his 
general  conduct  shows  that  he  is  still  guided  by  his  intelli- 
gence; he  knows  quite  well  what  he  wants  to  say,  but  he  can- 
not set  the  proper  motor  apparatus  in  action  to  utter  the 
word :  if  he  speaks,  the  word  has  no  connection  with  that  in 
his  mind,  and  as  soon  as  he  hears  himself  speaking  it  he  often 
knows  that  the  word  he  uses  is  quite  wrong.  We  find  in  such 
cases  the  power  to  understand  words,  and  to  form  ideas  of 
words,  and  to  utter  words,  but  some  link  between  the  origin 
of  the  idea  and  the  discharge  of  the  motor  impulses  willed  to 
express  it  is  out  of  gear.  It  is  as  if  an  injured  reflex  centre 
should  give  a  wrong  or  inco-ordinate  efferent  response  to  an 
afferent  impulse.  Aphasia  is  almost  invariably  connected 
with  disease  of  the  area  marked  SP  in  Fig.  179  and  known 
as  the  third  or  lower  frontal  convolution,  and  the  pathological 
change  is  on  the  left  side  of  the  brain  only.  The  area,  as  will 
be  seen,  is  closely  associated  with  the  face  area  and  the  tongue 


THE  PHYSIOLOGY  OF  THE  BRAIN.  629 

and  partly  overlaps  them,  or  rather  is  intermixed  with  them ;  as 
pointed  out  above,  the  lesion  is  not  one  of  motor  sj)eech  cen- 
tres, bat  of  the  connection  between  these  and  other  cerebral 
areas  in  which  have  occurred  changes  accompanied  by  the 
desire  of  verbal  expression ;  something  wrong  probably  in  the 
gray  network.  Very  rarely  aphasia  has  been  known  to  follow 
disease  or  injury  of  the  corresponding  convolution  on  the 
right  side;  so  that  in  it  we  have  an  example  of  a  very  definite 
nexus  between  a  limited  area  of  the  cortex  and  the  expres- 
sion of  will  through  movements.  Cases  of  recovery  from 
aphasia  have  occurred,  but  are  extremely  rare.  In  the  ex- 
ceptional cases  it  has  been  supposed  that  the  right  side  of  the 
brain  takes  up  the  duty  of  connecting  the  material  changes 
in  the  gray  network  which  accompany  the  origination  of  an 
idea  in  one  or  more  cortical  areas,  with  the  other  changes 
which  result  in  speech.  This  view  gains  some  support  from 
the  fact  that  in  certain  cases  of  recovery  due  to  left-side  dis- 
ease, subsequent  disease  in  the  third  right  frontal  convolution 
has  been  followed  by  a  fresh  aphasia.  But  however  that  may 
be  we  have  in  aphasic  persons  definite  evidence  of  the  limita- 
tion of  definite  function  to  a  very  limited  area  or  areas  of 
the  cerebral  cortex. 

Much  less  is  known  as  to  other  regions  of  the  cortex  than 
of  the  motor  area:  most  of  them  do  not  respond  to  electrical 
stimulation  at  all,  and  those  areas  that  do,  only  show  it  by 
movements  lacking  in  precision.  We  are  reduced,  therefore, 
to  observation  on  animals  from  whom  certain  cortical  parts 
have  been  removed,  and  to  observations  on  diseased  persons. 
Certain  broad  regions  have  in  this  way  been  mapped  out  as 
connected  with  certain  main  groups  of  sensations  (Figs.  179, 
180),  probably  rather  with  the  combining  and  interpreting 
of  sensations,  with  their  ideation,  than  with  the  mere  raw 
sensation  itself.  The  latter  is  probably  more  dependent  on 
the  lower  brain  centres;  in  most  cases  it  is  secondary  changes 
in  these  which  lead  to  impulses  which  are  jiassed  on  to  excite 
the  cortical  sensory  areas. 

There  is  considerable  evidence  that  removal  or  extensive 
injury  of  the  left  occipital  lobe  causes  blindness  of  the  left 
half  of  each  retina,  and  vice  versa.  Also,  that  stimulation  of 
this  region  of  the  brain  may  cause  movements  of  the  eyes 
and  eyelids  which  have  been  described  as  Buch  as  an  animal 
would  make  if  it,  thought  it  saw  something,  though  obviously 


630 


THE  III  MAS  BODY. 


that  must  be  a  very  uncertain  deduction.  Also,  the  optic 
tract  of  each  side  has  through  the  anterior  corpus  quadri- 
geminum  and  some  other  gray  masses  a  close  connection  with 
the  cortex  of  the  occipital  lobe.     Probably,  therefore,  that 

region  has  some  close  connection  with  vision.  There  is  also 
some  evidence  that  the  angular  gyrus  {an,  Fig.  179)  has  con- 
nection with  sight. 

The  sense  of  smell  has  been  supposed  especially  connected 
with  the  uncinate  gyrus  of  median  side  of  the  temporal  lobe 
{un,  Fig.   ISO),  and  "the  sense   of  taste  with  a  neighboring 


Fig.  180. — Diagram  of  inner  surface  of  left  cerebral  hemisphere  to  illustrate 
cerebral  localization.  8q,  fissure  of  Sylvius:  Ro,  fissure  of  Rolando:  Ft\  frontal 
lobe;  Oc,  occipital  lobe:  Te.  temporal  "lobe;  C.cl,  corpus  callosum;  III,  third  ven- 
tricle.    Compare  with  Fig.  179. 

area,  but  the  evidence  is  unsatisfactory;  and  the  same  may  be 
said  of  the  reasons  which  led  to  designation  of  the  region  of 
the  temporal  lobe  close  behind  the  fissure  of  Sylvius  with 
hearing.  The  region  marked  on  the  diagram  as  that  of 
cutaneous  sensations  has  also  a  doubtful  claim:  there  is  some 
reason  to  believe  that  the  motor  area  of  the  cortex  has  con- 
nection with  the  muscular  sense;  also  to  some  extent  with 
tactile  feelings. 

Tactile  and  temperature  impulses  cross  the  middle  line 
somewhere  on  their  path  from  the  skin  to  the  brain.  An 
apoplectic  effusion  in  one  cerebral  hemisphere  causes  loss  of 
sensation  and  of  voluntary  movement  on  the  other  side  of 
the  Body. 

The  frontal  lobes  are  quite  irresponsive  to  excitation,  and 
considerable  parts  of  them  have  been  removed  without  ap- 


THE  PHYSIOLOGY  OF  THE  BRAIN.  631 

parent  diminution  of  motor  or  sensory  faculty.  By  a  sort  of 
process  of  exclusion,  the  rest  of  the  cortex  being  allotted 
(though  on  unsatisfactory  evidence)  to  motion  and  sensation 
the  frontal  regions  have  been  supposed  to  have  special  con- 
nection with  the  higher  intellectual  faculties. 

Mental  Habits.  Movements  which  are  commonly  exe- 
cuted together  tend  to  become  so  associated  that  it  is  difficult 
to  perforin  one  alone;  many  persons,  e.g.,  cannot  close  one 
eye  and  keep  the  other  open.  From  frequent  use,  the  paths 
of  conduction  between  the  co-ordinating  centi*es  for  both 
groups  of  muscles  have  become  so  easy  that  a  volitional  im- 
pulse reaching  one  centre  spreads  to  the  other  and  excites 
both.  This  association  of  movements,  dependent  on  the 
modification  of  brain  structure  by  use,  finds  an  interesting 
parallel  in  the  psychological  phenomenon  known  as  the  asso- 
ciation of  ideas;  and  all  education  is  largely  based  on  the 
fact  that  the  more  often  brain  regions  have  acted  together 
the  more  readily,  until  finally  almost  indissolubly,  do  they  so 
act.  If  we  always  train  up  the  child  to  associate  feelings  of 
disgust  with  wrong  actions  and  of  approbation  with  right, 
when  he  is  old  he  will  find  it  very  hard  to  do  otherwise:  such 
an  organic  nexus  will  have  been  established  that  the  activity 
of  the  one  set  of  centres  will  lead  to  an  excitation  of  that 
which  habit  has  always  associated  with  it.  The  higher  nerve- 
centres  are  throughout  eminently  plastic;  it  is  that  which 
marks  them  out  for  a  far  higher  utility  and  greater  adaptation 
to  the  varying  experiences  of  individual  life  than  the  more 
fixed  and  machine-like  lower  centres:  every  thought  leaves 
in  them  its  trace  for  good  or  ill;  and  the  moral  truism  that 
the  more  often  we  yield  to  temptation — the  more  often  an 
evil  solicitation,  sensory  or  otherwise,  has  resulted  in  a  wrong 
act — the  harder  it  is  to  resist  the  repetition  of  it,  has  its  par- 
allel (and  we  can  hardly  doubt  its  physical  antecedent)  in  the 
marking  out  of  a  path  of  easier  conduction  from  perceptive 
to  volitional  centres  in  the  brain.  The  knowledge  that  every 
weak  yielding  degrades  our  brain  structure  and  leaves  its  trail 
in  that  organ  tli rough  which  man  is  the  "  paragon  of  animals," 
while  every  resistance  makes  Less  close  tin;  bond  between  the 
thought  and  the  act  for  all  future  time,  ought  surely  to 
"give  as  pause: "  on  the  other  hand,  every  right  action  helps 
tablish  a  "path  of  leasl  resistance,"  and  makes  its  sub- 
sequent performance  easier. 


632  TIIE  HUMAN  BODY. 

The  brain,  like  the  muscles,  is  improved  and  strengthened 
by  exercise  and  injured  by  overwork  or  idleness;  and  just  as 
a  man  may  specially  develop  one  set  of  muscles  and  neglect 
the  rest  until  they  degenerate,  so  he  may  do  with  his  brain; 
developing  one  set  of  intellectual  faculties  and  leaving  the 
rest  to  lie  fallow  until,  at  last,  he  almost  loses  the  power  of 
usins:  them  at  all.  The  fierceness  of  the  battle  of  life  nowa- 
days  especially  tends  to  produce  such  lopsided  mental  de- 
velopments; how  often  does  one  meet  the  business  man,  so 
absorbed  in  money-getting  that  he  has  lost  all  power  of  ap- 
preciating any  but  the  lower  sensual  pleasures;  the  intel- 
lectual joys  of  art,  science,  and  literature  have  no  charm  for 
him;  he  is  a  mere  money-making  machine.  One,  also,  not 
unfrequently  meets  the  scientific  man  with  no  appreciation 
of  art  or  literature;  and  literary  men  utterly  incapable  of 
sympathy  with  science.  A  good  collegiate  education  in  early 
life,  on  a  broad  basis  of  mathematics,  languages,  and  the 
natural  sciences,  is  a  great  security  against  such  imperfect 
mental  growth;  one  danger  in  American  life  is  the  tendency 
to  put  lads  in  a  technical  college,  or  to  start  them  in  business 
before  they  have  attained  any  broad  general  education.  An- 
other danger,  no  doubt,  is  the  opposite  one  of  making  the 
training  too  broad;  a  man  who  knows  one  or  two  literatures 
fairly  well,  and  who  has  mastered  the  elements  of  mathemat- 
ics and  of  one  of  the  observational  or  experimental  sciences, 
is  likely  to  have  a  better  and  more  utilizable  brain  than  he 
who  has  a  smattering  of  half  a  dozen  languages  and  a  con- 
fused idea  of  all  the  "  ologies."  The  habits  of  mental  sloven- 
liness, the  illogical  thinking,  and  the  incapacity  to  know 
when  a  thing  really  is  mastered  and  understood,  which  one  so 
often  finds  as  the  results  of  such  an  education,  are  far  worse 
than  the  narrowness  apt  to  follow  the  opposite  error,  which 
is  often  associated  with  the  power  of  accurate  logical  thought. 
Those  who  are  deprived  of  the  advantages  of  a  general  colle- 
giate education  may  now,  more  easily  than  at  any  previous 
period,  cultivate  mental  breadth  by  reading  some  of  the  many 
excellent  general  reviews  and  magazines,  and  the  readable 
but  exact  popular  expositions  now  available  on  nearly  all 
subjects,  which  are  such  a  feature  of  our  age.  Associating, 
out  of  working  hours,  with  those  whose  special  pursuits  are 
different  from  our  own  is  almost  necessary  to  those  who 
would  avoid  such  an  asymmetrical  development  as  almost 
amounts  to  intellectual  deformity. 


CHAPTER  XXXVIII. 
VOICE  AND  SPEECH. 

Voice  consists  of  sounds  produced  by  the  vibrations  of 
two  elastic  bands,  the  true  vocal  cords,  placed  in  the  larynx, 
an  upper  modified  portion  of  the  passage  which  leads  from  the 
pharynx  to  the  lungs.  When  the  vocal  cords  are  put  in  a  cer- 
tain position,  air  driven  past  them  sets  them  in  periodic  vibra- 
tion, and  they  emit  a  musical  note;  the  lungs  and  respiratory 
muscles  are,  therefore,  accessory  parts  of  the  vocal  apparatus: 
the  strength  of  the  blast  produced  by  them  determines  the 
loudness  of  the  voice.  The  larynx  itself  is  the  essential  voice- 
organ  :  its  size  primarily  determines  the  pitch  of  the  voice, 
which  is  lower  the  longer  the  vocal  cords;  and,  hence,  shrill 
in  children,  and  usually  higher  pitched  in  women  than  in 
men:  the  male  larynx  grows  rapidly  at  commencing  man- 
hood, causing  the  change  commonly  known  as  the  "  breaking 
of  the  voice."  Every  voice,  while  its  general  pitch  is  de- 
pendent on  the  length  of  the  vocal  cords,  has,  however,  a 
certain  range,  within  limits  which  determine  whether  it  shall 
be  soprano,  mezzo-soprano,  alto,  tenor,  baritone,  or  bass. 
This  variety  is  produced  by  muscles  within  the  larynx  which 
alter  the  tension  of  tho  vocal  cords.  Those  characters  of 
voice  which  we  express  by  such  phrases  as  harsh,  sweet,  or 
sympathetic,  depend  on  the  structure  of  the  vocal  cords  of 
the  individual;  cords  which  in  vibrating  emit  only  harmonic 
partial  tones  (Chap.  XXXV)  are  pleasant;  while  those  in 
which  inharmonic  partials  are  conspicuous  are  disagreeable. 

The  vocal  oords  alone  would  produce  but  feeble  sounds; 
those  that  they  emit  are  strengthened  by  sympathetic  reso- 
nance  of  the  air  in  the  pharynx  and  mouth,  the  action  of 
which  may  be  compared  to  that  of  the  sounding-board  of  a 
violin.  By  movements  of  throat,  soft  palate,  tongue,  cheeks, 
and  lips  the   sounds  emitted    from   the  larynx   are  altered  or 

633 


634 


THE  HUMAN  BODY. 


supplemented  in  various  ways,  and  converted  into  articulate 
language  or  speech. 

The  Larynx  lies  in  front  of  the  neck,  beneath  the  hyoid 
bone  and  above  the  windpipe;  in  many  persons  it  is  promi- 
nent, causing  the  projection  known  as  "  Adam's  apple."  It 
consists  of  a  framework  of  cartilages,  partly  joined  by  true 
synovial  joints  and  partly  bound  together  by  membranes; 


Fig.  181.— The  more  important  cartilages  of  the  larynx  from  behind,  t,  thy- 
roid:  Cs,  its  superior,  and  Ci.  its  inferior,  horn  of  the  right  side;  *,,  cricoid  carti- 
lage; t,  arytenoid  cartilage;  Pv,  the  coiner  to  which  the  posterior  eud  of  a  vocal 
cord  is  attached;  Pm,  corner  on  which  the  muscles  which  approximate  or  sepa- 
rate the  vocal  cords  are  inserted;  co,  cartilage  of  Sautorim. 

muscles  are  added  which  move  the  cartilages  with  reference 
to  one  another;  and  the  whole  is  lined  by  a  mucous  mem- 
brane. 

The  cartilages  of  the  larynx  (Fig.  181)  are  nine  in  num- 
ber; three  single  and  median,  and  three  pairs.  The  largest 
(t)  is  called  the  thyroid',  and  consists  of  two  halves  which 
meet  at  an  angle  in  front,  but  separate  behind  so  as  to  inclose 
a  V-shaped  space,  in  which  most  of  the  remaining  cartilages 
lie.  The  epiglottis  (not  represented  in  the  figure)  is  fixed 
to  the  top  of  the  thyroid  cartilage  and  overhangs  the  entry 
from  the  pharynx  to  the  larynx;  it  may  be  seen,  covered 
by  mucous  membrane,  projecting  at  the  base  of  the  tongue, 
if  the  latter  be  pushed  down  while  the  mouth  is  held  open  in 
front  of  a  mirror;  and  is,  similarly  covered,  represented,  as 
seen  from  behind,  at  a   in  Fig.  182.     The  cricoid,  the  last 


VOICE  AND  SPEECH.  635 

of  the  impaired  cartilages,  has  the  shape  of  a  signet-ring;  its 
broad  part  (**,  Fig.  181)  is  on  the  posterior  side  and  lies  at 
the  lower  part  of   the  opening  between  the  halves  of  the 
thyroid;  in  front  and  on  the  sides  it  is  narrow,  and  a  space, 
occupied  by  the  crico-thyroid  membrane,  intervenes  between 
its  upper  border.and  the  lower  edge  of  the  thyroid  cartilage. 
The  angles  of  the  latter  are  produced  above  and  below  into 
projecting  horns  {Cs  and  Ci,  Fig.  181),  and  the  lower  horn 
on  each  side  forms  a  joint  with  the  cricoid.     The  thyroid  can 
be  rotated  on  an  axis,  passing  through  the  joints  on  each 
side  and  rolled  down  so  that  its  lower  front  edge  shall  come 
nearer  the  cricoid  cartilage,  the  membrane  there  intervening 
being  folded.     The  arytenoids  (f,  Fig.  181)  are  the  largest  of 
the  paired  cartilages;  they  are  seated  on  the  upper  edge  of 
the   posterior  wide  portion   of   the    cricoid,    and  form   true 
joints  with  it.     Each  is  pyramidal  with  a  triangular  base, 
and  has  on  its  tip  a  small  nodule  (co,  Fig.  181),  the  cartilage 
of  Santorini.     From  the  tip  of  each  arytenoid  cartilage  the 
aryteno-epiglottidean  fold  of  mucous  membrane  (10,  Fig.  182) 
extends  to  the  epiglottis;  the  cartilage  of  Santorini  causes  a 
projection  (8,  Fig.  182)  in  this,  and  a  little  farther  on  (9) 
is  a  similar  eminence  on  each  side,  caused  by  the  remaining 
pair  of  cartilages,  known  as  the  cuneiform,  or  cartilages  of 

Wrisbery. 

The  Vocal  Cords  are  bands  of  elastic  tissue  which  reach 
from  the  inner  angle  (Pv,  Fig.  181)  of  the  base  of  each  aryte- 
noid cartilage  to  the  angle  on  the  inside  of  the  thyroid  where 
the  sides  of  the    V unite;  they  thus  meet  in  front  but   are 
separated  at  their  other  ends.     The  cords  are  not,  however, 
bare  strings,  like  those  of  a  harp,  but  covered  over  with  the 
lining  mucous  membrane  of  the  larynx,  a   slit,  called  the 
glottis  (c,  Fig.  182),  being  left  between  them.     It  is  the  pro- 
jecting cushions  formed  by  them   on  each  side  of  this  slit 
which   are  set  in  vibration   during   phonation.     Above  each 
vocal  cord  is  a  depression,  the  ventricle  of  the  larynx  (b', 
Pig.   L82);   this  is  bounded  above  by  a  somewhat  prominent 
edge,  the  false  vocal  cord.     Over  most  of  the  interior  of  the 
larynx  its  mucous  membrane  is  thick  and  covered  by  ciliated 
epithelium,  and    has  many  mucous   glands  imbedded   in  it. 
Over   the  vofal  cords,  however,  it  is  represented  only  by  a 
thin  layer  of  flat  non-ciliated  cells,  and  contains  no  glands. 
In  quiet  breathing,  and  after  death,  the  free  inner  edges  of  the 


636 


THE  III. MAX  BODY. 


vocul  cords  are  thick  and  rounded,  and  .-rem  very  unsuitable 
for  being  readily  set  in  vibration.  They  are  also  tolerably 
widely  separated   behind,  the  arytenoid   cartilages,  to  which 

their  posterior  ends  are  attached,  being  separated.     Air  under 


Fig.  182.— The  larynx  viewed  from  its  pharyngeal  opening.  The  back  wall  of 
the  pharynx  has  been  divided  and  its  edges  (11)  turned  aside.  1,  body  of  hyoid; 
2,  its  small,  and  3.  its  great,  horns;  4,  upper  and  lower  horns  of  thyroid  cartilage; 
5,  mucous  membrane  of  front  of  pharynx,  covering  the  back  of  the  cricoid  carti- 
lage; ti,  upper  end  of  gullet;  7,  windpipe,  lying  in  front  of  the  gullet;  8,  eminence 
caused  by  cartilage  of  Santorini;  9.  eminence  caused  by  cartilage  of  Wrisberg; 
both  lie  in,  10,  the  aryteno-epiglottideaii  fold  of  mucous  membrane,  surrounding 
i lie  opening  (aditus  laryngis)  from  pharynx  to  larynx,  a,  projecting  tip  of  epi- 
glottis; c.  the  glottis,  the  lines  leading ■from  the  letter  point  to  the  free  vibratory 
edges  of  the  vocal  cords,  b',  the  ventricles  of  the  larynx:  their  upper  edges,  mark- 
ing them  off  from  the  eminences  b,  are  the  false  vocal  cords. 

these  conditions  passes  through  without  producing  voice.  If 
they  are  watched  with  the  laryngoscope  during  phonation,  it 
is  seen  that  the  cords  approximate  behind  so  as  to  narrow  the 
glottis;  at  the  same  time  they  become  more  tense,  and  their 
inner  edges  project  more  sharply  and  form  a  better-defined 
margin  to  the  glottis,  and  their  vibrations  can  be  seen. 
These   changes    are    brought   about    by    the    delicately   co- 


VOICE  AND   SPEECH.  63? 

ordinated  activity  of  a  number  of  small  muscles,  which  move 
the  cartilages  to  which  the  cords  are  fixed. 

The  Muscles  of  the  Larynx.  In  describing  the  direc- 
tion and  action  of  these  it  is  convenient  to  use  the  words 
front  or  anterior  and  back  or  posterior  with  reference  to  the 
larynx  itself  (that  is,  as  equivalant  to  ventral  and  dorsal)  and 
not  with  reference  to  the  head,  as  usual.  The  base  of  each 
arytenoid  cartilage  is  triangular  and  fits  on  a  surface  of  the 
cricoid,  on  which  it  can  slip  to  and  fro  to  some  extent,  the 
ligaments  of  the  joint  being  lax.  One  corner  of  the  tri- 
angular base  is  directed  inwards  and  forwards  (i.e.  towards 
the  thyroid)  and  is  called  the  vocal  process  (Pv,  Fig.  181),  as 
to  it  the  vocal  cords  are  fixed.  The  outer  posterior  angle 
(Pm,  Fig,  181)  has  several  muscles  inserted  on  it  and  is 
called  the  muscular  process.  If  it  be  pulled  back  and 
towards  the  middle  line  the  arytenoid  cartilage  will  rotate  on 
its  vertical  axis,  and  roll  its  vocal  processes  forwards  and  out- 
wards, and  so  widen  the  glottis;  the  reverse  will  happen  if 
the  muscular  process  be  drawn  forwards.  The  muscle  pro- 
ducing the  former  movement  is  the  posterior  crico-arytenoid 
(Cap,  Fig.  183);  it  arises  from  the  back  of  the  cricoid  carti- 
lage, and  narrows  to  its  insertion  into  the  muscular  process 
of  the  arytenoid  on  the  same  side.  The  opponent  of  this 
muscle  is  the  lateral  crico-arytenoid,  which  arises  from  the 
side  of  the  cricoid  cartilage,  on  its  inner  surface,  and  passes 
upwards  and  backwards  to  the  muscular  process.  The  pos- 
terior crico-arytenoid s,  working  alone,  pull  inwards  and  down- 
wards the  muscular  processes,  turn  upwards  and  outwards 
the  vocal  processes,  and  separate  the  posterior  ends  of  the 
vocal  cords.  The  lateral  crico-thyroid,  working  alone,  pulls 
downwards  and  forwards  the  muscular  process,  and  rotates 
inwards  and  upwards  the  vocal  process,  and  narrows  the 
glottis;  it  is  the  chief  agent  in  producing  the  approximation 
of  the  cords  necessary  for  the  production  of  voice.  When 
both  pairs  of  muscles  act  together,  however,  each  neutralizes 
the  tendency  of  the  other  to  rotate  the  arytenoid  cartilage; 
the  downward  part  of  the  pull  of  each  is,  thus,  alone  left,  and 
this  causes  the  arytenoid  to  slip  downwards  and  outwards,  off 
the  eminence  on  the  cricoid  with  which  it  articulates,  as  far 
as  the  loose  capsular  ligament  of  the  joint  will  allow.  The 
arytenoid  cartilages  are  thus  moved  apart  and  the  glottis 
greatly    widened    and    brought  into  its   state  in   deep  quiet 


638 


TllK  HUMAN  BODY. 


breathing.  Other  muscles  approximate  the  arytenoid  carti- 
lages after  the  cartilages  have  been  separated.  The  most  im- 
portant is  the  transverse  arytenoid  (A,  Fig.  183),  which  runs 
across  from  one  arytenoid  cartilage  to  the  other.  Another  is 
the  oblique  arytenoid  ( Taep),  which  runs  across  the  middle 
line  from  the  base  of  one  arytenoid  to  the  tip  of  the  other; 


Taep 


Fig.  183.—  The  larynx  seen  from  behind  and  dissected  so  as  to  display  some  of 
its  muscles.  The  mucous  membrane  of  the  front  of  the  pharynx  (5,  Fig.  157)  has 
been  dissected  away,  so  as  to  display  the  laryngeal  muscles  beneath  it.  Part  of 
the  left  hair  of  the  thyroid  cartilage  has  been  cut  away,  co,  cartilage  or  San- 
toriui;  cu,  cartilage  of  Wrisberg. 

thence  certain  fibres  continue  in  the  aryteno-epiglottidean 
fold  (10,  Fig.  182)  to  the  base  of  the  epiglottis;  this,  with  its 
fellow,  embraces  the  whole  entry  to  the  larynx;  when  they 
contract  they  bend  inwards  the  tips  of  the  arytenoid  car- 
tilages, approximate  the  edges  of  the  aryteno-epiglottidean 
fold,  and  draw  down  the  epiglottis,  and  so  close  the  passage 
from  the  pharynx  to  the  larynx.  When  the  epiglottis  has 
been  removed,  food  and  drink  rarely  enter  the  larynx  in 
swallowing,  the  folds  of  mucous  membrane  being  so  brought 
together  as  to  effectually  close  the  aperture  between  them. 

Increased  tension  of  the  vocal  cords  is  produced  by  the 
crivo-thyroid  muscles,  one  of  which  lies  on  each  side  of  the 
larynx,  over  the  crico-thyroid  membrane.     Their  action  may 


VOICE  AND  SPEECH. 


639 


J 


If 


Fig.  184. 


be  understood  by  help  of  the  diagram,  Fig.  184,  in  which  t 
represents  the  thyroid  cartilage,  c 
the  cricoid,  a  an  arytenoid,  and  vc 
a  vocal  cord.  The  muscle  passes 
obliquely  backwards  and  upwards 
from  about  d  near  the  front  end  of 
c,  to  t,  about  I,  near  the  pivot  (which 
represents  the  joint  between  the 
cricoid  cartilage  and  the  inferior 
horn  of  the  thyroid).  When  the 
muscle  contracts  it  pulls  together 
the  anterior  ends  of  t  and  c;  either 
by  depressing  the  thyroid  (as  rep- 
resented by  the  dotted  lines)  or  by  raising  the  front  end  of 
the  cricoid;  and  thus  stretches  the  vocal  cord,  if  the  ary- 
tenoid cartilages  be  held  from  slipping  forwards.  The  an- 
tagonist of  the  crico-thyroid  is  the  thyro-arytenoid  muscle; 
it  lies,  on  each  side,  imbedded  in  the  fold  of  elastic  tissue 
forming  the  vocal  cord,  and  passes  from  the  inside  of  the 
angle  of  the  thyroid  cartilage  in  front,  to  the  anterior  angle 
and  front  surface  of  the  arytenoid  behind.  If  the  latter  be 
held  firm,  the  muscle  raises  the  thyroid  cartilage  from  the 
position  into  which  the  crico-thyroid  pulls  it  down,  and  so 
slackens  the  vocal  cords.  If  the  thyroid  be  held  fixed  by  the 
crico-thyroid  muscle,  the  thyro-arytenoid  will  help  to  approxi- 
mate the  vocal  cords,  rotating  inwards  the  vocal  processes  of 
the  arytenoids. 

The  lengthening  of  the  vocal  cords  when  the  thyroid 
cartilage  is  depressed  tends  to  lower  their  pitch;  the  in- 
creased tension,  however,  more  than  compensates  for  this 
and  raises  it.  There  seems,  however,  still  another  method 
by  which  high  notes  are  produced.  Beginning  at  the  bot- 
tom of  his  register,  a  singer  can  go  on  up  the  scale  some  dis- 
tance without  a  break;  but,  then,  to  reach  his  higher  notes, 
must  pause,  rearrange  his  larynx,  and  begin  again.  What 
happens  is  that,  at  first,  the  vocal  processes  are  turned  in,  so 
as  to  approximate  but  not  to  meet;  the  whole  length  of  each 
edge  of  the  glottis  then  vibrates,  and  its  tension  is  increased, 
and  the  pitch  of  the  note  raised,  by  increasing  contraction  of 
the  crico-thyroid.  At  last  this  attains  its  limit  and  a  new 
method  has  to  be  adopted.  The  vocal  processes  are  more 
rolled    in,  until    they    touch.       This    produces    a   node    (see 


640  THE  HUMAN  BODY. 

Pliysics)  at  that  point  and  shortens  the  length  of  vocal  cord 
which  vibrates.  The  shorter  string  emits  a  higher  note;  so 
the  crico-thyroid  is  relaxed,  and  then  again  gradually  tight- 
ened as  the  notes  sung  are  raised  in  pitch  from  the  new 
starting-point.  To  pass  easily  and  imperceptibly  from  one 
such  arrangement  of  the  larynx  to  another  is  a  great  art  in 
singing.  There  is  some  reason  to  helieve  that  a  second  node 
may,  for  still  higher  notes,  be  produced  at  a  more  anterior 
point  on  the  vocal  cords. 

The  method  of  production  of  falsetto  notes  is  uncertain; 
during  their  emission  the  free  border  of  the  vocal  cords 
alone  vibrates. 

The  range  of  the  human  voice  is  about  three  octaves, 
from  e  (80  vib.  per  1")  on  the  unaccented  octave,  in  male 
voices,  to  c  on  the  thrice-accented  octave  (1024  vib.  per  1"), 
in  female.  Great  singers  of  course  go  beyond  this  range; 
basses  have  been  known  to  take  a  on  the  great  octave  (55 
vib.  per  1") ;  and  Nilsson  in  "  II  Flauto  Magico  "  used  to  take 
/on  the  fourth  accented  octave  (1408  vib.  per  1").  Mozart 
heard  at  Parma,  in  1770,  an  Italian  songstress  whose  voice 
had  the  extraordinary  range  from  g  in  the  first  accented 
octave  (198  vib.  per  1")  to  c  on  the  fifth  accented  octave 
(2112  vib.  per  1").  An  ordinary  good  bass  voice  has  a  com- 
pass from  /(88  vib.  per  1")  to  d"  (297  vib.  per  1");  and  a 
soprano  from  V  (248  vib.  per  1")  to  g'"  (792). 

Vowels  are,  primarily,  compound  musical  tones  produced 
in  the  larynx.  Accompanying  the  primary  partial  of  each, 
which  determines  its  pitch  when  said  or  sung,  are  a  number 
of  upper  partials,  the  first  five  or  six  being  recognizable  in 
good  full  voices.  Certain  of  these  upper  partials  are  rein- 
forced in  the  mouth  to  produce  one  vowel,  and  others  for 
other  vowels;  so  that  the  various  vowel  sounds  are  really 
musical  notes  differing  from  one  another  in  timbre.  The 
mouth  and  throat  cavities  form  an  air-chamber  above  the 
larynx,  and  this  has  a  note  of  its  own  which  varies  with  its 
size  and  form,  as  may  be  observed  by  opening  the  mouth 
widely,  with  the  lips  retracted  and  the  cheeks  tense;  then 
gradually  closing  it  and  protruding  the  lips,  meanwhile  tap- 
ping the  cheek.  As  the  mouth  changes  its  form  the  note 
produced  changes,  tending  in  general  to  pass  from  a  higher 
to  a  lower  pitch  and  suggesting  to  the  ear  at  the  same  time  a 
change  from  the  sound  of  a  (father)  through  o  (more)  to  60 


VOICE  AND  SPEECH.  641 

(moor).  When  the  mouth  and  throat  chambers  are  so  ar- 
ranged that  the  air  in  them  has  a  vibratory  rate  in  unison 
with  any  partial  in  the  laryngeal  tone,  it  will  be  set  in  sym- 
pathetic vibration,  that  partial  will  be  strengthened,  and  the 
vowel  characterized  by  it  uttered.  As  the  mouth  alters  its 
form,  although  the  same  note  be  still  sung,  the  vowel  changes. 
In  the  above  series  (a,  o,  05)  the  tongue  is  depressed  and  the 
cavity  forms  one  chamber;  for  a  this  has  a  wide  mouth  open- 
ing; for  o  it  is  narrowed;  for  65  still  more  narrowed,  and  the 
lips  protruded  so  as  to  increase  the  length  of  the  resonance 
chamber.  The  partial  tones  reinforced  in  each  case  are,  ac- 
cording to  Helmholtz — 


In  other  cases  the  mouth  and  throat  cavity  is  partially  sub- 
divided, by  elevating  the  tongue,  into  a  wide  posterior  and  a 
narrow  anterior  part,  each  of  which  has  its  own  note;  and 
the  vowels  thus  produced  owe  their  character  to  two  rein- 
forced partials.  This  is  the  case  with  the  series  a  (man),  e 
(there),  and  i  (machine),  the  tones  reinforced  by  resonance 
in  the  mouth  being — 


t= 


The  usual  I  of  English,  as  in  spire,  is  not  a  true  simple 
vowel  but  a  diphthong,  consisting  of  a  (p«d)  followed  by  e 
(feet),  as  may  be  observed  by  trying  to  sing  a  sustained  note 
to  the  sound  I;  it  will  then  be  seen  that  it  begins  as  a  and 
ends  as  ee.  A  simple  vowel  can  be  maintained  pure  as  long 
as  tin;  breath  holds  out. 

In  uttering  true  vowel  sounds  the  soft  palate  is  raised  so 

0  cut  off  the  air  in  the  nose,  which,  thus,  does  not  take 

part  in  the  sympathetic  resonance.     For  some  other  sounds 

(the  aemi-voiveU  or  resonant*)  the  initial  step  is,  as  in  the 

case  of  the  true  vowels,  the  production  of  a  laryngeal  tone; 


642  yv/ E  III  MA.\  BODY. 

but  the  soft  palate  is  not  raised,  and  the  mouth  exit  is  more 
or  less  closed  by  the  lips  or  the  tongue;  hence  the  blast  partly 
issues  through  the  nose,  and  the  air  there  takes  part  in  the 
vibrations  and  gives  them  a  special  character;  this  is  the  ease 
with  vi,  n,  and  ug. 

Consonants  are  sounds  produced  not  mainly  by  the  vocal 
cords,  but  by  modifications  of  the  expiratory  blast  on  its  way 
through  the  mouth.  The  current  may  lie  interrupted  and 
the  sound  changed  by  the  lips  {labials);  or,  at  or  near  the 
teeth,  by  the  tip  of  the  tongue  [dentals),  or, in  the  throat,  by 
the  root  of  the  tongue  and  the  soft  palate  [gutturals).  Con- 
sonants are  also  characterized  by  the  kind  of  movement 
which  gives  rise  to  them.  In  explosives  an  interruption  to 
the  passage  of  the  air-current  is  suddenly  interposed  or  re- 
moved (P,  T,  B,  D,  K,  G).  Other  consonants  are  continudus 
(as  F,  S,  R),  and  may  be  subdivided  into — (1)  aspirates,  char- 
acterized by  the  sound  produced  by  a  rush  of  air  through  a 
narrow  passage,  as  when  the  lips  are  approximated  (F),  or  the 
teeth  (S),or  the  tongue  is  brought  near  the  palate  (Sh),or  its 
tip  against  the  two  rows  of  teeth,  they  not  being  quite  in 
contact  (Th).  For  L  the  tongue  is  put  against  the  hard 
palate  and  the  air  escapes  on  its  sides.  For  Ch  (as  in  the 
proper  Scotch  pronunciation  of  loch)  the  passage  between  the 
back  of  the  tongue  and  the  soft  palate  is  narrowed.  To 
many  of  the  above  pure  consonants  answer  others,  in  whose 
production  true  vocalization  [i.e.  a  laryngeal  tone)  takes  a 
part.  F  with  some  voice  becomes  V;  S  becomes  Z,  Th  soft 
[teeth)  becomes  Th  hard ;  and  Ch  becomes  Gh.  (2)  reso- 
nant*; these  have  been  referred  to  above.  (3)  vibratoriea 
(the  different  forms  of  R),  which  are  due  to  vibrations  of 
parts  bounding  a  constriction  put  in  the  course  of  the  air- 
current.  Ordinary  R  is  due  to  vibrations  of  the  tip  of  the 
tongue  held  near  the  hard  palate;  and  guttural  R  to  vibra- 
tions of  the  uvula  and  parts  of  the  pharynx. 

The  consonants  may  physiologically  be  classified  as  in  the 
following  table  (Foster) : 

Explosives.   Labials,  without  voice P. 

"        with  voice  B. 

Dentals,  without  voice..  .  .  .T. 

"  with   voice   P. 

Gutturals,  without  voice... K. 

"  with  voice G  (hard). 


VOICE  AND  SPEECH.  643 

Aspirates.       Labials,  without  voice F. 

"        with  voice V. 

Dentals,  without  voice S,  L,  Sh,  Th  (hard). 

with  voice. : Z,  Zh  (azure),  Th  (soft). 

Gutturals,  without  voice. .  .Ch  (loch). 

with  voice   Ch. 

Resonants.     Labial M. 

Dental N. 

Guttural NG. 

Vibratories.  Labial — not  used  in  European  languages. 

Dental R  (common). 

Guttural R  (guttural). 

H  is  a  laryngeal  sound:  the  vocal  cords  are  separated  for 
its  production,  yet  not  so  far  as  in  quiet  breathing.  The  air- 
current  then  produces  a  friction  sound  but  not  a  true  note, 
as  it  passes  the  glottis;  and  this  is  again  modified  when  the 
current  strikes  the  wall  of  the  pharynx.  Simple  sudden 
closure  of  the  glottis,  attended  with  no  sound,  is  also  a 
speech  element,  though  we  do  not  indicate  it  with  a  special 
letter,  since  it  is  always  understood  when  a  word  begins  with 
a  vowel,  and  only  rarely  is  used  at  other  times.  The  Greeks 
had  a  special  sign  for  it,/,  the  soft  breathing;  and  another, 
*,  the  hard  breathing,  answering  somewhat  to  our  h  and  indi- 
cating that  the  larynx  was  to  be  held  open,  so  as  to  give  a 
friction  sound,  but  not  voice. 

In  whispering  there  is  no  true  voice;  the  latter  implies 
true  tones,  and  these  are  only  produced  by  periodic  vibra- 
tions; whispering  is  a  noise.  To  produce  it  the  glottis  is 
considerably  narrowed  but  the  cords  are  not  so  stretched  as  to 
produce  a  sharply  defined  edge  on  them,  and  the  air  driven 
past  is  then  thrown  into  irregular  vibrations.  Such  vibra- 
tions as  coincide  in  period  with  the  air  in  the  mouth  and 
throat  are  always  present  in  sufficient  number  to  characterize 
the  vowels;  and  the  consonants  are  produced  in  the  ordinary 
way,  though  the  distinction  between  such  letters  as  P  and  B, 
F  and  V,  remains  imperfect. 


CHAPTER  XXXIX. 

REPRODUCTION. 

Reproduction  in  General.  In  all  cases  reproduction 
consists,  essentially,  in  the  separation  of  a  portion  of  living 
matter  from  a  parent;  the  separated  part  bearing  with  it,  or 
inheriting,  certain  tendencies  to  repeat,  with  more  or  less 
variation,  the  life  history  of  its  progenitor.  In  the  more 
simple  cases  a  parent  merely  divides  into  two  or  more  pieces, 
each  resembling  itself  except  in  size;  these  then  grow  and 
repeat  the  process;  as,  for  instance,  in  the  case  of  Amoeba  and 
our  own  white  blood  corpuscles  (pp.  23,  44).  Such  a  process 
may  be  summed  up  in  two  words  as  discontinuous  growth; 
the  mass,  instead  of  increasing  in  size  without  segmentation, 
divides  as  it  grows,  and  so  forms  independent  living  beings. 
In  some  tolerably  complex  multicellular  animals  we  find 
essentially  the  same  thing;  at  times  certain  cells  of  the  fresh- 
water Polype  multiply  by  simple  division  in  the  manner 
above  described,  but  there  is  a  certain  concert  between  them: 
they  build  up  a  tube  projecting  from  the  side  of  the  parent,  a 
mouth-opening  forms  at  the  distal  end  of  this,  tentacles 
sprout  out  around  it,  and  only  when  thus  completely  built  up 
and  equipped  is  the  young  Hydra  set  loose  on  its  own  career. 
How  closely  such  a  mode  of  multiplication  is  allied  to  mere 
growth  is  shown  by  other  polypes  in  which  the  young,  thus 
formed,  remain  permanently  attached  to  the  parent  stem,  so 
that  a  compound  animal  results.  This  mode  of  reproduction 
(known  as  gemmation  or  budding)  may  be  compared  to  the 
method  in  which  many  of  the  ancient  Greek  colonies  were 
founded;  carefully  organized  and  prepared  at  home,  they 
were  sent  out  with  a  due  proportion  of  artificers  of  various 
kinds;  so  that  the  new  commonwealth  had  from  its  first  sep- 
aration a  considerable  division  of  employments  in  it,  and  was, 
on  a  small  scale,  a  repetition  of  the  parent  community.  In 
the  great  majority  of  animals,  however  (even  those  which  at 
times  multiply  by  budding),  a  different  mode  of  reproduction 

644 


REPROD  UGTION.  645 

occurs,  one  more  like  that  by  which  our  western  lands  were 
settled  and  gradually  built  up  into  Territories  and  States. 
The  new  individual  in  the  political  world  began  with  little 
differentiation;  it  consisted  of  units,  separated  from  older  and 
highly  organized  societies,  and  these  units  at  first  did  pretty 
much   everything,  each   man  for  himself,  with  more  or  less 
efficiency.     As  growth  took  place  development  also  occurred; 
persons   assumed    different   duties    and    performed   different 
work   until,   finally,   a   fully   organized    State   was    formed. 
Similarly,  the  body  of  one  of  the  higher  animals  is,  at  an 
early  stage  of  life,  merely  a  collection  of  undifferentiated 
cells,  each  capable  of  multiplication  by  division,  and  more  or 
less  retaining  all   its  original   protoplasmic  properties;  and 
with   no   specific    individual    endowment  or  function.     The 
mass  (Chap.  III.)  then  slowly  differentiates  into  the  various 
tissues,  each  with  a  predominant  character  and  duty;  at  the 
same  time  the  majority  of  the  cells  lose  their  primitive  powers 
of  reproduction,  though  exactly  how  completely  is  a  problem 
not  yet  sufficiently  studied.     In  adult  Vertebrates  it  seems 
certain  that  the  white  blood  corpuscles  multiply  by  division: 
and  in   some  cases  (in  the  newts  or  tritons,  for  example)  a 
limb  is  reproduced  after  amputation.     But  exactly  what  cells 
take  part  in  such   restorative  processes  is  uncertain;  we  do 
not  know  if  the  old  bone  corpuscles  left  form  new  bones,  old 
muscle-fibres  new  muscles,  and  so  on;  though  it  is  probable 
that  the    little-differentiated  leucocytes  build  up  most  of  the 
new  limb.     In  Mammals  no  such  restoration  occurs;  an  am- 
putated leg  may  heal  at  the  stump  but  does  not  form  again. 
In  the  healing  processes  the  connective  tissues  play  the  main 
part,  as  we  might  expect;  their  cellular  elements  being  but 
little  modified  from  their  primitive  state  (p.  102)  can  still 
multiply  and  develop.    New  blood  capillaries,  however,  sprout 
out  from  the  sides  of  old,  and  new  epidermis  seems  only  to  be 
formed  by  the  multiplication  of  epidermic  cells;  hence  the 
practice,  frequently   adopted   by  surgeons,  of  transplanting 
little  bits   of  skin  to   points  on   flic  surface  of  an  extensive 
burn  or  ulcer.     In   blood   capillaries  and  epidermis  the  de- 
parture from  the  primary  undifferentiated  cell  is  hut  slight; 
and.  as  regards  the  cuticle,  one  of  the  permanent  physiologi- 
cal characters  of  the  cells  of  the  rete  mucoswm  is  their  multi- 
plication throughout  the  whole  of  life;  thatisamaiu  physio- 
logical characteristic  of  Hi'-  tissue:  the  same  is  very  probably 


04(5  THE  HUMAN  BODY. 

true  of  the  protoplasmic  cells  forming  the  walls  of  the  capil- 
laries. When  a  highly  differentiated  tissue  is  replaced  in 
the  body  of  mammals  after  breaking  down  or  removal,  it  is 
usually  by  the  activity  of  special  cells  set  apart  for  that  pur- 
pose, or  by  repair  or  outgrowth  of  the  cells  affected  and  not 
by  their  division.  The  red  blood  corpuscles  are  constantly 
being  broken  down  and  replaced,  but  the  new  ones  are  not 
formed  *by  the  division  of  already  fully  formed  corpuscles  but 
by  certain  special  licematoblastic  cells  retained  throughout 
life  in  the  red  marrow  of  bone  and  perhaps  in  the  spleen. 
The  nervous  tissues  are  highly  differentiated  and  a  nerve  is 
often  regenerated  after  division,  but  this  is  by  outgrowth  of 
the  ends  of  axis  cylinders  still  attached  to  their  cells  and  by 
secondary  formation  of  a  medullary  sheath  around  these,  and 
not  by  division  or  multiplication  of  already  existing  fibres. 
A  striped  muscle  when  cut  across  is  healed  by  the  formation 
of  a  band  of  connective  tissue;  after  a  very  long  time  it  is 
said  that  true  muscular  fibres  may  be  found  in  the  cicatrix, 
but  their  origin  is  not  known;  it  is  probably  not  from  pre- 
viously developed  muscle  fibres.  On  the  other  hand,  the  less 
differentiated  unstriated  muscle  has  been  observed  to  be  re- 
paired in  some  eases  after  injury  by  true  karyokinetic  division 
of  previously  formed  muscle  cells.  Although  many  gland- 
cells  in  the  performance  of  their  physiological  work  are  par- 
tially broken  down  and  lost  in  their  secretion,  and  then 
repaired  by  the  residue  of  the  cell,  multiplication  by  division 
of  fully  differentiated  gland-cells  does  not  appear  to  occur,  if 
we  except  such  organs  as  the  testes,  the  secretion  of  which 
consists  essentially  of  cells.  An  excised  portion  of  a  salivary 
or  parotid  gland  is  never  regenerated:  the  wound  is  repaired 
by  connective  tissues. 

We  find,  then,  as  we  ascend  in  the  animal  scale  a  diminish- 
ing reproductive  power  in  the  tissues  generally:  with  the 
increasing  division  of  physiological  labor,  with  the  changes 
that  fit  pre-eminently  for  one  work,  there  is  a  loss  of  other 
faculties,  and  this  one  among  them.  The  more  specialized  a 
tissue  the  less  the  reproductive  power  of  its  elements,  and  the 
most  differentiated  tissues  are  either  not  reproduced  at  all  after 
injury,  or  only  by  the  specialization  of  amoeboid  cells,  and  not 
by  a  progenitive  activity  of  survivors  of  the  same  kind  as  those 
destroyed.  In  none  of  the  higher  animals,  therefore,  do  we 
find  multiplication  by  simple  division,  or  by  budding:  no  one 


REPROD  UCTION.  647 

cell,  and  no  group  of  cells  used  for  the  physiological  mainte- 
nance of  the  individual,  can  build  up  a  new  complete  living 
being;  but  the  continuance  of  the  race  is  specially  provided 
for  by  setting  apart  certain  cells  which  shall  have  this  one 
property — cells  whose  duty  is  to  the  species  and  not  to  any  one 
representative  of  it — an  essentially  altruistic  element  in  the 
otherwise  egoistic  whole. 

Sexual  Reproduction.  In  some  cases,  especially  among 
insects,  the  specialized  reproductive  cells  can  develop,  each  for 
itself,  under  suitable  conditions,  and  give  rise  to  new  indi- 
viduals; such  a  mode  of  reproduction  is  called  parthenogenesis: 
but  in  the  majority  of  cases,  and  always  in  the  higher  animals, 
this  is  not  so;  the  fusion  of  two  cells,  or  of  products  of  two 
cells,  is  a  necessary  preliminary  to  development.  Commonly 
the  coalescing  cells  differ  considerably  in  size  and  form,  and 
one  takes  a  more  direct  share  in  the  developmental  processes; 
this  is  the  egg-cell  or  ovum;  the  other  is  the  sperm-cell  or 
spermatozoon.  The  fusion  of  the  two  is  known  as  fertiliza- 
tion. Animals  producing  both  ova  and  spermatozoa  are 
hermaphrodite;  those  bearing  ova  only,  female;  and  those 
spermatozoa  only,  male :  hermaphroditism  is  not  found  in 
Vertebrates,  except  in  rare  and  doubtful  cases  of  monstrosity. 

Accessory  Reproductive  Organs.  The  organ  in  which 
ova  are  produced  is  known  as  the  ovary,  that  forming  sperma- 
tozoa, as  the  testis  or  testicle;  but  in  different  groups  of  animals 
many  additional  accessory  parts  may  be  developed.  Thus, 
in  all  but  the  very  lowest  Mammalia,  the  offspring  is  nourished 
for  a  considerable  portion  of  its  early  life  within  the  body  of 
its  mother,  a  special  cavity,  the  uterus  or  womb,  being  pro- 
vided for  this  purpose:  the  womb  communicates  with  the 
exterior  by  a  passage,  the  vagina;  and  two  tubes,  the  oviducts 
or  Fallopian  lubes,  convey  the  eggs  to  it  from  the  ovaries. 
In  addition,  mammary  glands  provide  milk  for  the  nourish- 
ment of  the  young  in  the  first  months  after  birth.  In  the 
male  mamma]  we  find  as  accessory  reproductive  organs,  vasa 
deferentia  which  convey  from  the  testes  the  seminal  fluid  con- 
taining spermatozoa;  vesiculce  seminales  (not  present  in  all 
Mammalia),  glands  whose  secretion  is  mixed  with  that  of  the 
testes  or  is  expelled  after  it  in  the  sexual  act;  &  prostate  gland, 
whose  secretion  is  added  to  the  semen;  and  an  erectile  organ, 
the  penis,  by  which  the  fertilizing  liquid  is  conveyed  info  the 
vagina  of  the  female. 


648 


THE  HUMAN  BODY. 


The  Male  Reproductive  Organs.  The  testes  in  man  arc 
paired  tubular  glands,  which  lie  in  a  pouch  of  skin  called  the 
scrotum.  This  pouch  is  subdivided  internally  by  a  partition 
into  right  and  left  chambers,  in  each  of  which  a  testicle  lies. 
The  chambers  are  lined  inside  by  a  serous  membrane,  the 
tunica  vaginalis,  and  this  doubles  back  (like  the  pleura  round 
the  lung)  and  covers  the  exterior  of  the  gland.  Between  the 
external  and  reflected  layers  of  the  tunica  vaginalis  is  a  space 
containing  a  small  quantity  of  lymph. 

The  testicles  develop  in  the  abdominal  cavity,  and  only 
later  (though  commonly  before  birth)  descend  into  the  scrotum, 
passing  through  apertures  in  the  muscles,  etc.,  of  the  abdom- 
inal wall,  and  then  sliding  down  over  the  front  of  the  pubes, 
beneath  the  skin.  The  cavity  of  the  tunica  vaginalis  at  first 
is  a  mere  offshoot  of  the  peritoneal  cavity,  and  its  serous  mem- 
brane is  originally  a  part  of  the  peritoneum.  In  the  early  years 
of  life  the  passage  along  which  the  testis  passes  usually  becomes 
nearly  closed  up,  and  the  communication  between  the  peri- 
toneal cavity  and  that  of  the  tunica  vaginalis  is  also  obliterated. 
Traces  of  this  passage  can,  however,  readily  be  observed  in 
male  infants;  if  the  skin  inside  the  thigh  be  tickled  a  muscle 
lying  beneath  the  skin  of  the  scrotum  is 
made  to  contract  reflexly,  and  the  testis 
is  jerked  up  some  way  towards  the 
abdomen  and  quite  out  of  the  scrotum. 
Sometimes  the  passage  remains  per- 
manently open  and  a  coil  of  intestine 
may  descend  along  it  and  enter  the 
scrotum,  constituting  an  inguinal 
hernia  or  rupture.  A  In/drocele  is  an 
excessive  accumulation  of  liquid  in  the 
serous  cavity  of  the  tunica  vaginalis. 

Beneath  its  covering  of  serous  mem- 
brane each  testis  has  a  proper  fibrous 
tunic  of  its  own.  This  forms  a  thick 
mass  on  the  posterior  side  of  the  gland, 
from  which  partitions  or  septa  (i,  Fig. 
185)  radiate,  subdividing  the  gland 
into  many  chambers.  In  each  chamber 
lie  several  greatly  coiled  seminiferous  tubules,  «,  a,  averaging  in 
length  0.68  metre  (27  inches)  and  in  diameter  only  0.14  mm. 
{^-  inch).     Their  total  number  in  each  gland  is  about  800. 


Fig.    185.— Diagram    of   a 
Vertical  section  through  tlie 

testis.  a,  a,  tiibiili  Bernini- 
feri;  l>,  vasa  recta:  d,  vasa 
efferentia  ending  in  the 
coni  vasculosi:  e,  e.  epidi- 
dymis,   h,  vas  deferens. 


REPROD  UGTION.  649 

Near  the  posterior  side  of  the  testis  the  tubules  unite  to  form 
about  20  vasa  recta  (d),  and  these  pass  out  of  the  gland  at  its 
upper  end,  as  the  vasa  efferentia  (d),  which  become  coiled  up 
into  conical  masses,  the  eotii  vasculosis  these,  when  unrolled, 
are  tubes  from  15  to  20  cm.  (6-8  inches)  in  length;  they  tajier 
somewhat  from  their  commencements  at  the  vasa  efferentia, 
where  they  are  0.5  mm.  (-£s  inch)  in  diameter,  to  the  other 
end  where  they  terminate  in  the  epididymis  (e,  e,  Fig.  185). 
The  latter  is  a  narrow  mass,  slightly  longer  than  the  testicle, 
which  lies  along  the  posterior  side  of  that  organ,  near  the  lower 
end  of  which  it  passes  (g)  into  the  vas  deferens,  h.  If  the 
epididymis  be  carefully  unravelled  it  is  found  to  consist  of  a 
tube  about  6  metres  (20  feet)  in  length,  and  varying  in  diam- 
eter from  0.35  to  0.25  mm.  (^  to  ^  inch). 

The  vas  deferens  (h,  Fig.  185)  commences  at  the  lower 
part  of  the  epididymis  as  a  coiled  tube,  but  it  soon  ceases  to  be 
convoluted  and  passes  up  beneath  the  skin  covering  the  inner 
part  of  the  groin,  till  it  gets  above  the  pelvis  and  then,  passing 
through  the  abdominal  walls,  turns  inwards,  backwards,  and 
downwards,  to  the  under  side  of  the  urinary  bladder,  where  it 
joins  the  duct  of  the  seminal  vesicle;  it  is  about  0.6  meters 
(2  feet)  in  length  and  2.5  mm.  (TV  inch)  in  diameter.  Its 
lining  epithelium  is  ciliated. 

The  vesieulce  seminales,  two  in  number,  are  membranous 
receptacles  which  lie,  one  on  each  side,  beneath  the  bladder, 
between  it  and  the  rectum.  They  are  commonly  about  5  cm. 
(2  inches)  long  and  a  little  more  than  a  centimetre  wide  (or 
about  0.5  inch)  at  their  broadest  part.  The  narrowed  end  of 
each  enters  the  vas  deferens  on  its  own  side,  the  tube  formed 
by  the  union  being  the  ejaculatory  duct,  which,  after  a  course 
of  about  an  inch,  enters  the  urethra  near  the  neck  of  the 
bladder.  In  some  animals  the  vesieulce  seminales  form  a  liquid 
which  is  added  to  the  secretion  of  the  testis.  In  man  they 
appear  to  be  merely  reservoirs  in  which  tbe  semen  collects. 

The  prostate  gland  is  a  dense  body,  about  Hie  size  of  a 
large  chestnut,  which  surrounds  the  commencement  of  the 
urethra;  the  ejaculatory  ducts  pass  through  it.  It  is  largely 
made  up  of  fibrous  and  imstriped  muscular  tissues,  but  con- 
tain- also  ;i  Dumber  of  small  secreting  saccules  whose  ducts 
open  into  the  urethra.  The  prostatic  secretion  though  small 
in  amount  would  appear  to  he  of  importance:  ;it  least  the 
gland  remains  undeveloped  in  persons  who  have  been  castrated 


650  7  HE   HUM  AX  BODY. 

in  childhood;  and  atrophies  after  removal  of  the  testicles  later 
in  life. 

The  male  urethra  leads  from  the  bladder  to  the  end  of  the 
penis,  where  it  terminates  in  an  opening,  the  nival  us  urinarius. 
It  is  described  by  anatomists  as  made  ii]>  of  three  portions, 
the  prostatic,  the  membranous,  and  the  spongy.  The  firsl  is 
surrounded  by  the  prostate  gland  and  receives  the  ejaculatory 
duets.  On  its  posterior  wall,  close  to  the  bladder,  is  an  eleva- 
tion containing  erectile  tissues  (see  below)  and  supposed  to  be 
dilated  during  sexual  congress,  so  as  to  cut  off  the  passage  to 
the  urinary  receptacle.  On  this  crest  is  an  opening  leading 
into  a  small  recess,  the  utricle,  which  is  of  interest,  since  the 
study  of  its  embryology  shows  it  to  be  an  undeveloped  male 
uterus.  The  succeeding  membranous  portion  of  the  urethra 
is  about  1.8  cm.  (f  inch)  long;  the  spongy  portion  lies  in  the 
penis. 

The  penis  is  composed  mainly  of  erectile  tissue,  i.e.,  tissues 
so  arranged  as  to  inclose  cavities  which  can  be  distended  by 
blood.  Covered  outside  by  the  skin,  internally  it  is  made  up 
of  three  elongated  cylindrical  masses,  two  of  which,  the  corpora 
cavernosa,  lie  on  its  anterior  side;  the  third,  the  corpus  spongi- 
osum, surrounds  the  urethra  and  lies  on  the  posterior  side  of 
the  organ  for  most  of  its  length;  it,  however,  alone  forms 
the  terminal  dilatation,  or  glans,  of  the  penis.  Each  corpus 
cavernosum  is  closely  united  to  its  fellow  in  the  middle  line 
and  extends  from  the  pubic  bones,  to  which  it  is  attached 
behind,  to  the  glans  penis  in  front.  It  is  enveloped  in  a  dense 
connective-tissue  capsule  from  which  numerous  bars,  contain- 
ing white  fibrous,  elastic,  and  unstriped  muscular  tissues, 
radiate  and  intersect  in  all  directions,  dividing  its  interior  into 
many  irregular  chambers  called  venous  sinuses.  Into  these 
blood  is  conveyed  partly  through  open  capillaries,  partly 
directly  by  the  open  ends  of  small  arteries;  this  blood  is  car- 
ried off  by  veins  proceeding  from  the  sinuses. 

The  arteries  of  the  penis  are  supplied  with  vaso-dilator 
nerves,  the  nerri  erigentes,  derived  from  the  sacral  plexus. 
Under  certain  conditions  these  are  stimulated  and,  the 
arteries  expanding,  blood  is  poured  into  the  venous  sinuses 
faster  than  the  veins  drain  it  off;  the  latter  are  probably  also 
at  the  same  time  compressed  where  they  leave  the  penis  by  the 
contraction  of  certain  muscles  passing  over  them.  Simul- 
taneously the  involuntary  muscular  tissue  of  the  bars  ramify- 


REPRODUCTION.  651 

irtg  through  the  erectile  masses  relaxes.  As  a  result  the  whole 
organ  becomes  distended  and  finally  rigid  and  erect.  The 
co-ordinating  centre  of  erection  lies  in  the  lumbar  region  of  the 
spinal  cord,  and  may  be  excited  reflexly  by  mechanical  stimu- 
lation of  the  penis,  or  under  the  influence  of  nervous  impulses 
originating  in  the  brain  and  associated  with  sexual  emotions. 
The  corpus  spongiosum  resembles  the  corpora  cavernosa  in 
essential  structure  and  function. 

The  skin  of  the  penis  is  thin  and  forms  a  simple  layer  for 
some  distance;  towards  the  end  of  the  organ  it  separates  and 
forms  a  fold,  the  foreskin  or  prepuce,  which  doubles  back, 
and,  becoming  soft,  moist,  red,  and  very  vascular,  covers  the 
glands  to  the  meatus  urinarius,  where  it  becomes  continuous 
with  the  mucous  membrane  of  the  urethra;  in  it,  near  the 
projecting  posterior  rim  of  the  glans,  are  imbedded  many 
sebaceous  glands.  It  possesses  nerve  end  organs  (genital 
corpuscles)  which  much  resemble  end  bulbs  in  structure. 

The  Seminal  Fluid.  The  essential  elements  of  the  tes- 
ticular secretion  are  much  modified  cells,  the  spermatozoa, 
which  are  passed  out  with  some  albuminous  liquid.  The 
spermatozoa  (Fig.  186)  are  motile  bodies  about 
0.01  m.m.  (-g^-g-  inch)  in  length.  They  have  a 
flattened  clear  body  or  head  and  a  long  vibratile 
tail  or  cilinm;  the  portion  of  the  tail  nearest  to 
the  head  is  thicker  than  the  rest,  and  is  known 
as  the  neck.  The  mode  of  development  of  a 
spermatozoon  shows  that  the  head  is  a  cell-nu-  FlQ  iso.— sper- 
cleus  and  the  neck  and  tail  a  modified  cell-  \ "^ "£°£:0* te.,^'} 

ljodv  in     si.ie    view,     a, 

head ;    b,   neck ;    c, 

On  cross-section  a  seminiferous  tubule  pre-  'ab- 
sents externally  a  well-marked  basement  membrane,  upon 
which  are  borne  several  layers  of  cells;  the  lumen  or  bore  of 
the  tubule  is  in  great  part  occupied  by  the  tails  of  sper- 
matozoa projecting  from  some  of  the  lining  cells.  The  outer 
cells,  those  next  the  basement  membrane,  are  arranged  in  a 
single  layer,  and  are  usually  found  in  one  or  other  stage  of 
active  karyokinetic  division  (p.  19).  The  result  of  the  divi- 
sion is  an  outer  cell,  which  remains  next  the  basement  mem- 
brane  to  repeat  the  process,  and  an  inner,  which  is  the  mother- 
ed1 of  spermatozoa.  The  latter  cell  by  repeated  mitotic  divi- 
sion give  rise  to  a  number  of  cells  lying  side  by  side  and  each 
having  a  relatively  large  nucleus  and  small  cell-body.     These 


Qi)2  THE  HUMAN  BODY. 

cells  elongate,  t  lie  nucleus  remaining  near  the  deeper  end  and 
the  protoplasm  extending  towards  the  lumen  of  the  tubule, 
into  which  it  ultimately  projects.  Such  cells  are  sperma- 
toblasts, and  lie  in  bunches  side  by  side  and  several  rows 
deep.  Interlaced  among  them  are  other  granular  supporting 
cells  of  the  epithelium,  which  are  probably  concerned  with 
the  nutrition  of  the  essential  cells.  The  final  step  by  which 
the  spermatoblast  is  converted  into  a  spermatozoon  is  a  kary- 
okinetic  division  into  two  unecjual  (.-ells:  a  part  of  the  nu- 
cleus with  a  little  of  the  protoplasm  separates  and  appears  to 
have  no  further  function;  the  remaining  part  of  the  nucleus 
{male  pronucleus)  remains  as  the  head  of  the  spermatozoon 
and  the  cell  protoplasm  develops  into  the  neck  and  tail. 
The  spermatozoa  appear  frequently  to  be  cast  off  before 
their  development  is  completed :  at  least  many  spermato- 
blasts which  have  not  gone  through  the  final  stages  are 
found  in  the  vasa  recta,  and  even  in  the  vas  deferens. 
Probably  the  secretion  normally  collects  in  the  vesicular 
seminales,  and  there  undergoes  its  final  elaboration. 

The  Reproductive  Organs  of  the  Female.  Each  ovary 
(o,  Fig.  187)  is  a  dense  oval  mass  about  3.25  cm.  (1.5  inches) 
in  length,  2  cm.  (0.75  inch)  in  width,  and  1.27  cm.  (0.5  inch) 
in  thickness;  it  weighs  from  4  to  7  grams  (60-100  grains). 
The  organs  lie  in  the  pelvic  cavity  enveloped  in  a  fold  of 
peritoneum  (the  broad  ligament),  and  receive  blood-vessels 
and  nerves  along  one  border.  From  time  to  time  ova  reach 
the  surface,  burst  through  the  enveloping  peritoneum,  and 
are  received  by  the  wide  fringed  aperture,^?,  of  the  oviduct 
or  Fallopian  tube,  od.  This  tube  narrows  towards  its  inner 
end,  where  it  communicates  with  the  uterus,  and  is  lined  by 
a  mucous  membrane,  covered  by  ciliated  epithelium;  plain 
muscular  tissue  is  also  developed  in  its  wall.  The  uterus 
(«,  c,  Fig.  187)  is  a  hollow  organ,  with  relatively  thick  mus- 
cular walls  (left  unshaded  in  the  figure);  it  contains  the 
foetus  during  pregnancy  and  expels  it  at  birth;  it  lies  in  the 
pelvis  between  the  urinary  bladder  and  the  rectum  (Fig.  l.sS); 
the  Fallopian  tubes  open  into  its  anterior  corners.  It  is  free 
above,  but  its  lower  end  is  attached  to  and  projects  into  the 
vagina.  In  the  fully  developed  virgin  state  the  organ  is 
somewhat  pear-shaped,  but  flattened  from  before  back;  about 
7.5  cm.  (3  inches)  in  length,  5  cm.  (2  inches)  in  breadth  at 
its  upper  widest  part,  and  2.5  cm.  (1  inch)  in  thickness;  it 


REPRODUCTION. 


65i 


weighs  from  25  to  42  grams  (£  to  li  oz.).  The  upper  wider 
portion  of  the  womb  is  known  as  its  body;  the  cavity  of  this 
is  produced  at  each  side  to  meet  the  openings  of  the  Fallo- 
pian tubes,  and  narrows  below  to  the  neck,  or  cervix  uteri, 
opposite  c  (Fig.  187),  the  communication  between  neck  and 
body  cavities  beiug  known  as  the  os  inter  mem.  Below  this 
the  neck  dilates  somewhat:  it  forms  no  part  of  the  cavity  in 


Fiq.  187.— The  uterus,  in  section,  with  the  right  Fallopian  tube  and  ovary,  as 
seen  from  behind,  about  %  the  natural  size,  u,  upper  part  of  uterus;  c,  cervix; 
v,  upper  part  of  vasrina;  od,  Fallopian  tube;  fi,  its  fimbriated  extremity;  o, 
ovary;  po,  parovarium. 

which  the  embryo  is  retained  and  nourished.  The  lowest 
part  of  the  cervix  reaches  into  the  vagina  and  communicates 
with  it  by  a  transverse  aperture,  the  os  uteri.  During  gesta- 
tion or  pregnancy  the  foetus  develops  in  the  body  of  the 
womb,  which  becomes  greatly  enlarged  and  rises  high  into 
the  abdomen :  the  virgin  womb  lies  mainly  below  the  level  of 
th<-  bones  of  the  pelvis. 

The  chief  bulk  of  the  non-gravid  uterus  consists  of  a  coat 
of  plain  muscular  tissue,  arranged  in  a  thin  outer  longitudinal 
layer,  and  an  inner,  thicker,  consisting  of  oblique  and  cir- 
cular fibres.  Between  the  layers  is  an  extensive  vascular  net- 
work, with  many  dilated  veins  or  venous  sinuses.  The  mus- 
cular coat  is  lined  internally  by  a  ciliated  mucous  membrane, 
and  is  covered  externally  by  the  peritoneum,  bands  of  which 
project  from  each  side  of  it  as  the  broad  ligaments  (11,  Fig. 
The  outer  layer  of  the  mncous  membrane  presents  a 
very  well  developed  mU8CUlaHs  'mucosa',  much  thicker  than 
the  corresponding  layer  in  the  gastric  or  intestinal  mucous 
membranes  and  much  less  sharply  marked  off  from  the 
true  muscular  coat  outside  it.     The   main   thickness  of  the 


054 


THE  //r.l/.l.Y   Unity. 


mucous  membrane  consists  of  closely  set,  simple  or  slightly 
branched,  tubular  glands;  between  these  is  a  close  blood- 
vascular  and  lymphatic  network.  The  glands  open  on  the 
interior  of  the  womb;  they  and  the  mucous  membrane  be- 
tween their  mouths  are  lined  by  a  Bingle  layer  of  columnar 
ciliated  cells,  with  some  goblet  cells  between  them.  In  the 
cervix  the  glands  are  shorter,  and  many  of  the  epithelial 
cells  not  ciliated.  The  viscid  mucus  secreted  by  the  uterine 
glands  is  alkaline  or  neutral. 


Fig.  188.— The  viscera  of  the  female  pelvis  as  exposed  by  a  dorso-ventral  me- 
dian section,  g,  symphysis  pubis;  v,  v',  urinary  bladder;  it,  urethra;  u,  uterus; 
rii.  vagina;  r,  r',  rectum;  a.  anal  opening;  /,  right,  labium  major;  >i,  right uympha; 
h,  hymen;  cl,  divided  cilitoris. 

The  vagina  is  a  distensible  passage,  extending  from  the 
uterus  to  the  exterior;  dorsally  it  rests  on  the  rectum,  and 
ventrally  is  in  contact  with  the  bladder  and  urethra.  It  is 
lined  by  mucous  membrane*,  the  epithelium  of  which  is  much 
like  the  epidermis  but  thinner;  outside  the  mucous  membrane 
the  vagina  is  made  up  of  areolar,  erectile,  and  unstriped  mus- 
cular tissues.  Around  its  lower  end  is  a  ring  of  striated  mus- 
cular tissue,  the  sphincter  vagina. 


REPRODUCTION. 


655 


The  vulva  is  a  general  term  for  all  the  portions  of  the  female 
generative  organs  visible  from  the  exterior.  Over  the  front  of 
the  pelvis  the  skin  is  elevated  by  adipose  tissue  beneath  it,  and 
forms  the  mons  Veneris.  From  this  two  folds  of  skin  (I,  Fig. 
188),  the  labia  majora,  extend  downwards  and  backwards  on 
each  side  of  a  median  cleft,  beyond  which  they  again  unite. 
On  separating  the  labia  majora  a  shallow  genito-urinary  sinus, 
into  which  the  urethra  and  vagina  open,  is  exposed.  At  the 
upper  portion  of  this  sinus  lies  the  clitoris,  a  small  and  very 
sensitive  erectile  organ,  resembling  a  miniature  penis  in  struc- 
ture, except  that  it  has  no  corpus  spongiosum  and  is  not 
traversed  by  the  urethra.  From  the  clitoris  descend  two  folds 
of  mucous  membrane,  the  nymplm  or  labia  interna,  between 
which  is  the  vestibule,  a  recess  containing,  above,  the  opening 
of  the  short  female  urethra,  and,  below,  the  aperture  of  the 
vagina,  which  is  in  the  virgin  more  or  less  closed  by  a  thin 
duplicature  of  mucous  membrane,  the  hymen. 


Fro  189  —  A  section  of  a  Mammalian  ovarv,  considerably  masmifled.  1,  outer 
car^ule  of  ovarv:  2.  3,  3',  stroma:  4,  blood-vessels;  5,  rudimentary  Graafian  fol- 
licles: 0  7  8  follicles  bes-innine-  to  enlare-e  and  mature,  and  receding:  from  the  sur- 
face; 0  a  nearly  ripe  follicle  which  i«  extending;  towards  the  surface  preparatory  to 
dlncharjrfne  the  ovum:  <t ,  membrnnn  granulosa:  b.  discus  proligrerus:  r,  ovum  with 
d  "'-rminal  vpsiHe.  and  e.  terminal  snot  The  general  cavity  of  the  follicle  (in 
which  0  is  printed)  is  filled  with  lymph-like  transudation  liquid  during;  life. 

Microscopic  Structure  of  the  Ovary-  The  main  mass  of 
the  ovary  consists  of  a  dense  oonneetive-tissne  stroma,  eon- 
tainimr  unstriped  musele,  blood-vessels,  and  nerves;  it  is 
covered  externally  by  a  peculiar  germinal  epithelium^  and  con- 
tains imbedded  in  it  many  minnte  cavities,  the  Graafian  folli- 
cles,  in  which  ova  lie.  Tf  a  thin  section  of  an  ovary  be  examined 
with  the  microscope  many  hundreds  of  small  Graafian  follicles, 


656  THE  HUMAN  BODY. 

each  about  0.25  mm.  (,,',,,  inch)  in  diameter,  will  be  found 
imbedded  in  it  near  thesurface.  These  are  lined  by  cells,  and 
each  contains  a  Bingle  ovum.  In  a  woman  of  child-bearing 
age  there  will  be  found  also,  deeper  in,  larger  follicle-  (7,  8, 
9,  Fig.  L89),  their  cavities  being  distended,  during  life,  by 
liquid;  in  these  the  essential  structure  may  he  more  readily 
made  out.  Each  has  an  external  fibrous  coat  constituted  by 
a  dense  and  vascular  layer  of  the  ovarian  stroma;  within  this 
come  Beveral  layers  of  lining  cells  (9,  a,  Fig.  L89)  constituting 
the  merribrana  granulosa.  At  one  point,  b,  the  cells  of  this 
layer  are  heaped  up,  forming  the  discus  proligerus,  which 
projects  into  the  liquid  Idling  the  cavity  of  the  follicle.  Buried 
among  the  cells  of  the  discus  proligerus  the  ovum,  c,  lies. 

The  Mammalian  Ovum.  As  the  Graafian  follicles  enlarge 
the  ova  grow  but  not  proportionately,  so  that  they  occupy 
relatively  less  of  the  cavities  of  the  larger  follicles:  the  cells  of 
the  discus  proligerus  probably  elaborate  food  for  the  egg  cell 
from  material  derived  from  the  blood-vessels  which  form  a 
close  network  around,  most  of  each  enlarging  Graafian  follicle 
and  transude  crude  nutritive  matter  into  the  liquid  filling 
most  of  the  follicle.  The  fully  formed 
ovum  (Fig.  190)  is  about  0.2  mm. 
(T|¥  inch)  in  diameter:  it  has  a  well- 
marked  outer  coat  or  sac,  a,  the  zona 
radiata,  zona  pellucida  or  vitelline 
membrane,  surrounding  a  very  granu- 
lar cell-budy  or  vitellus,  l>,  in  which  is  a 
conspicuous  nucleus,  e,  here  named 
the  germinal  vesicle  and  possessing 
JSvZatiSSXUS^   a  nucleolus  or  germinal  spot.      The 

zona  pellucida;  h,  vitellus;  c,  ZOna  pellucida  exhibits  distinct  radial 
germinal  vesicle,  with  distinct  -1  . 

reticulum  of  karyopiasm  and    markings  which  probably  are  due  to 

a  nucleolus  or  germinal  spot.       „  ,  .        .        r^,  .     ,     ,, 

tine  tubes  traversing  it.    the  mam  bulk 

of  the  vitellus  or  yelk  consists  of  highly  refracting  spheroidal 
particles  of  nutritive  matter  (deutoplasm)  imbedded  in  and 
concealing  a  true  protoplasmic  reticulum.  In  the  eggs  of  birds 
and  reptiles  the  deutoplasm  is  in  very  large  amount  and  forms 
nearly  all  of  the  yelk,  the  protoplasm  being  for  the  most  part 
aggregated  around  the  germinal  vesicle  at  a  small  area  on  one 
side  of  the  yelk.  It  is  in  this  area  that  new  cell-formation 
occurs  and  the  embryo  is  built  up,  the  rest  of  the  yelk  being 
gradually  absorbed  by  it:  such  eggs  are  known  as  mesoblastie 


REPROD  XJCTION.  657 

or  partly-dividing  eggs.  In  all  the  higher  mammalia  the 
dentoplasm  is  relatively  sparse  and  tolerably  evenly  mingled 
with  the  protoplasm,  and  the  whole  fertilized  ovum  divides  to 
form  the  first  cells  of  the  embryo:  such  eggs  are  named 
holoblastic. 

The  Maturation  of  the  Ovum.  From  time  to  time, 
usually  at  intervals  of  about  four  weeks,  in  a  woman  of  child- 
bearing  age,  certain  ova  after  attaining  the  size  and  struc- 
ture described  in  the  preceding  paragraph  undergo  further 
changes  by  which  the  egg-cell  is  rendered  capable  of  fertiliza 
tion.  These  phenomena,  known  as  the  maturation  of  the 
ovum,  result  in  separation  of  small  parts  of  the  nucleus  or 
germinal  vesicle  and  cell  protoplasm  from  the  rest.  They  are 
essentially  typical  cases  of  indirect  cell  division  (p.  19).  The 
cell-body  shrinks  a  little  so  as  to  not  quite  fill  the  zona  pellu- 
cida,  and  the  germinal  vesicle  approaches  one  side.  Meanwhile 
the  nuclear  membrane  and  karyoplasm  form  the  chromatic 
loop  and  this  divides  into  the  usual  two  sets  of  V  s.  One 
set  of  these,  with  part  of  the  nuclear  plasm,  now  separates 
with  a  little  of  the  cell  protoplasm 
to  form  a  small  cell,  the  first  polar 
{/lobule  or  directive  corpuscle  (c,  Fig 
191).  The  much  larger  cell  result 
ing  from  the  division  and  represent- 
ing the  remainder  of  the  vitellus 
and  nucleus  now  repeats  the  process, 
and  gives  rise  to  the  second  polar 
globule.  In  Fig.  191  the  first  polar 
globule  is  shown  at  c,  as  already 
separated,    and   the   nucleus,    d,  is    form  the  second  poiai  globule. 

1       ,  '  a,  zona  peilucida  ;  0.  space  filled 

dividing,  preparatory  to    the  forma-     with  liquid  and  left  by  the  shrink- 

p     ,,  n"     -,.         ,  .  age  of  the  vitellus;  c,  first  polar 

tion    of    tne  second    directive   cor-  giobnie;  d,  nucleus  of  ovum  divid- 

i  mi  i  j>   1  i  •  ing  preparatory  to  the  separation 

pUSCle.       lhe    stage  Of    karyokmeslS  of  fee  second  polar  globule;  v, 

i  i    i.i  i_i  vitellus.  showing  radial  arrange- 

is  more  advanced  than  those  repre-  ment  oMts  granule*  near  the  end 
sented   in   Fig.   10.      The   two   polar     of  the  nuclear  spindle. 

globules  lie  \'<>r  a  time  (Fig.  192)  within  the  zona  peilucida 
in  the  space  left  by  the  shrinkage  of  the  vitellus,  but  take  no 
part  in  the  formation  of  the  embryo  and  soon  disappear.  The 
real  of  the  original  ovum  is  now  mature  and  ready  for  fertili- 
zation; its  nucleus  is  known  as  the  female  pronucleus,  fn, 
Pig.  L92.     It  passes  towards  the  centre  of  the  ovum  and  forms 


Qi)S  THE  HUMAN  BODY. 

the  usual  recticulum  of  karyoplasm  found  in  norma]  resting 
nuclei  (Fig.  8). 

Ovulation.  From  puberty,  during  the  whole  child-bearing 
period  <>f  Life,  certain  comparatively  very  large  Graafian  follicles 
nia\  nearly  always  be  found  either  close  to  the  surface  of  the 
ovary  or  projecting  on  its  exterior.  These,  by  accumulation 
of  liquid  within  them,  have  become  distended  to  a  diameter 
of  about  4  mm.  (i  inch);  finally,  the  thinned  projecting  por- 
tion of  the  wall  of  the  follicle,  which  differs  from  the  rest  in 
containing  few  blood-vessels,  gives  way  and  the  ovum  is  dis- 
charged, surrounded  by  some  cells  of  the  discus  proligerus. 
The  emptied  follicle  becomes  filled  up  with  a  reddish-yellow 
mass  of  cells,  and  constitutes  the  corpus  luteum,  which  recedes 
again  to  the  interior  of  the  ovary  and  disappears  in  three  or 
four  weeks,  unless  pregnancy  occur;  in  that  case  the  corpus 
luteum  increases  for  a  time,  and  persists  during  the  greater 
j>art  of  the  gestation  period. 

Menstruation.  Ovulation  occurs  during  the  sexual  life  of 
a  healthy  woman  at  intervals  of  about  four  weeks,  and  i- 
attended  with  important  changes  in  other  portions  of  the  gen- 
erative apparatus.  The  ovaries  and  Fallopian  tubes  become 
congested,  and  the  fimhriaB  of  the  latter  are  erected  and  come 
into  contact  with  the  ovary  so  as  to  receive  any  ova  discharged. 
Whether  the  fimbria?  embrace  the  ovary  and  catch  the  ovum, 
or  merely  touch  it  at  various  points  and  the  ova  are  swept  along 
them  by  their  cilia  to  the  cavity  to  the  oviduct,  is  not  certain. 
Having  entered  the  Fallopian  tube  the  egg  slowly  passes  on 
to  the  uterus,  probably  moved  by  the  cilia  lining  the  oviduct ; 
its  descent  probably  takes  about  four  or  five  days;  if  not  fertil- 
ized, it  dies  and  is  passed  out.  In  the  womb  important  changes 
occur  at  or  just  before  the  periods  of  ovulation;  its  mucous 
membrane  becomes  swollen  and  soft,  and  minute  hemorrhages 
occur  in  its  substance.  The  superficial  layers  of  the  uterine 
mucous  membrane  are  broken  down,  and  discharged  along  with 
more  or  less  blood,  constitufi  i%  the  menses,  or  monthly  sick- 
ness, which  commonly  lust-  from  three  to  five  days.  During 
this  time  the  vaginal  secretion  is  also  increased,  and,  mixed  with 
the  blood  discharged,  more  or  less  alters  its  color  and  usually 
destroys  its  coagulating  power.  Except  during  pregnancy  and 
while  suckling,  menstruation  occurs  at  the  above  intervals, 
from  puberty  up  to  about  the  forty-fifth  year;  the  periods 
then  become  irregular,  and   finally  the  discharges  cease;  this 


REPRODUCTION.  659 

is  an  indication  that  ovulation  lias  come  to  an  end,  and  that 
the  sexual  life  of  the  woman  is  completed.  This  time,  the 
climacteric  or  "  turn  of  life,"  is  a  critical  one;  various  local 
disorders  are  apt  to  supervene,  and  even  mental  derangement. 

Hygiene  of  Menstruation.  During  menstruation  there  is 
apt  to  be  more  or  less  general  discomfort  and  nervous  irrita- 
bility; the  woman  is  not  quite  herself,  and  those  responsible 
for  her  happiness  ought  to  watch  and  tend  her  with  special 
solicitude,  forbearance,  and  tenderness,  and  protect  her  from 
anxiety  and  agitation.  Any  strong  emotion,  especially  of  a 
disagreeable  character,  is  apt  to  check  the  flow,  and  this  is 
always  liable  to  be  followed  by  serious  consequences.  A  sudden 
chill  often  has  the  same  effect;  hence  a  menstruating  woman 
ought  always  to  be  warmly  clad,  and  take  more  than  usual 
care  to  avoid  draughts  or  getting  wet.  At  these  periods,  also, 
the  uterus  is  enlarged  and  heavy,  and  being  (as  may  be  seen 
in  Fig.  188)  but  slightly  supported,  and  that  near  its  lower 
end,  it  is  especially  apt  to  be  displaced  or  distorted;  it  may 
tilt  forwards  or  sideways  (versions  of  the  uterus),  or  be  bent 
where  the  neck  and  body  of  the  organ  meet  (flexion).  Hence 
violent  exercise  at  this  time  should  be  avoided,  though  there 
is  no  reason  why  a  properly  clad  woman  should  not  take  her 
usual  daily  walk. 

The  absence  of  the  menstrual  flow  (amenorrhea)  is  normal 
during  pregnancy  and  while  suckling;  and  in  some  rare  cases 
it  never  occurs  throughout  life,  even  in  healthy  women  capa- 
ble of  child-bearing.  Usually,  however,  the  non-appearance 
of  the  menses  at  the  proper  periods  is  a  serious  symptom,  and 
one  which  calls  for  prompt  measures.  In  all  such  cases  it 
cannot  be  too  strongly  impressed  upon  women  that  the  most 
dangerous  thing  to  do  is  to  take  drugs  tending  to  induce  the 
discharge,  except  under  skilled  advice;  to  excite  the  flow,  in 
many  cases,  as  for  example  occlusion  of  the  os  uteri,  or  in 
general  debility  (when  its  absence  is  a  conservative  effort  of 
the  system),  may  have  the  most  disastrous  results. 

Fertilization.  As  the  ovum  descends  the  Fallopian  tube 
the  changes  of  menstruation  are  taking  place  in  the  uterus. 
Fertilization  usually  takes  place  in  a  Fallopian  tube.  The 
spermatozoa  are  carried  along  partly,  perhaps,  by  the  contrac- 
tions of  the  muscular  walls  of  the  female  cavities,  hut  mainly 
by  their  own  activity.      Occasionally  the   ovum    is    fertilized 


660 


THE  HUMAN  BODY. 


before  reaching  the  Fallopian  tube  and  fails  to  enter  it,  giving 
rise  to  an  extra-uterine  pregnancy. 

The  actual  process  of  the  fertilization  of  the  ovum  has  only 
been  observed  in  the  lower  animals,  but  there  is  no  doubt  that 
the  phenomena  are  the  same  in  all  essentials  in  all  cases.  Some 
of  the  spermatozoa  penetrate  the  zona  pellucida  and  the  head 
of  one  of  them  enters  the  ovum,  when  it  grows  and  forms  the 

male  pronucleus  (mn,  Fig.  192). 
This  travels  towards  the  nucleus 
of  the  matured  ovum  or  female 
pronucleus,/^,  and  in  each  pro- 
nucleus a  karyoplastic  filament 
forms  and  breaks  up  into  a  set 
of  V's;  in  the  pronuclei  repre- 
sented in  Fig.  192  this  has  not 
yet  taken  place,  the  karyoplasm 
being  still  arranged  in  a  retic- 
ulum. The  tail  of  the  sperma- 
tozoon (which  represents,  it  will 

Fig.    192.— An  ovum   shortly  before  i  rommn  hprprl     flip    iH-ntr>nl-i«m 

the  fusion  of  the  pronuclei.    «,  zona  De  lememoeieu,  me    pioiopiasm 

Pellucida;  b,  polar  globules;  >,  female  0f  a      ma]e     cep\      disappears; 

pronucleus;  m  it,  male  pronucleus;  pp,  ill 

attraction  bodies,  with  the  nuclear  whether  it  is  cast  off  when  the 

spindle  lying  between  them;  s,  sper- 

matozoa  which  have  not  taken  part  in    head  enters    the  VltellllS '01'  1)1  i  II- 

gles  with  the  protoplasm  of  the 
latter  is  not  known.  As  the  pronuclei  approach  one  another 
two  attraction  particles,  /;,  p,  appear  in  the  protoplasm  of  the 
ovum;  around  these  the  granules  of  the  vitellus  show  a  radial 
arrangement  and  a  nuclear  spindle  (p.  19)  unites  them.  The 
spindle  lies  with  its  long  axis  at  right  angles  to  a  line  joining 
the  pronuclei.  The  latter  next  completely  fuse  across  the 
middle  of  the  spindle  and  form  a  new  single  nucleus.  Fertili- 
zation is  then  complete,  and  the  ovum  capable  of  dividing  or 
segmenting  (Fig.  11)  to  form  the  cells  which  by  multiplication 
and  differentiation  build  up  the  embryo.  The  zona  pellucida 
takes  no  part  in  the  segmentation  and  is  gradually  absorbed. 

The  Signification  of  the  Polar  Globules.  The  union  of 
the  male  and  female  pronuclei  is  the  essential  fact  in  fertiliza- 
tion and  the  material  basis  of  all  the  phenomena  of  heredity; 
therefore  everything  pertaining  to  it  is  of  very  great  interest. 
There  is  reason  to  believe  that  each  half  of  the  nucleus  of  the 
fertilized  egg  contains  karyoplasm  from  both  pronuclei,  and 
that  in  all  subsequent  cell-divisions  each  new  cell  gets  nuclear 


REPRODUCTION.  661 

karyoplasm  from  both,  and  therefore  contains  both  male  and 
female  morphological  elements.  If  this  be  so,  every  cell  of  the 
adult  Body  contains  a  material  representative  of  both  father 
and  mother,  and  may  be  regarded  as  hermaphrodite.  Upon 
this  supposition  explanations  of  the  unequal  cell-divisions  of 
the  ovum  giving  rise  to  the  polar  globules  have  been  based. 
The  ovum  before  maturation  and  the  spermatoblast  before  final 
formation  of  the  spermatozoon  being  bisexual,  each  must,  it 
has  been  suggested,  get  rid  of  material  derived  from  one 
parent  before  it  can  fuse  with  a  residuum  of  the  other  to  make 
a  new  cell.  The  spermatoblast  therefore  in  its  first  cell- 
division  separates  female  nuclear  matter,  and  the  spermatozoon 
is  a  purely  male  cell ;  the  ovum  on  the  other  hand  gets  rid  of 
male  material  in  the  polar  globules,  and  the  mature  ovum  is  a 
solely  female  cell ;  the  union  of  the  two  makes  a  complete  her- 
maphrodite cell  from  which  the  new  animal  develops.  This 
view  was  supported  by  the  belief  that  certain  insect  eggs  which 
develop  parthenogenetically  did  not  separate  polar  globules 
before  commencing  to  form  the  embryo.  It  is  now  known, 
however,  that  such  eggs  do  separate  one  polar  globule,  so  the 
theory  requires  modification.  We  cannot  here  go  into  the  dis- 
cussion of  this  matter,  which  is  one  of  the  most  interesting 
biological  questions.  The  argument  gathers  mainly  round 
the  theory  (Weismann)  that  each  complete  cell  apart  from 
male  and  female  elements  contains  two  kinds  of  living  mate- 
rial: one  (nuclear  plasma)  with  controlling,  reproducing,  and 
hereditary  functions;  the  other  (nutritive  plasma)  with  as- 
similative duties  and  other  powers  in  various  cells,  as  con- 
tractility, irritability,  and  so  forth,  but  exercising  these  under 
the  influence  and  direction  of  the  nuclear  plasma.  In  the 
nuclear  plasma  itself  are  two  distinct  substances — a  germinal 
plasma  with  hereditary  functions,  and  alone  found  in  the  just 
fertilized  ovum,  and  a  histogenetic  or  tissue-building  plasma, 
which  is  formed  by  and  from  the  germinal  plasma  and  controls 
cell-growth,  division,  and  differentiation.  The  ovum  in  the 
first  polar  globule  gets  rid  of  some  of  its  histogenetic  plasma, 
and  then  in  (he  second  polar  globule  of  the  male  portion  of 
it-  germinal  plasma,  and  these  are  replaced  by  the  material 
brought  by  the  spermatozoon,  which  is  a  cell  that  lias  in  a 
similar  way  goi  rid  of  some  of  its  histogenetic  and  germinal 
plasma.  Ob  this  theory,  moreover,  the  proportion  of  the 
ovum  extruded  in  the  polar  globules  and   the  ratio  of  that 


662  THE   HUMAN  BODY. 

remaining  to  the  germ  plasma  bronghl  by  the  spermatozoon 

may  be  supposed  to  differ  in  dilTeivnt  instances  and  account 
for  individual  differences  in  the  offspring:  thus  sonic  physical 
basis  for  the  facts  of  variation  as  well  as  of  heredity  would 
be  obtained. 

Impregnation.  The  fertilized  ovum  continues  its  descent 
to  the  uterine  cavity,  but,  instead  of  lying  dormant  like  the 
unfertilized,  segments  (p.  39),  and  forms  a  morula.  This, 
entering  the  womb,  becomes  imbedded  in  the  soft,  vascular 
mucous  membrane  from  winch  it  imbibes  nourishment,  and 
which,  instead  of  being  cast  oil'  in  subsequent  menstrual  dis- 
charges, is  retained  and  grows  during  the  whole  of  pregnancy, 
having  important  duties  to  discharge  in  connection  with  the 
nutrition  of  the  embryo. 

Sexual  congress  is  most  apt  to  be  followed  by  pregnancy  if 
it  occur  immediately  after  a  menstrual  period;  at  those  times 
a  ripe  ovum  is  usually  in  the  Fallopian  tube,  near  the  upper 
end  of  which  it  is  probably  fertilized  in  the  majority  of  cases. 
There  is  some  difference  of  opinion  as  to  whether  the  rupture 
of  the  Graafian  follicle  occurs  most  frequently  immediately 
before  the  appearance  of  the  menstrual  flow,  or  towards  its 
close ;  but  the  preponderance  of  evidence  favors  the  latter  view. 
The  menstrual  process  probably  is  a  special  preparation  of  the 
womb  for  the  reception  of  an  embryo  and  its  nourishment. 
There  is,  however,  evidence  that  ova  are  ocasionally  discharged 
at  other  than  the  regular  monthly  periods  of  ovulation  and 
may  be  fertilized  and  cause  a  pregnancy. 

Pregnancy.  When  the  mulberry  mass  reaches  the  uterine 
cavity  the  mucous  membrane  lining  the  latter  grows  rapidly 
and  forms  a  new,  thick,  very  vascular  lining  to  the  womb, 
known  as  the  decidua.  At  one  point  on  this  the  morula  be- 
comes attached,  the  decidua  growing  up  around  it.  As  preg- 
nancy advances  and  the  embryo  grows,  it  bulges  out  into  the 
uterine  cavity  and  pushes  before  it  that  part  of  the  decidua 
which  has  grown  over  it  (the  decidua  reflexa);  at  about  the 
end  of  the  third  month  this  coalesces  with  the  decidua  lining 
the  opposite  sides  of  the  uterine  cavity  so  that  the  two  can  no 
longer  be  separated.  That  part  of  the  decidua  (decidua 
serotina)  against  which  the  morula  is  first  attached  subsequently 
undergoes  a  great  development  in  connection  with  the  forma- 
tion of  the  placenta  (see  below).  Meanwhile  the  whole  uterus 
enlarges;  its  muscular  coat  especially  thickens.     At  first  the 


REPROD  UCTION.  663 

organ  still  lies  within  the  })elvis,  where  there  is  but  little  room 
for  it;  it  accordingly  presses  on  the  bladder  and  rectum  (see 
Fig,  188)  and  the  nerves  in  the  neighborhood,  frequently 
causing  considerable  discomfort  or  pain;  and,  reflexly,  often 
exciting  nausea  or  vomiting  (the  morning  sickness  of  preg- 
nancy). Later  on,  the  pregnant  womb  escapes  higher  into  the 
abdominal  cavity,  and  although  then  larger,  the  soft  abdominal 
walls  more  readily  make  room  for  it,  and  less  discomfort  is 
usually  felt,  though  there  may  be  shortness  of  breath  and 
palpitation  of  the  heart  from  interference  with  the  diaphrag- 
matic movements.  All  tight  garments  should  at  this  time  be 
especially  avoided ;  the  woman's  breathing  is  already  suffi- 
ciently impeded,  and  the  pressure  may  also  injure  the  develop- 
ing child.  Meanwhile,  changes  occur  elsewhere  in  the  Body. 
The  breasts  enlarge  and  hard  masses  of  developing  glandular 
tissue  can  be  felt  in  them ;  and  there  may  be  mental  symptoms : 
depression,  anxiety,  and  an  emotional  nervous  state. 

During  the  whole  period  of  gestation  the  woman  is  not 
merely  supplying  from  her  blood  nutriment  for  the  fcetus,  but 
also,  through  her  lungs  and  kidneys,  getting  rid  of  its  wastes; 
the  result  is  a  strain  on  her  whole  system  which,  it  is  true, 
she  is  constructed  to  bear  and  will  carry  well  if  in  good  health, 
but  which  is  severely  felt  if  she  be  feeble  or  suffering  from  dis- 
ease. The  healthy  married  woman  who  endeavors  to  evade 
motherhood  because  she  thinks  she  will  thus  preserve  her  per- 
sonal appearance,  or  because  she  dislikes  the  trouble  of  a 
family,  deserves  but  little  sympathy;  she  is  trying  to  escape  a 
duty  voluntarily  undertaken,  and  owed  to  her  husband,  her 
country,  and  her  race;  but  she  whose  strength  is  undermined 
and  whose  life  is  made  one  long  discomfort  for  the  sexual 
gratification  of  her  husband  deserves  every  consideration,  and 
the  family  physician  ought  perhaps  to  warn  the  husband  more 
frequently  than  he  does  of  the  risk  to  a  delicate  wife's  health, 
or  even  life,  of  frequent  pregnancies:  and  the  husband  should 
control  himself  accordingly.  The  professor  of  gynecology  in 
a  leading  medical  school,  gives  it  as  his  deliberate  opinion 
that  the  majority  of  American  women  must  at  some  periods  of 
their  liv<s  choose  between   freedom   from  pregnancy  or  early 

death. 

Apart  from  pregnancy,  moreover,  a  woman's  health  is  often 
injun-d  by  frequenl  sexual  intercourse.     A  physician  who  has 

unusual  opportunities  of  knowing  states  that  he  has  reason  to 


664  THE  HUMAN  BODY. 

believe  that  not  only  is  the  act  of  sexual  congress  at  best,  from 
a  physical  poini  of  view,  a  mere  nuisance  to  the  majority  of 
women  belonging  to  the  more  luxurious  classes  of  society  after 
they  attain  the  age  of  twenty-two  or  twenty-three,  but  that  a 
wry  considerable  proportion  suffer  acute  pain  from  it  such  as, 
if  frequent,  breaks  down  the  general  health.  A  loving  woman, 
finding  her  highest  happiness  in  suffering  for  those  dear  to  her, 
is  very  unlikely  to  let  her  husband  know  this,  so  long  as  Bhe 
can  bear  it;  but  if  the  possibility  is  known  it  will  not,  per- 
haps, need  much  acuteness  in  him  to  discover  such  suffering 
when  it  exists,  nor  very  much  real  affection  to  direct  him- 
self accordingly.  In  the  class  of  eases  referred  to,  rest  of  the 
over-irritable  and.  congested  female  organs  is  above  all  essen- 
tial. The  cause  is  frequently  removable  by  simple,  but  skilled, 
treatment;  the  desirability  of  rendering  this  available  to  a 
woman  in  members  of  her  own  sex  is  now  generally  recog- 
nized. 

The  Intrauterine  Nutrition  of  the  Embryo.  At  first 
the  embryo  is  nourished  by  absorption  of  materials  from  the 
soft  vascular  lining  of  the  womb;  as  it  increases  in  size  this 
is  not  sufficient,  and  a  new  organ,  the  2^laceuta,  is  formed  for 
the  purpose.  A  foetal  outgrowth,  the  allantois,  plants  itself 
firmly  against  the  decidua  serotina,  and  villi  developed  on  it 
burrow  from  its  surface  into  the  uterine  mucous  membrane. 
In  the  deeper  layer  of  this  latter  are  large  sinuses  through 
which  the  maternal  blood  flows,  and  into  which  the  allantoic 
villi  project.  Blood  is  brought  from  the  foetus  to  the  allantois 
by  arteries  and  carried  back  by  veins  after  traversing  the 
capillaries  of  the  villi,  and  while  flowing  through  these  re- 
ceives, by  dialysis,  oxygen  and  food  materials  from  the  mater- 
nal blood,  and  gives  up  to  it  carbon  dioxide,  urea,  and  other 
wastes.  There  is  thus  no  direct  intermixture  of  the  two 
bloods;  the  embryo  is  from  the  first  an  essentially  separate 
and  independent  organism.  The  allantois  and  decidua  sero- 
tina becoming  inseparably  united  together  form  the  placenta, 
which  in  the  human  species  is,  when  fully  developed,  a  round 
thick  mass  about  the  size  of  a  large  saucer,  connected  to  the 
embryo  by  a  narrow  stalk,  the  umbilical  cord,  in  which  blood- 
vessels run  to  and  from  the  placenta. 

Parturition.  At  the  end  of  from  275  to  280  days  from 
fertilization  of  the  ovum  (concejjtion)  pregnancy  terminates, 
and   the  child   is  expelled   by  powerful  contractions  of  the 


REPRODUCTION.  665 

uterus,  assisted  by  those  of  the  muscles  in  the  abdominal 
walls.  When  the  child  is  born,  it  has  attached  to  its  navel 
the  umbilical  cord,  which  is  then  usually  ligatured  and  cut 
across:  some  good  authorities,  however,  maintain  that  this 
should  not  be  done  until  after  the  contractions  which  expel 
the  placenta,  as  otherwise  a  quantity  of  the  infant's  blood 
remains  in  that  organ;  the  loss  of  which  might  be  serious  to 
a  feeble  infant.  Shortly  after  the  birth  of  the  child  renewed 
uterine  contractions  detach  and  expel  the  placenta,  both  its 
foetal  or  allantoic  and  maternal  or  decidual  part,  as  the  after- 
birth. Where  it  is  torn  loose  from  the  uterine  wall  large 
blood  sinuses  are  left  open;  hence  a  certain  amount  of  bleed- 
ing occurs,  but  in  normal  labor  this  is  speedily  checked  by 
firm  contraction  of  the  uterus.  Should  this  fail  to  take 
place  profuse  haemorrhage  occurs  {flooding)  and  the  mother 
may  bleed  to  death  in  a  few  minutes  unless  prompt  measures 
are  adopted. 

For  a  few  days  after  delivery  there  is  some  discharge  (the 
lochia)  from  the  uterine  cavity:  the  whole  decidua  being 
broken  down  and  carried  off,  to  be  subsequently  replaced  by 
new  mucous  membrane.  The  muscular  fibres  developed  in 
the  uterine  wall  in  such  large  quantities  during  pregnancy 
undergo  rapid  fatty  degeneration  and  are  absorbed,  and  in  a 
few  weeks  the  organ  returns  almost  to  its  original  size.  The 
parturient  woman  is  especially  apt  to  take  infectious  diseases; 
and  these,  should  they  attack  her,  are  fatal  in  a  very  large 
percentage  of  cases.  Very  special  care  should  therefore  be 
taken  to  keep  all  contagion  from  her. 

There  is  a  current  impression  that  a  pregnancy,  once 
commenced,  can  be  brought  to  a  premature  end,  especially  in 
its  early  stages,  without  any  serious  risk  to  the  woman.  That 
belief  is  erroneous.  Premature  delivery,  early  or  late  in 
pregnancy,  is  always  more  dangerous  than  natural  labor  at 
the  proper  term;  the  physician  has  sometimes  to  induce  it, 
as  when  ;i  malformed  pelvis  makes  normal  parturition  impos- 
sible, or  tin:  general  derangement  of  health  accompanying 
the  pregnancy  is  such  as  to  threaten  flic  mother's  life;  but 
the  occasional  necessity  of  deciding  whether  it  is  his  duty  to 
procure  an  abortion  ifl  one  of  the  most  serious  responsibilities 
he  meets  with  in  tin;  course  of  his  professional  work. 

Dr.  Storer,  an  eminent  gynaecologist,  states  emphatically, 


660  TEE  HUMAN  HO  1)7. 

from  extended  observation,  that  despite  apparent  and  isolated 
instances  to  the  contrary — 

1.  A  larger  proportion  of  women  die  during  or  in  con- 
sequence of  an  abortion,  than  during  or  in  consequence  of 
child-bed  at  the  full  term  of  pregnancy. 

2.  A  very  much  larger  number  of  women  become  con- 
firmed invalids,  perhaps  for  life;  and — 

'.).  The  tendency  to  serious  and  often  fatal  organic  disease, 
as  cancer,  is  rendered  very  much  greater  at  the  so-called 
"turn  of  life,''  by  previous  artificially  induced  premature 
delivery. 

During  pregnancy  there  is  a  close  connection  between  the 
placenta  and  uterus;  nature  makes  preparation  for  the  safe 
dissolution  of  this  at  the  end  of  the  normal  period,  but  "  its 
premature  rupture  is  usually  attended  by  profuse  Inemor- 
rhage,  often  fatal,  often  persistent  to  a  greater  or  less  degree 
for  many  months  after  the  act  is  completed,  and  always  at- 
tended with  more  or  less  shock  to  the  maternal  system,  even 
though  the  full  effect  of  this  is  not  noted  for  years."  The 
same  authority  states  again:  "  Any  deviation  from  this  proc- 
ess at  the  full  term"  {i.e.,  the  process,  associated  with  lacta- 
tion, by  which  the  uterus  is  restored  to  its  small  non-gravid 
dimensions)  "lays  the  foundation  of,  and  causes,  a  wide 
range  of  uterine  accidents  and  disease,  displacements  of 
various  kinds;  falling  of  the  womb  downwards  or  forwards 
or  backwards,  with  the  long  list  of  neuralgic  pains  in  the 
back,  groin,  thighs,  and  elsewhere  that  they  occasion;  con- 
stant and  inordinate  leucorrhcea;  sympathetic  attacks  of 
ovarian  irritation,  running  even  into  dropsy,"  etc.,  etc. 
There  is,  thus,  abundant  reason  for  bearing  most  things 
rather  than  the  risks  of  an  avoidable  abortion. 

Lactation.  The  mammary  glands  for  several  years  after 
birth  remain  small,  and  alike  in  both  sexes.  Towards 
puberty  they  begin  to  enlarge  in  the  female,  and  when  fully 
developed  form  in  that  sex  two  rounded  eminences,  the 
breasts,  placed  on  the  thorax.  A  little  below  the  centre  of 
each  projects  a  small  eminence,  the  nipple,  and  the  skin 
around  this  forms  a  colored  circle,  the  areola.  In  virgins 
the  areolae  are  pink;  they  darken  in  tint  and  enlarge  during 
the  first  pregnancy  and  never  quite  regain  their  original  hue. 
The  mammary  glands  are  constructed  on  the  compound 
racemose   type.      Each   consists  of   from  fifteen   to    twenty 


REPRODUCTION.  667 

distinct  lobes,  made  up  of  smaller  divisions;  from  each  main 
lobe  a  separate  galactophorous  duct,  made  by  the  union  of 
smaller  branches  from  the  lobules,  runs  towards  the  nipple, 
all  converging  beneath  the  areola.  There  each  dilates 
and  forms  a  small  elongated  reservoir  in  which  the  milk 
may  temporarily  collect.  Beyond  this  the  ducts  narrow 
again,  and  each  continues  to  a  separate  opening  on  the  nip- 
ple. Imbedding  and  enveloping  the  lobes  of  the  gland  is  a 
quantity  of  firm  adipose  tissue  which  gives  the  whole  breast 
its  rounded  form. 

During  maidenhood  the  glandular  tissue  remains  imper- 
fectly developed  and  dormant.  Early  in  pregnancy  it  begins 
to  increase  in  bulk,  and  the  gland  lobes  can  be  felt  as  hard 
masses  through  the  superjacent  skin  and  fat.  Even  at  par- 
turition, however,  their  functional  activity  is  not  fully  estab- 
lished. The  oil-globules  of  the  milk  are  formed  by  a  sort  of 
fatty  degeneration  of  the  gland-cells,  which  finally  fall  to 
pieces;  the  cream  is  thus  set  free  in  the  watery  and  albu- 
minous secretion  formed  simultaneously,  while  newly  de- 
veloped gland-cells  take  the  place  of  those  destroyed.  In  the 
milk  first  secreted  after  accouchment  (the  colostrum)  the  cell 
destruction  is  incomplete,  and  many  cells  still  float  in  the 
liquid,  which  has  a  yellowish  color;  this  first  milk  acts  as  a 
purgative  on  the  infant,  and  probably  thus  serves  a  useful 
purpose,  as  a  certain  amount  of  substances  (biliary  and 
other),  excreted  by  its  organs  during  development,  are  found 
in  the  intestines  at  birth. 

Human  milk  is  undoubtedly  the  best  food  for  an  infant  in 
the  early  months  of  life ;  and  to  suckle  her  child  is  useful  to 
the  mother  if  she  be  a  healthy  woman.  There  is  reason  to 
e  that  the  processes  of  involution  by  which  the  large 
masW)f  muscular  and  other  tissues  developed  in  the  uterine 
walls  anring  pregnancy  are  broken  down  and  absorbed,  take 
place  more  safely  to  health  if  the  natural  milk  secretion  is 
encouraged.  Many  women  refuse  to  suckle  their  children 
from  a  belief  that  so  doing  will  injure  their  personal  appear- 
ance, but  skilled  medical  opinion  is  to  the  contrary  effect;  the 
natural  four-"  of  events  is  the  best  fur  this  purpose,  unless 
lactation  be  too  prolonged.  Of  course  in  many  cases  there  are 
justifiable  grounds  for  a  mother's  not  undertaking  this  part,  of 
her  duties;  a  physician  is  the  proper  person  to  decide. 

Jn  a  healthy  woman,  not  suckling  her  child,  ovulation  and 


66S  THE  HUMAN  BODY. 

menstruation  recommence  about  six  weeks  after  childbirth;  a 
nursing  mother  usually  does  not  menstruate  for  ten  or  twelve 
months;  the  infant  should  then  be  weaned. 

When  an  infant  cannot  be  suckled  by  its  mother  or  a  wet- 
nurse  an  important  matter  is  to  decide  what  is  the  best  food 
to  substitute.  Good  cow's  milk  contains  rather  more  fats  than 
that  of  a  woman,  and  much  more  casein;  the  following  table 
gives  averages  in  1000  parts  of  milk : 

Woman.  Cow. 

Casein   28.0  54.0 

Butter 33.5  43.0 

Milk  sugar 44.5  42.5 

Inorganic  matters 4.75  7. To 

The  inorganic  matters  of  human  milk  yield,  on  analysis,  in 
100  parts — calcium  carbonate  6.9;  calcium  phosphate,  70.6; 
sodium  chloride,  9.8;  sodium  sulphate,  7.4;  other  salts,  5.3. 
The  lime  salts  are  of  especial  importance  to  the  child,  which 
has  still  to  build  up  nearly  all  its  bony  skeleton. 

When  undiluted  cow's  milk  is  given  to  infants  they  rarely 
bear  it  well;  the  too  abundant  casein  is  vomited  in  loose 
coagula.  The  milk  should  therefore  be  diluted  with  half  or, 
for  very  young  children,  even  two  thirds  its  bulk  of  water. 
Tins,  however,  brings  down  the  percentage  of  sugar  and 
butter  below  the  proper  amount.  The  sugar  is  commonly 
replaced  by  adding  cane  sugar;  but  sugar  of  milk  is  readily 
obtainable  and  is  better  for  the  purpose.  If  used  at  all  it 
should,  however,  be  employed  from  the  first ;  it  sweetens  much 
less  than  cane  sugar,  and  infants  used  to  the  latter  refuse  milk 
in  which  milk  sugar  is  substituted.  Cream  from  cow's  milk 
may  be  added  to  raise  the  percentage  of  fats  to  the  normal,  but 
must  be  perfectly  fresh  and  only  added  to  the  milk  immediately 
before  it  is  given  to  the  child.  "While  milk  is  standing  for  the 
cream  to  rise  it  is  very  apt  to  turn  a  little  sour;  the  amount  of 
this  sour  milk  carried  off  with  the  cream  is  itself  no  harm 
when  mixed  with  a  large  bulk  of  fresh  milk;  it  carries  with  it, 
however,  some  of  the  fungus  whose  development  causes  the 
souring,  and  this  will  rapidly  develop  and  sour  all  the  milk  it 
is  added  to  if  the  mixture  be  let  stand.  As  the  infant  grows 
older  less  diluted  cow's  milk  may  gradually  be  given;  after 
the  seventh  or  eighth  month  no  addition  of  water  is  necessary. 

In  the  first  weeks  after  birth  it  is  no  use  to  give  an  infant 
starchy  foods,  as  arrowroot.     The  greater  part  of  the  starch 


REPRODUCTION.  669 

passes  through  the  bowels  unchanged ;  apparently  because  the 
pancreas  has  not  yet  fully  developed,  and  has  not  commenced 
to  make  its  starch-converting  ferment.  Later  on,  starchy 
substances  may  be  added  to  the  diet  with  advantage,  but  it 
should  be  borne  in  mind  that  they  cannot  form  the  chief  part 
of  the  child's  food;  it  needs  proteids  for  the  formation  of 
its  tissues,  and  amyloid  foods  contain  none  of  these.  Many 
infants  are,  ignorantly,  half  starved  by  being  fed  almost  en- 
tirely on  such  things  as  corn-flour  or  arrowroot. 

Puberty.  The  condition  of  the  reproductive  organs  of 
each  sex  described  in  preceding  pages  is  that  found  in  adults; 
although  mapped  out,  and,  to  a  certain  extent,  developed 
before  birth  and  during  childhood,  these  parts  grow  but 
slowly  and  remain  functionally  incapable  during  the  early 
years  of  life ;  then  they  comparatively  rapidly  increase  in  size 
and  become  physiologically  active ;  the  boy  or  girl  becomes  man 
or  woman. 

This  period  of  attaining  sexual  maturity,  known  as  puberty, 
takes  place  from  the  eleventh  to  the  sixteenth  year,  and  is 
accompanied  by  changes  in  many  parts  of  the  Body.  Hair 
grows  more  abundantly  on  the  pubes  aud  genital  organs,  and 
in  the  armpits;  in  the  male  also  on  various  parts  of  the  face. 
The  lad's  shoulders  broaden;  his  larynx  enlarges,  and  lengthen- 
ing of  the  vocal  cords  causes  a  fall  in  the  pitch  of  his  voice ; 
all  the  reproductive  organs  increase  in  size ;  fully  formed  seminal 
fluid  is  secreted,  and  erections  of  the  penis  occur.  As  these 
changes  are  completed  spontaneous  nocturnal  seminal  emis- 
sions take  place  from  time  to  time  during  sleep,  being  usually 
associated  with  voluptuous  dreams.  Many  a  young  man  is 
alarmed  by  these;  he  has  been  kept  in  ignorance  of  the  whole 
matter,  is  too  bashful  to  speak  of  it,  and  getting  some  quack 
advertisement  thrust  into  his  hand  in  the  street  is  alarmed  to 
lean]  that  his  strength  is  being  drained  off,  and  that  he  is  on 
the  high-road  to  idiocy  and  impotence  unless  he  place  himself 
in  the  hands  of  the  advertiser.  Lads  at  this  period  of  life 
uliould  have  been  taught  that  such  emissions,  when  not  too 
frequent  and  noi  excited  by  any  voluntary  act  of  their  own, 
are  natural  and  healthy.  They  may,  however,  occur  too  of  ten ; 
if  there  is  any  reason  to  suspect  this,  the  family  physician 
should  be  consulted,  as  the  healthy  activity  of  the  sexual 
organs  varies  so  much  in  individuals  as  to  make  it  impossible 
to  lay  down  numerical  rules  on  the  subject.     The  best  proven- 


670  T11E  HUMAN  BODY. 

tives  in  any  case  arc,  however,  not  drugs,  bul  an  avoidance  of 

too  warm  and  soft  a  bed,  plenty  <d  muscular  exercise,  and 
keeping  out  of  the  way  of  anything  likely  to  excite  the  sexual 
instincts. 

In  the  woman  the  pelvis  enlarges  considerably  at  puberty, 
and,  commonly,  more  subcutaneous  adipose  tissue  develops 
over  the  Body  generally,  but  especially  on  the  breasts  and  hips; 
consequently  the  contours  become  more  rounded.  The  exter- 
nal generative  organs  increase  in  size,  and  the  clitoris  and 
nymphse  become  erectile.  The  uterus  grows  considerably,  the 
ovaries  enlarge,  some  Graafian  follicles  ripen,  and  menstruation 
commences. 

The  Stages  of  Life.  Starting  from  the  ovum  each  human 
being,  apart  from  accident  or  disease,  runs  through  a  life-cycle 
which  terminates  on  the  average  after  a  course  of  from  75  to 
80  years.  The  earliest  years  are  marked  not  only  by  rapid 
growth  but  by  differentiating  growth  or  development;  then 
comes  a  more  stationary  period,  and  finally  one  of  degenera- 
tion. The  life  of  various  tissues  and  of  many  organs  is  not, 
however,  coextensive  with  that  of  the  individual.  During  life 
all  the  formed  elements  of  the  Body  are  constantly  being 
broken  down  and  removed;  either  molecularly  (i.e.,  bit  by  bit 
while  the  general  size  and  form  of  the  cell  or  fibre  remains 
unaltered),  or  in  mass,  as  when  hairs  and  the  cuticle  are  shed. 
The  life  of  many  organs,  also,  does  not  extend  from  birth  to 
death,  at  least  in  a  functionally  active  state.  At  the  former 
period  numerous  bones  are  represented  mainly  by  cartilage. 
The  pancreas  has  not  attained  its  full  development ;  and  some 
of  the  sense-organs  seem  to  be  in  the  same  case;  at  least  new- 
born infants  appear  to  hear  very  imperfectly.  The  reproductive 
organs  only  attain  full  development  at  puberty,  and  degenerate 
and  lose  all  or  much  of  their  functional  importance  as  years  ac- 
cumulate. Certain  organs  have  even  a  still  shorter  range  of 
physiological  life;  the  thymus,  for  example,  attains  its  fullest 
development  at  the  end  of  the  second  year  and  then  gradually 
dwindles  away,  so  that  in  the  adult  scarcely  a  trace  of  it  is  to 
be  found.  The  milk-teeth  are  shed  in  childhood,  and  their 
so-called  permanent  successors  rarely  last  to  ripe  old  age. 

During  early  life  the  Body  increases  in  mass,  at  first  very 
rapidly,  and  then  more  slowly,  till  the  full  size  is  attained, 
except  that  girls  make  a  sudden  advance  in  this  respect  ;it 
puberty.      Henceforth  the  woman's  weight  (excluding  excep- 


REPRODUCTION.  671 

tional  cases  of  accumulation  of  non- working  adipose  tissue) 
remains  about  the  same  until  the  climacteric.  After  that 
there  is  often  an  increase  of  weight  for  several  years  due 
mainly  to  increased  formation  of  fat;  a  man's  weight  usually 
slowly  increases  until  forty. 

As  old  age  comes  on  a  general  decline  sets  in,  the  rib  car- 
tilages become  calcified,  and  lime  salts  are  laid  down  in  the 
arterial  walls,  which  thus  lose  their  elasticity ;  the  refracting 
media  of  the  eye  become  more  or  less  opaque;  the  physiological 
irritability  of  the  sense-organs  in  general  diminishes;  and  fatty 
degeneration,  diminishing  their  working  power,  occurs  in 
many  tissues.  In  the  brain  we  find  signs  of  less  plasticity ;  the 
youth  in  whom  few  lines  of  least  resistance  have  been  firmly 
established  is  ready  to  accept  novelties  and  form  new  associa- 
tions of  ideas ;  but  the  longer  he  lives,  the  more  difficult  does 
this  become  to  him.  A  man  past  middle  life  may  do  good, 
or  even  his  best  work,  but  almost  invariably  in  some  line  of 
thought  which  he  has  already  accepted :  it  is  extremely  rare 
for  an  old  man  to  take  up  a  new  study  or  change  his  views, 
philosophical,  scientific,  or  other.  Hence,  as  we  live,  we  all 
tend  to  lag  behind  the  rising  generation. 

Death.  After  the  prime  of  life  the  tissues  dwindle  (or  at 
least  the  most  important  ones)  as  they  increased  in  childhood ; 
it  is  conceivable  that,  without  death,  this  process  might  occur 
until  the  Body  was  reduced  to  its  original  microscopic  dimen- 
sions. 

Before  any  great  diminution  takes  place,  however,  a  break- 
down occurs  somewhere,  the  enfeebled  community  of  organs 
and  tissues  forming  the  man  is  unable  to  meet  the  contingen- 
cies of  life,  and  death  supervenes.  "  It  is  as  natural  to  die  as 
to  be  born,"  Bacon  wrote  long  since;  but  though  we  all 
know  it,  few  realize  the  fact  until  the  summons  comes.  To 
the  popular  imagination  the  prospect  of  dying  is  often  asso- 
ciated with  thoughts  of  extreme  suffering;  personifying  life, 
men  picture  a  forcible  and  agonizing  rending  of  it,  as  an 
entity,  from  the  bodily  frame  with  which  it  is  associated.  As 
a  matter  of  fact,  death  is  probably  rarely  associated  with  any 
immediate  suffering.  The  sensibilities  are  gradually  dulled  as 
the  end  approaches;  the  nervous  tissues,  with  the  rest,  loue 
their  functional  capacity,  and,  before  the  heart  ceases  to  beat, 
the  individual  has  commonly  [oat  consciousness. 

The  actual  momenl  of  death  is  hard  to  define:  that  of  the. 


672  THE  HUMAN  BODY. 

Body  generally,  of  the  mass  as  a  whole,  may  he  taken  to  be  the 
moment  when  the  heart  makes  its  last  heat;  arterial  pressure 
then  falls  irretrievably,  the  capillary  circulation  ceases,  and 
the  tissues,  no  longer  nourished  from  the  blood,  gradually  die, 
not  all  at  one  instant,  but  one  after  another,  according  as  their 
individual  respiratory  or  other  needs  are  great  or  little. 

AYhile  death  is  the  natural  end  of  life,  it  is  not  its  aim — we 
should  not  live  to  die,  but  live  prepared  to  die.  Life  has  its 
duties  and  its  legitimate  pleasures,  and  we  better  play  our  part 
by  attending  to  the  fulfilment  of  the  one  and  the  enjoyment 
of  the  other,  than  by  concentrating  a  morbid  and  paralyzing 
attention  on  the  inevitable,  with  the  too  frequent  result  of 
producing  indifference  to  the  work  which  lies  at  hand  for  each. 
Our  organs  and  faculties  are  not  talents  which  we  may  justifi- 
ably leave  unemployed;  each  is  bound  to  do  his  best  with 
them,  and  so  to  live  that  he  may  most  utilize  them.  An  active, 
vigorous,  dutiful,  unselfish  life  is  a  good  preparation  for  death ; 
when  that  time,  at  which  we  must  pass  from  the  realm  con- 
trolled by  physiological  laws,  approaches,  when  the  hands 
tremble  and  the  eyes  grow  dim,  when  "  the  grasshopper  shall 
be  a  burden  and  desire  shall  fail,"  then,  surely,  the  conscious- 
ness of  having  quitted  us  like  men  in  the  employment  of  our 
faculties  while  they  were  ours  to  use,  will  be  no  mean  consola- 
tion. 


INDEX. 


Abdomen,  contents  of,  5. 

Abdominal  respiration,  393. 

Abducens  nerve,  174. 

Aberration,  cbrornatic,  527. 

Aberration,  spberical,  527. 

Absorbents,  349. 

Absorption  from  intestines,  373. 

Absorption  of  gases,  405. 

Absorption  of  oxygen  bv  blood, 
407. 

Accelerator  nerves  of  heart,  271. 

Accessory  reproductive  organs,  647. 

Accommodation,  522. 

Acetabulum,  77. 

Acbromatin  filaments,  20. 

Acid,  acetic,  13;  butyric,  13;  car- 
bonic, see  carbon  dioxide ;  for- 
mic, 13  ;  glycero-pbosphoric, 
13  ;  glycocbolic,  12  ;  bippuric, 
434  ;  lactic,  13;  oleic,  12  ;  pal- 
mitic, 12;  sarcolactic,  13,  123; 
stearic,  12  ;  taurocbolic,  370 ; 
uric,  434. 

Action  current  (negative  varia- 
tion), 139.  199. 

Actions,  reflex,  188,  GOO. 

Addison's  Disease,  359. 

Adenoid  tissue,  103,  353. 

Adipose  tissue,  107. 

Adrenals  (  supra-renal  capsules), 
359. 

Advantage  of  mixed  diet,  320,  474. 

After-birth  (placenta),  665. 

After  images,  548. 

Air,  chemical  composition  <>f,  I'M). 

Air-cells,  888. 

Air,  changes  produced  in,  by 
breathing,  899. 

Air,  complements!,  tidal,  etc.,  392. 

Air-passages,  381. 

Albumin,  serum,  59. 


Albuminoids  (gelatinoids),  10,  319. 

Albuminous  bodies,  9. 

Albumose,  366. 

Alcohol,  323. 

Alimentary  canal,  328. 

Alimentary  principles,  319. 

Amoeboid  cells,  110. 

Amoeboid  movements,  23,  48. 

Amyloids  (carbohydrates),  12,  319. 

Amyloids,  digestion  of,  362,  368, 
375. 

Anabolism,  22. 

Anaemia,  60. 

Anatomical  systems,  39. 

Anatomy  of  alimentary  canal,  328; 
of  brain,  166,  610;  of  ear,  557; 
of  eye,  504;  of  joints,  92;  of 
lymphatic  system,  349;  of  mus- 
cular system,  112;  of  nervous 
system,  158,  172;  of  respiratory 
organs,  380;  of  skeleton,  63;  of 
skin,  441;  of  urinary  organs, 
427;  of  vascular  system,  211. 

Animal  heat,  source  of,  477. 

Anvil  bone,  558. 

Agraphia,  628. 

Aorta,  215,  219. 

Aphasia,  628 

Apoplexy,  170. 

Appendicular  skeleton,  77. 

Appendix  vermiformis,  342. 

Appetite,  377. 

Aqueduct  of  Sylvius,  or  iter,  171, 

618. 
Aqueous  humor,  516. 
Arachnoid,  5,  161. 
Arbor  vit;u,  171. 
Area,  motor,  623. 

Areolar  tissue,  100. 

Areolar  tissue,  subcutaneous,  442. 

Arm,  skeleton  of,  77. 

673 


674 


INDEX. 


Arterial  blood,  225,  404. 

Arterial  pressure,  242,  267. 

Arteries,  distribution  of,  218. 

Arteries,  structure  of,  225. 

Artery,  axillary,  219  ;  brachial, 
219;  bronchial,  220;  carotid, 
219;  cceliac,  220;  coronary,  216, 
219;  femoral,  220;  hepatic,  345; 
iliac,  220  ;  innominate,  219  ;  in- 
tercostal, 220  ;  mesenteric,  2,'1 1  ; 
radial,  219;  renal,  220,  427; 
subclavian,  219;  ulnar,  219;  ver- 
tebral, 219. 

Articular  cartilage.  92. 

Articulations,  64,  91. 

Arytenoid  cartilages,  635. 

Ascending  antero-lateral  tract,  598. 

Asphyxia,  422. 

Aspiration  of  thorax,  251,  393. 

Assimilation,  22. 

Assimilative  tissues,  32. 

Associated  movements,  631. 

Association  of  ideas,  631. 

Astigmatism,  528. 

Astragalus,  81. 

Atlanto-axial  articulation,  95. 

Atlas  vertebra,  69. 

Attraction  particle,  19. 

Auditory  nerve,  174. 

Auditory  ossicles,  558. 

Auditory  perceptions,  574. 

Augmentor  nerve-fibres,  270. 

Auriculo-ventricular  valves,  217. 

Automatic  centres,  189. 

Automatic  movements,  25. 

Automatic  tissues,  34. 

Automatism  of  heart,  258. 

Axial  current,  235. 

Axial  ligament,  559. 

Axial  skeleton,  64,  67. 

Axillary  artery,  219. 

Axis  vertebra,  69. 

Axis,  visual,  541. 

Ball-and-socket  joints,  94. 

Basement  membrane,  103,  283. 

Basilar  membrane,  561,  573. 

Bathing,  449. 

Beat  of  heart,  227,  258. 

Beef-tea,  125. 

Biceps  muscle  of  arm,  113. 

Bile,  370. 

Bilirubin,  357,  370. 

Blackness,  sensation  of,  542. 

Bladder,  urinarv,  427. 

Blind  spot,  532.' 

Blood,   41  ;    arterial   and    venous, 

225,    380;    composition    of,    59; 

clotting    of,    51  ;    crystals,    47 ; 


gases  of,  404;  histology  of,  44; 
laky,  46  ;  quantity  of,  61 ;  se- 
rum, 51. 

Blood  corpuscles,  44,  60. 

Blood-rlow  in  capillaries,  234;  in 
kidneys,  431  ;  in  liver,  347. 

Blood-vessels,  anatomy  of,  211. 

Blood-vessels,  nerves  of,  273. 

Blood-vessels,  structure  of,  225. 

Blushing,  277. 

Bone,  composition  of,  89;  histology 
of,  87;  gross  structure  of,  85. 

Bones  of  face,  75;  of  fore-limb, 
77;  of  hind-limb,  78;  of  pectoral 
arch,  77 ;  of  pelvic  arch,  77  ;  of 
skull,  72. 

Brachial  artery,  219. 

Brachial  plexus,  164. 

Brain,  anatomy  of,  166,  610;  physi- 
ology of,  609 ;  membranes  of, 
160." 

Bread,  323. 

Breast-bone,  72. 

Bronchial  arteries,  220. 

Bronchial  tubes,  382. 

Bronchus,  382. 

Brunner's  glands.  341. 

Buccal  cavity,  328. 

Buffy  coat  on  blood-clot,  52. 

Caecum,  342. 

Calcaneum,  79. 

Calcium  salts,  relation  to  blood- 
clotting,  55;  to  heart-beat,  271. 

Camera  obscura,  521. 

Canals,  semicircular,  560,  563,  574, 
615. 

Capacity  of  lungs,  391. 

Capillaries,  blood,  220,  226. 

Capillaries,  lymphatic,  352. 

Capillary  circulation,  234. 

Capsule,  internal,  619. 

Capsule  of  Glisson,  345. 

Carbamide,  see  Urea. 

Carbohydrates,  see  Amyloids. 

Carbon  dioxide,  13;  in  blood,  411; 
production  of,  in  muscle,  457. 

Carbon  monoxide  haemoglobin,  422. 

Cardiac  impulse,  228. 

Cardiac  muscular  tissue,  123,  255. 

Cardiac  nerves,  257. 

Cardiac  orifice  of  stomach,  338. 

i  'ardiac  plexus,  176. 

Cardio-inhibitory  nerves,  264,  269. 

Carotid  artery,  219. 

Carpus,  77. 

Casein,  10. 

Caseinogen,  10. 

Cartilage,  98  ;  articular,  92 ;  ela» 


INDEX. 


675 


tic,  104  ;  fibro-,   104  ;    histology 
of,  99  ;  inter-articular,  104. 
Cartilages  of  larynx,  684. 
Cataract,  528. 
Cauda  equina,  164. 
Caudate  nucleus,  619. 
Cells,   17;  amoeboid,   110;  ciliated; 
35,   110;    division  of,   18;   differ- 
entiation of,  29  ;  growth  of,  1?  ; 
oxyntic,    338  ;     secretory,     288 ; 
vaso-motor,  273. 
Cement,  of  tootb,  331. 
Central  fissure,  623. 
Centre,      cardio-inbibitory,      269 ; 
cerebro-spinal,  5,    159 ;    convul- 
sive, 424;  respiratory,  414. 
Centre  of  gravity  of  body,  149. 
Centres,   nerve,    general  functions 

of,  188. 
Centrosome,  19. 
Cephalic  vein,  222. 
Cerebellar  tract,  598. 
Cerebellum,  167,  613. 
Cerebral  cortex,  622  ;  motor  areas 

in,  623;  sensory  areas  in,  629. 
Cerebral  hemispheres,  166. 
Cerebral    hemispheres,    functions 

of,  622. 
Cerebral  localization,  623. 
Cerebral  ventricles,  168 
Cerebro-spinal  centre,  5,  159. 
Cerebro-spinal  liquid,  161,  169. 
Cervical  plexus,  164. 
Cervical  vertebra?,  68. 
Characteristics  of  human  skeleton, 

83. 
Chemical  changes  in  breathed  air, 

399. 
Chemical      combinations,     energy 

liberated  in,  304. 
Chemical  composition  of  body,  7. 
Chemistrv,  of  bile,  370  ;  of  blood, 
59;  of  *b<me,  89;  of  fats,  108;  of 
gastric  juice,  365;  of  lymph,  62; 
of  muscle,  123,  457;  of  pancreatic 
secretion,   368;    of    respiration, 
398;  of  secretion,  287;  of  teeth, 
331  ;    of   urine,   433  ;    of   white 
fibrous  tissue,  102  ;    of  working 
muscle,  454. 
Chest,  tee  Thorax. 
Chondrin,  11. 
Chorda  tympani  nerve,  293. 

Choroid,'  510. 

Choroid  plexus,  189. 
Chromatic  aberration,  527. 
Chroiiioplasm,  19. 
Chyle,  3';:. 
Chyme,  807. 


Ciliary  muscle,  516,  524. 

Ciliary,  processes,  510. 

Ciliated  cells,  110. 

Circulation,  211,  223,  234;  during 

asphyxia,   423  ;    influence  of  re- 
spiratory movements  on,  394;  in- 
fluence of  nerves  on,  253;  portal, 
223,  347;  pulmonary,  223;  renal, 
431. 
Circulatory  organs,  211. 
Circumvallate  papillae,  333. 
Classification  of  the  tissues,  31. 
Classification  of  nerve-fibres,  191. 
Clavicle,  77. 
Clothing,  486. 
Coagulated  proteid,  10. 
Coagulation  of  blood,  51. 
Coccyx,  71. 
Cochlea,  560,  573. 
Coeliac  axis,  220. 
Cold-blooded  animals,  477. 
Collagen,  105. 
Collar-bone,  77. 
Colon,  342. 
Color  blindness,  545. 
Color  mixing,  543. 
Color  vision,  541. 
Comma  tract,  598. 
Combustible  foods,  453. 
Commissure,  optic,  512. 

Commissures,  cerebral,  170,  620. 

Common  bile-duct,  344. 

Common  sensation,  490,  585,  587. 

Complemental  air,  392. 

Complementary  colors,  543. 

Concha,  557. 

Conduction  in  spinal  cord,  594,  599. 

Conductive  tissues,  35 

Conductivity,  physiological,  24. 

Congestion,  487. 

Conjunctiva,  506. 

Connective  tissue,  63,  100,  106. 

Connective-tissue  corpuscles,  102. 

Conservation  of  energy,  302. 

Consonants,  642. 

Contractile  tissues,  35,  117. 

Contractility,  23,  127. 

Contraction,  muscular,  130. 

Contrasts,  visual,  547. 

Convulsive  centre,  424. 

Cooking  of   meats,    321  ;    of   vege- 
tables, 323. 

Co-ordinating  tissues,  34. 

Co-ordination,  24,  188. 

Cord,  spinal,  161,  594. 

fords,  vocal,  635. 

Coriuin,  6,  442. 

Corn,  822. 

Cornea,  509. 


676 


INDEX. 


Coronary  artery,  216;  sinus,  215. 
( lorpora  albicantia,  173. 
Corpora  geniculata,  017. 
Corpora  quadrigemina,  107,  til 7. 
Corpora  striata,  107,  018. 
Corpus  callosum,  1 7<>. 
Corpuscli'S  of  blood,  red,  44  ;  color- 
less, 47;  platelets,  49. 
Corresponding  retinal  points,  554. 
Cortex  of  cerebrum,  022. 
Corti,  organ  of,  502. 
Costal  cartilages,  72. 
Costal  respiration,  393. 
Coughing,  425. 
Cranial  nerves,  172. 
Cranium,  73. 
Cream,  322. 
Cretinism,  358. 
Cricoid  cartilage,  634. 
Crura  cerebri,  107. 
Crying,  420. 

Crypts  of  Lieberkiihn,  341. 
Crystalline  lens,  516. 
Curare  poisoning,  129. 
Cutaneous  secretions,  447. 
Cutis  vera,  442. 
Cystic  duct,  344. 

Daltonism,  546. 

Death,  671. 

Death  stiffening,  123,  457. 

Defects  (optical)  of  eye,  525,  526. 

Degeneration  of  nerve-fibres,  209. 

Degenerations,  in  brain,  619  ;  in 
spinal  cord,  596. 

Deglutition,  363. 

Dentine,  331. 

Depressor  nerve,  276. 

Dermis,  5,  576. 

Descemet,  membrane  of,  510. 

Deutoplasm,  26. 

Development,  29. 

Diabetes,  355,  470 

Dialysis,  42. 

Diapedesis,  281. 

Diaphragm,  4,  386. 

Dietetics,  474. 

Diet,  mixed,  advantages  of,  325. 

Differentiation  of  the  tissues,  29. 

Digestion,  361. 

Digestion  of  a  typical  meal,  375. 

Diplofi.  89. 

Dislocation,  96. 

Dispersion  of  light,  519. 

Dissimilation,  22. 

Distance,  perception  of,  552,  574. 

Division  of  physiological  employ- 
ments, 30. 

Dorsal  (neural)  cavity,  5. 


Dorsal  (thoracic)  vertebras,  66. 

Drum  of  ear,  557. 

Ductless  glands,  :i">4 

Duodenum,  339 

Dura  mater,  100. 

Duration   of    luminous  sensations, 

539. 
Dyspepsia,  377. 

Ear,  557. 

Eggs,  321. 

Elasticity  of  muscle,  137. 

Elastic  cartilage,  104. 

Elastic  tissue,  102. 

Electrical  currents,  of  muscle,  138; 
of  nerve,  198. 

Elements  found  in  body,  8. 

Eliminative  (excretory)  tissues,  32. 

Emmetropia,  525. 

Emulsification,  369. 

Enamel,  331. 

End  bulbs,  576. 

Endocardium,  213. 

Endo-lymph,  500. 

Endo-skeleton,  63. 

End  plates,  121. 

Energy,  conservation  of,  302  ;  ki- 
netic, 303;  lost  from  body  daily, 
300,  480;  of  chemical  affinity, 
304;  potential,  303  ;  muscular, 
source  of,  140,  454;  source  of, 
in  body,  304  ;  utilization  of,  in 
body,  310. 

Energy-yielding  foods,  452. 

Enzymes,  11,  303. 

Epidermis,  5,  441. 

Epiglottis,  634. 

Epithelium,  6,  36. 

Epithelium,  ciliated,  110. 

Equilibrium  sensations,  614, 

Erect  posture,  149. 

Ethmoid  bone,  75. 

Eustachian  tube,  558. 

Excretion,  282. 

Exercise,  154. 

Exoskeleton,  63. 

Expiration,  390. 

Expiratory  centre,  421. 

External  auditory  meatus,  557. 

External  ear,  557. 

External  respiration,  380. 

Extrinsic  reference  of  sensations, 
501. 

Eye,  504;  appendages  of,  505;  op- 
tical defects  of.  520  ;  physiology 
of,  530;  refraction  of  light  in, 
521. 

Eyeball,  509. 

Eyeball,  muscles  of,  507. 


INDEX. 


677 


Eyelids,  506. 

Facial  nerve,  174. 
False  vocal  cords,  635. 
Fat,  12,  108. 

Fat,  source  of,  in  body,  472. 
Fatigue  of  retina,  546. 
Fatty  tissue,  107. 
Fauces,  335. 
Fechner's  law,  499. 
Feeding  of  infants,  667. 
Femoral  artery,  220. 
Femur,  78,  93. 
Ferments,  11,  363. 
Fertilization,  659. 
Fever,  485. 
Fibrin,  10,  52,  54. 
Fibrin  ferment,  55. 
Fibrinogen,  10,  54. 
Fibro-cartilage,  104. 
Fibula,  79. 

Fick  and  Wislecenus,  455. 
Filiform  papillae,  334. 
Flesb  foods,  321. 
Follicles  of  bairs,  444. 
Fontanelles,  92. 
Food  of  plants,  315. 
Foods,    definition  of,   317;  energy 
yielding,  452  ;  flesby,  321  ;  non- 
oxidizable,     316,     320;     tissue- 
forming,    313,    452;    vegetable, 
322. 
Foot,  skeleton  of,  79. 
Foramen,  intervertebral,  71;  mag- 
num, 75  ;    of  Monro,  171,   619  ; 
oval,  558  ;  round,  558  ;  thyroid, 
78 ;  vertebral,  68. 

Forebrain,  166,  609,  618. 

Forelimb,  skeleton  of,  77. 

Fornix,  170. 

Frog,  heart  of,  256. 

Frontal  bone,  75. 

Fuel  of  body,  308. 

Fundamental  pbysiological  actions, 
28. 

Fungiform  papillae,  333. 

Fur  on  tongue,  334. 

Gall  bladder,  344. 

Ganglia,  100,  175,  176,  184  ;  of 
cranial  nerves,  175;  of  heart, 
257,  368  ;  of  spinal  nerve-roots, 
16::,  184  ;  sporadic.  170. 

Ganglion,  Gasserian,  174;  spinal, 
184 

( la  >--  of  blood,  404. 
I .      erian  ganglion,  174. 
<  i:,  - 1  ic  dij  e  ition,  366. 
I  ic  glands,  888, 


Gastric  juice,  365. 

Gelatin,  11. 

Gelatinoids,  10,  319. 

Gemmation,  644. 

Glands,  284. 

Glenoid  fossa,  77. 

Gliding  joints,  96. 

Glisson.  capsule  of,  346. 

Globe  of  eve.  509. 

Globulin,  10,  47. 

Glossopharyngeal  nerve,  174. 

Glottis,  635. 

Glucose  (grape  sugar),  12. 

Gluten,  322. 

Glycerine,  12. 

Glycocholic  acid,  12. 

Glycogen,  12,  466. 

Gmelin's  test,  370. 

Goitre,  357. 

Golgi's  tendon  organs,  122. 

Grape  sugar,  12. 

Great  omentum,  339. 

Growth,  17. 

Gullet,  336. 

Ha?mal  (ventral)  cavity,  4,  6. 

Haematin,  12,  47. 

Hoematoblasts,  62. 

Haemoglobin,  46,  405. 

Hairs,  444. 

Hair-cells,  562. 

Hammer-bone,  558. 

Hand,  skeleton  of,  77. 

Haversian  canals,  87. 

Hearing,  557. 

Heart,  213,  271 ;  beat  of,  227,  256, 

262  ;  nerves  of,  253. 
Heat  production  and  regulation  in 

Body,  477. 
Heat  lost  from  lungs,  399. 
Heel-bone,  79. 
Hemianopia,  512. 

Hemispheres,  cerebral,  166  ;  func- 
tions of,  622. 
Hensen,  band  of,  119. 

Hepatic  artery,  343. 

Hepatic  cells,  344,  467. 

Hepatic  duct,  344. 

Hepatic  veins,  224,  344. 

Heredity,  theories  of,  661. 

Hering's    theory  of    color   vision, 
548. 

Hiccough,  425. 

Hind  limb,  skeleton  of,  78. 

Hinge  joints,  05. 

Hip-joint,  92. 

Hippuric  acid,  434. 

Histology,    1  ;    of    adipose    tissue 
107  ;   Of    areolar   tissue,    100  ;   of 


678 


INDEX. 


blood,  44;  of  hone,  87;  of  cardiac 
muscle,  123  ;  of  cartilage,  99  ;  of 
connective  tissues,  100  ;  of  ear, 
561;  of  elastic  issue,  102;  of 
hairs,  444;  of  heart,  123;  of  kid- 
ney, 429;  of  liver,  344;  of  lungs, 
383,  of  lymph,  49;  of  lymph 
glands,  852;  of  nails,  445;  of 
nerve-cells,  179,  1S4  ;  of  nerve- 
fibres,  176;  of  nose,  588;  of 
plain  muscular  tissue,  122;  of 
skin,  441;  of  small  intestine, 
339;  of  retina,  511;  of  spinal 
cord,  181;  of  stomach,  122,  337; 
of  striped  muscle,  117;  of  teeth, 
331;  of  tactile  organs,  576;  of 
tongue,  589 ;  of  white  fibrous 
tissue.  101. 

Homologies  of  supporting  tissues, 
105. 

Homology,  68 ;  of  limbs,  80. 

Horopter,  554. 

Humerus,  77,  86. 

Humor,  aqueous,  516  ;  vitreous, 
516. 

Hunger,  587. 

Hyaloplasm,  19,  120. 

Hydrocarbons.     See  Fats. 

Hydrogen,  9. 

Hygiene,  1;  of  blood,  60;  of  brain, 
632;  of  clothing,  392,  486;  of 
exercise,  153;  of  eye,  525;  of 
growing  skeleton,  90,  106 ;  of 
joints,  96  ;  of  muscles,  153  ;  of 
respiration,  392,  401  ;  of  sight, 
525 ;  of  skeleton,  90;  of  skin, 
448;  of  supporting  tissues,  106. 

Hyoid  bone,  76. 

Hypermetropia,  525. 

Hypoglossal  nerve,  175. 

Ideas,  association  of,  631. 

Idio-retinal  light,  531. 

Ileum,  339. 

Ileocolic  valve,  342. 

Iliac  artery,  220. 

Ilium,  77. 

Illusions,  sensory,  502. 

Images,  after,  548. 

Impulse,  cardiac,  228. 

Impregnation,  662. 

Impulse,  nervous,  203. 

Incus,  558. 

Indigestion,  377. 

Inert  layer,  235. 

Inferior  laryngeal  nerve,  420. 

Inferior  maxillary  nerve,  174. 

Inferior  mesenteric  artery,  220. 

Inferior  vena  cava,  215. 


[nflammation,  280. 

[nfuudibulum,  171. 

Inhibition  of  reflexes,  606. 

Inhibitory  nerves,  191). 

Innervation  sensations,  591. 

innominate  artery.  219. 

Innominate  bone,  77. 

Innominate  vein,  223. 

Inogen,  141. 

Inorganic  constituents  of  Body,  13; 
foods,  320. 

Inosit,  13. 

Inspiration,  how  effected,  385. 

Intensity  of  sensations,  499. 

Interarticular  cartilage,  104. 

Intercostal  arteries,  220, 

Intercostal  muscles,  388. 

Internal  ear.  559. 

Internal  medium,  40. 

Internal  respiration,  411. 

Intervertebral  disks,  92. 

Intervertebral  foramina,  71. 

Intestinal  digestion,  372;  move- 
ments, 378. 

Intestines,  339. 

Intrinsic  heart-nerves,  257,  263. 

Intussusception,  18. 

Iris,  510. 

Irritability,  23. 

Irritability,  muscular,  128. 

Irritable  tissues,  33. 

Ischium,  77. 

Iter,  618. 

Jaw-bones,  75. 

Jejunum,  339. 

Jelly-like  connective  tissue,  103. 

Joints,  92. 

Jugular  vein,  223. 

Karyokinesis,  18. 
Karyoplasm,  19. 
Katabolism,  22. 
Kidneys,  427. 
Kinetic  energy,  303. 
Knee-cap  or  knee-pan,  79. 
Kreatin,  11,  123,  460. 

Labyrinth,  560. 
Lachrymal  apparatus,  507. 
Lachrymal  bone,  76. 
Lactation,  666. 
Lacteals,  44,  341. 
Lactose,  13. 

Lacume,  lymphatic,  350. 
Lamina  spiralis,  561. 
Large  intestine,  342,  374. 
Laryngeal  nerves,  420. 
Larynx,  634. 


INDEX. 


679 


Laughing,  426. 

Law,  the  psycho-physical,  499. 
Leaping,  153. 

Least-resistance  hypothesis,  603. 
Lecithin,  13,  321. 
Lens,  crystalline,  516. 
Lenses,  refraction  of  light  by,  520. 
Lenticular  nucleus,  619. 
Leucin,  368.  462. 
Leucocytes,  49. 
Levers  "in  the  Body,  145. 
Lieberkiihn,  crypts  of,  341. 
Liebig's  classification  of  foods,  453. 
Liebig's  extract,  125. 
Ligament,  round,  93. 
Ligament,  suspensory,  of  lens,  516, 
524. 

Ligaments,  93. 

Light,  dispersion  of,  519. 

Light,  properties  of,  516. 

Light,  refraction  of,  518. 

Limbs,  7. 

Limbs,  skeleton  of,  77. 

Iiiquid  extract  of  meat,  125. 

Liquor  sanguinis,  44. 

Liver,  344;  glycogenic  function  of, 
466;  histology  of,  346,  467. 

Local  sign  of  sensations,  492. 

Local  temperatures,  484. 

Localization  of  cerebral  functions, 
623. 

Localizing  powers  of  retina,  539. 

Localizing  power  of  skin,  580. 

Locomotion,  151. 

Locus  niger,  616. 

Long  saphenous  vein,  222. 

Long  sight,  525. 

Losses  of  energy  daily,  301,  480. 

Losses  of  material  from  Body,  299. 

Lower  maxilla,  75. 

Lumbar  plexus,  165. 

Lumbar  vertebrae,  69. 

Lungs,  383. 

Lungs,  capacity  of,  391. 

Luxus  consumption,  459,  462. 

Lymph,  42;  canaliculi,  103,  351 
chemistry    of,    62;   hearts,    353 
histology   of,    49;   lacunae,    350 
movement  of,  353,  397;  renewal 
of,  42;  v.-ss.-ls.  43,  350. 
Lymphatic  glands,  852. 
Lymphatic  system,  349. 
Lymphoid  tissue,  49,  351. 

Macula  lutea,  511. 
Millar  hone,  76. 
Malleus,  558. 

Malpigbian  corpuscles  of  spleen, 
867. ' 


Malpighian  layer  of  epidermis,  441, 
Malpighian   pyramids  of   kidney, 

429. 
Maltose,  362. 
Mammalia,  4. 
Mandible,  75. 
Manometer,  267. 
Marrow  of  bone,  87. 
Marrow,  spinal  (spinal  cord),  160, 

181,  594. 
Material  daily  losses  of  Body,  299. 
Maxilla,  75. 
Measurement  of  arterial  pressure, 

266. 
Meatus,  external  auditory,  557. 
Mechanisms,  physiological,  38. 
Media,  refracting,  in  eye,  515. 
Median  posterior  tract,  598. 
Medulla  oblongata,  167,  610. 
Medullary  cavity,  87. 
Membrane,  basilar,  561;  of  Desce- 
met,  510;  of  Krause,  119;  retic- 
ular, 563;  synovial,  93;  tectorial, 
563;  tympanic,  558. 
Menstruation,  658. 
Mesentery,  339. 
Metabolic  tissues,  33,  288. 
Metacarpus,  77. 
Metatarsus,  79. 

Microscopic  anatomy,  2,  see  Histol- 
ogy. 
Mid-brain,  functions  of,  616. 
Midriff,  see  Diaphragm. 
Migration,  281. 
Milk,  322;  for  infants,  667. 
Millon's  test,  9. 
Mitosis,  18. 

Mixed  diet,  advantage  of,  325. 
Modality  of  sensation,  492,  494. 
Modiolus,  561. 
Monro,  foramen  of,  171,  619. 
Morula,  29. 
Motion,  144. 

Motor  area  of  cortex,  623. 
Motor  organs,  109. 
Motor  tissues,  35. 
Motores  oculi,  172. 
Mouth,  329. 
Movements,    associated,    63,    593  ; 

voluntary,  622. 
Movements,    intestinal,    378  ;    re- 
spiratory, 385. 
Mucin,  11. 

Mucous  layer  of  epidermis,  441. 
Mucous  membranes,  6,  '.'>'.'>'.),  341. 
Mulberry  mass,  29. 
Mumps,  334. 
Muscffi  volitantes,  529. 
Muscle,  biceps,   118;  <ardiac,  123, 


(580 


INDEX. 


255,   262  ;   ciliary,    516;    stape- 
dius, 559;  tensor  tympani,  559. 
Muscles,  chemistry  of,    123,  457; 
histology  of,  117",  122;  of  eyeball, 

507,  of  larynx,  ti;i7;  of  respi- 
ration, 387;  physiology  of,  127; 
skeletal,  113;  structure  of,  112, 
117;  visceral,  122. 

Muscle  spindle,  121. 

Muscular  contraction,  130. 

Muscular  energv,  source  of,  140. 

Muscular  sense,  591. 

Muscular  tissue,  35,  117,  123, 
127. 

Muscular  work,  135,  454. 

Myocardium,  256. 

Myopia,  525. 

Myosin.  10,  124. 

Myosinogen,  124. 

Nails,  445. 

Nasal  bone,  76. 

Negative  variation,  139,  199. 

Nerve-cells,  179. 

Nerve-centres,  158,  181,  594. 

Nerve-fibres,  35,  176. 

Nerve-fibres,  classification  of,  191. 

Nerve  plexuses,  158. 

Nerve  stimuli,  194. 

Nerve  trunks,  158. 

Nerves,  158;  accelerator  or  aug- 
nientor,  270;  cranial,  172,  207; 
cardiac,  253,  264 ;  laryngeal, 
420  ;  optic,  512 ;  respiratory, 
414;  secretory,  292;  spinal,  163; 
sympathetic,  160,  175;  thermo- 
genic, 484 ;  trophic,  192,  295; 
vaso-constrictor,  273;  vaso-dila- 
tor,  279,  293;  vaso-motor,  273, 
280. 

Nervous  impulses,  203. 

Nervous  system,  anatomy  of,  158. 

Nervous  system,  physiology  of, 
186. 

Neural  tube  (dorsal  cavity),  5. 

Neurilemma,  177. 

Neuroglia,  180. 

Nitrogenous  compounds  in  bodv, 
9. 

Nodal  points  of  eye,  530. 

Noises,  504. 

Non-vascular  tissues,  41. 

Notes,  musical,  5(14. 

Nuclear  spindle,  20. 

Nuclein,  27. 

Nucleoalbumins,  28. 

Nucleolus,  17,  19. 

Nucleoplasm,  19. 

Nucleus,  17,  20. 


Nucleus,  caudate,  619;  lenticular, 

619. 
Nucleus,  red,  of  mid  brain,  617. 
Nutrition,  451. 
Nutrition  of  embryo,  664. 
Nutritive  tissues,  32. 
Nystagmus,  614. 

Occipital  bone,  75. 

Oculo-motor  nerves,  172. 

Odontoid  process,  95. 

Odorous  bodies,  588. 

(Esophagus,  336. 

Olecranon,  81. 

Olein,  12. 

Olfactory  lobe,  167. 

Olfactory  nerves,  172. 

Olfactory  organs,  588. 

Omentum,  339. 

Ophthalmic  nerve,  174. 

Optical  defects  of  eye,  526. 

Optic  commissure,  512. 

Optic  nerves,  172,  512. 

Optic  thalami.  167,  170,  618. 

Optic  tracts,  512. 

Organ  of  Corti,  562. 

Organs,  1,  36;  of  animal  life,  110; 
of  circulation,  211;  of  common 
sensation,  587;  of  digestion,  328; 
of  movement,  109;  of  relation, 
110;  of  reproduction,  647;  of 
respiration,  380;  of  secretion, 
282;  of  special  sense,  490,  493; 
urinary,  427;  of  vegetative  life, 
110. 

Os  calcis,  79. 

Os  innominatum,  77. 

Os  orbiculare,  559. 

Os  pubis,  79. 

Osmazome,  321. 

Ossicles,  auditorv,  558. 

Otoliths,  564. 

Oval  foramen,  558. 

Ovary,  652,  655. 

Over-tones  (upper  partial  tones), 
568. 

Ovulation,  658. 

Ovum,  29,  656. 

Oxidation  by  stages,  309. 

Oxidations  in  the  bodv,  307,  451 
454. 

Oxygen  in  the  blood,  407. 

Oxygen  consumed  daily,  400. 

Oxyhemoglobin,  405. 
Oxyntic  cells,  338. 

Pacinian  bodies,  576,  578. 
Pain,  585. 
Palate,  329. 


INDEX. 


681 


Palate  bones,  75. 

Pahnatin,  12. 

Pancreas,  290,  346,  358. 

Pancreatic  secretion,  368. 

Papillary  muscles,  217,  230. 

Papilla?  of  tongue,  334. 

Paraglobulin,  10.  54. 

Parapeptone,  366. 

Paraplasm,  26. 

Parietal  bone,  75. 

Parotid  gland,  296,  334. 

Partial  tones.  568. 

Parturition,  664. 

Patella,  79. 

Patbeticus,  173. 

Patbology,   1. 

Peas,  323. 

Pectoral  arcb,  77. 

Pelvic  arcb,  77. 

Pendular  vibrations,  565.- 

Pepsin,  365. 

Peptones,  10,  365,  372. 

Perceptions,  500 ;  visual,  552  ; 
auditory,  574. 

Pericardium,  213. 

Pericbondrium,  98. 

Perilymph,  560. 

Perimysium,  117. 

Perineurium,  177. 

Periosteum,  85. 

Peripberal  reference  of  sensations, 
491,  501. 

Peristaltic  movements,  365. 

Peritoneum,  5,  337. 

Pes  of  cms  cerebri,  616. 

Pettenkofer's  test,  370. 

Peyer's  patcbes,  352. 

Pbagocytes,  48. 

Pbalanges  of  fingers  and  toes,  77, 
79. 

Pbarynx,  335. 

Pbrenic  nerve,  165,  390. 

Pbysiological  chemistry,  8. 

Physiological  mecbanisms,  38. 

Pbysiological  properties,  16. 

Physiology,  1;  of  blood-vessels, 
227;  of  brain,  609;  of  connective 
tissues,  102;  of  digestion,  361: 
of  ear,  564,  571;  of  eye,  530;  of 
heart,  253;  of  kidneys,  435;  of 
muscles,  127,  144;  of  nerves, 
186;  of  nerve-centres,  186,  594; 
of  nutrition,  451;  ol*  respiration, 
385,  898,  414;  of  skin,  446,  578; 
of  smell,  687;  of  spinal  cord, 
504;  of  taste,  589;  of  touch,  578. 

Pia  mater,  160. 

Pineal  eland,  171. 

Pitch  of  notes,  564. 


Pitch  of  voice,  640. 

Pituitary  body,  171,  359. 

Pivot-joints,  95. 

Placenta,  665. 

Plain  muscular  tissue,  123,  142. 

Plastic  foods,  453. 

Platelets,  or  plaques  of  blood,  49. 

Pleura,  5,  383. 

Plexus,  158;  bracbial,  164;  car- 
diac, 176;  cervical,  164;  choroid, 
169;  lumbar,  165;  sacral,  166; 
solar,  176. 

Pneumogastric  nerves,  174,  264, 
419. 

Polar  globules,  660. 

Pons  Varolii,  167,  613. 

Popliteal  artery,  220. 

Portal  circulation,  223,  347. 

Portal  vein,  343. 

Posterior  tibial  artery,  220. 

Postures,  149. 

Potatoes,  323. 

Potential  energy,  303. 

Pregnancy,  662. 

Presbyopia,  526. 

Pressure,  arterial,  242. 

Pressure,  intra-tboracic,  384. 

Pressure  sense,  578. 

Primates,  2. 

Production  of  heat  in  body,  477„ 

Pronation,  96. 

Proofs  of  circulation,  251. 

Prostate,  649. 

Protective  tissues,  36. 

Proteids,  9,  314,  453. 

Proteids,  oxidation  of,  455. 

Protoplasm,  19,  26. 

Psychical  activities  of  cord,  607. 

Psycho-physical  law,  499. 

Ptosis,  509. 

Ptyalin,  362. 

Puberty,  669. 

Pubis,  77. 

Pulmonary  artery,  214. 

Pulmonary  circulation,  223. 

Pulmonary  veins,  216. 

Pulse,  246. 

Purkinje's  experiment,  533. 

Pus,  48. 

Pylorus,  336,  338. 

Pyramidal  tracts,  597. 

Pyramids  of  Malpigbi,  429. 

Pyrexia,  485. 

Qualities  of  sensation,  401 
Quantity  of  air  breathed  daily,  392. 

Quantity  of  blood,  61. 
Quantity  of  food  needed  daily,  326, 
476. 


682 


INDEX. 


Radial  artery,  210. 
Radio-ulnar  articulation,  95. 

Radius,  77. 

Range  of  voice,  640. 

Elate  of  blood  How,  248. 

Receptaculum  chyli,  1550. 

Receptive  tissues,  32. 

Rectum,  342. 

Red  blood-corpuscles,  44.  60. 

Reduced  haemoglobin,  405. 

Keflex  actions,  188,  600. 

Reflex  convulsions,  602. 

Reflex  time,  608. 

Reflexes,  acquired,  613. 

Reflexes,  inhibition  of,  606. 

Refracting  media  of  eye,  515. 

Refraction  <<f  lenses,  520. 

Refraction  of  light,  518. 

Refraction  in  tin-  eve,  521. 

Regulation  of  temperature,  482. 

Renal  artery,  220,  427. 

Renal  organs,  427. 

Renal  secretion,  432. 

Rennet,  365. 

Reproduction,  22,  644. 

Reproductive  tissues,  36. 

Residual  air,  392. 

Resistance  theory,  419. 

Resonance,  sympathetic,  569. 

Respiration,  22,  380. 

Respiration,  chemistry  of,  398. 

Respiration,  nerves  of,  414. 

Respiratory  centre,  414. 

Respiratory  foods,  453. 

Respiratory  movements,  385;  influ- 
ence of,  on  circulation  and  on 
flow  of  lymph,  394. 

Respiratory  sounds,  391. 

Reticular  membrane,  563. 

Reticulum  of  cells,  19. 

Retiform  (adenoid)  connective  tis- 
sue, 103. 

Retina,  511,  514. 

Rhythmic  movements,  417. 

Ribs,  73. 

Rib  cartilage,  72. 

Rice.  323. 

Right  lymphatic  duct,  350. 

Rigor  mortis,  123,  457. 

Rods  and  cones,  513,  532. 

Rolando,  fissure  of,  623. 

Round  foramen,  558. 

Running,  152. 

Sacculus,  561. 
Sacral  plexus,  166. 
Sacral  vertebrae,  69. 
Sacrum,  69. 
Saliva,  uses  of,  361. 


Salivary  glands,  •!'!•!. 

Salivary  glands,  nerves  of,  293. 

Salivin  (ptyalinj,  862. 
Santorini,  cartilages  of,  635. 
Sarcolactic  acid,  18,  123. 
Barcolemma,  118. 
Sarcomere,  119. 
Sarcoplasm,  120. 
Sarcous  element,  120. 
Sarcosome,  120. 
Sarcostyle,  119. 
Scal.-e  of  cochlea,  561. 
Scalene  muscles,  388. 

Scapula,   77. 

Sciatic  nerve,  166. 

Sclerotic,  509. 

Sebaceous  glands,  446. 

Secondary  (acquired)  reflexes,  613. 

Secretion,  282. 

Secretion,  cutaneous,  447. 

Secretion,  renal,  432. 

Secretory  nerves,  292. 

Secretory  tis-ues,  32,  283. 

Sections  of  Rudy,  (J. 

Segmentation  of  ovum,  29. 

Segmentation  of  skeleton,  67. 

Semicircular  canals,  560,  563,  574, 
615. 

Semilunar  valves,  217. 

Sensation,  488;  color,  541;  com- 
mon, 490.  585;  intensity  of,  499; 
of  equilibrium,  614;  of  hunger, 
587;  special,  490;  of  thirst,  587; 
pain,  585;  peripheral  reference 
of,  491,  501;  qualities  of,  491. 

Sense-organs,  493. 

Sense,  muscular,  591;  of  bearing, 
557;  of  pain,  585;  of  sight,  580; 
of  smell,  587;  of  taste,  5*9;  of 
temperature,  582;  of  touch,  576. 

Sensory  illusions,  502. 

Serous  or  lvmph  canaliculi,  103, 
351. 

Serous  cavities,  351. 

Serous  membranes,  5. 

Serum,  51,  59. 

Serum  albumin,  9,  59. 

Shimbone,  79. 

Shine,  555. 

Shingles,  192. 

Short  sight,  525. 

Shoulder-blade,  77. 

Shoulder-girdle,  77. 

Sighing,  425. 

Sight,  sense  of,  530. 

Sijrht,  hygiene  of,  525. 

Sigmoid  flexure,  342. 

Single  vision,  553. 

Si/.e,  perception  of,  553. 


INDEX. 


683 


Skeleton,  63  ;  appendicular,  77  ; 
axial,  64,  67  ;  of  face,  75 ;  of 
skull,  72;  peculiarities  of  huruan, 
83;  of  thorax,  386. 

Skin,  5,  441:  glauds  of,  446;  hy- 
giene of,  448;  nerve  endings  iu, 
576 

Skull,  72. 

Small  intestine,  339. 

Smell,  587. 

Sneezing,  426. 

Solar  plexus,  176. 

Solar  spectrum,  519. 

Solidity,  visual  perception  of,  554. 

Soluble  ferments,  11. 

Sounds,  564. 

Sounds  of  the  heart,  230. 

Sounds,  respiratory,  391. 

Source  of  animal  heat,  479. 

Source  of  fats,  472. 

Source  of  glycogen,  468. 

Source  of  muscular  work,  454. 

Source  of  urea,  460. 

Sources  of  energy  to  Body,  304. 

Special  senses,  490. 

Specific  elements,  283. 

Specific  nerve  energies,  197. 

Spectacles,  526. 

Speech,  633. 

Spermatozoa,  651. 

Sphenoid  bone,  75. 

Spherical  aberration,  527. 

Spinal  column,  71. 

Spinal  cord,  161,594;  conduction 
in,  594  ;  functions  of,  187,  594, 
600 ;  histology  of,  181  ;  mem- 
branes of,  160;  psychical  activi- 
ties of,  607. 

Spinal  accessory  nerve,  174. 

Spinal  marrow,  see  Spinal  cord. 

Spinal  nerves.  163. 

Spinal  nerve-roots,  163,  206. 

Spleen,  356. 

Spongioplasm,  19. 

Spontaneity,  25. 

Sporadic  ganglia,  176. 

Sprains,  97. 

Squinting,  509. 

Stasres  of  life,  670. 

Stapedius  muscle,  559. 

Stapes,  558. 

Starch,  828;  digestion  of,  362,  307. 
Starvation,  proteid,  468, 
Stationary  air,  392. 
Stearin,  12, 

Stereoscopic  vision,  555. 
Sternum,  72. 

Stimuli,  muscular,  128;  nervous, 
194. 


Stimulus,  24. 

Stirrup-bone,  558. 

Stomach,  122,  336. 

Stomata,  lymphatic,  351. 

Storage  tissues,  33,  464. 

Strabismus  (squinting),  509. 

Structure  of  bone,  85. 

Strychnine  poisoning,  602. 

Subclavian  artery,  219. 

Subcutaneous  areolar  tissue,  442. 

Sublingual  gland,  336. 

Submaxillary  gland,  293,  334. 

Succus  entericus,  371. 

Sudoriparous  glands,  446. 

Superior  laryngeal  nerve,  420. 

Superior  maxillary  nerve,  174. 

Superior  mesenteric  artery,  220. 

Supination,  96. 

Supplemental  air,  392. 

Supporting  tissues,  32,  105. 

Supra-renal  capsules,  359. 

Sutures,  92. 

Swallowing,  363. 

Sweat,  446. 

Sweat-glands,  446. 

Sweat-glands,  nerves  of,  292. 

Sweetbread,  290,  346,  358. 

Sympathetic  nervous  svstem,  5, 
160,  175. 

Sympathetic  resonance,  569. 

Sympathetic  resonance  in  ear,  573. 

Synovial  membranes,  93. 

Syntonin,  125. 

System,  alimentary,  328  ;  circula- 
tory, 211;  muscular,  116,  127; 
nervous,  158.  186  ;  osseous,  63  ; 
respiratory,  380  ;  renal,  427. 

Systemic  circulation,  223. 

Systems,  anatomical,  39. 

Tactile  organs,  576. 

Taking  cold,  278. 

Tarsus,  79. 

Taste,  589. 

Taste-buds.  589. 

Taurocholicacid,  12. 

Tear-glands,  5(17. 

Tectorial  membrane,  563. 

Teeth,  329. 

Teeth,  structure  of,  331. 

Tegmentum,  616 

Temperature  of  Body,  478. 

Temperature,  bodily,  regulation  of, 

482. 
Temperature,  influence  of ,  pn  pulse 

rate,  271. 
Temperature  sense,  582. 
Temperatures,  local,  484. 
Temporal  artery,  219. 


684 


INDEX. 


Temporal  bone,  75. 

Tendons,  113. 

Tension  of  blood  gases,  407. 

Tensor  tympani  muscle,  559. 

Testis,  648. 

Tests  for  proteids,  10. 

Tetanus,  133,  140. 

Thalamencephalon,  171,  618. 

Theory,  resistance,  419. 

Theory  of  color  vision,  542,  548. 

Thermogenic  nerves,  484. 

Thigh-bone,  78. 

Thirst,  587. 

Thoracic  duct,  350. 

Thoracic  vertebra?,  66. 

Thorax,  aspiration  of,  251,  393  ; 
contents  of,  5;  movements  of,  in 
respiration,  385  ;  skeleton  of, 
387 

Throat,  335. 

Thyroid  body,  357. 

Thyroid  cartilage,  634. 

Thvroid  foramen,  78. 

Thymus,  358. 

Tibia,  79. 

Timbre,  564. 

Tissues,  1  ;  adenoid,  103  ;  adipose 
107  ;  areolar,  100  ;  assimilative 
32;  automatic,  34;  bony,  87;  car 
tilaginous,  100;  classification  of 
31;  conductive,  35;  connective 
63,  100;  contractile,  35,  117;  co 
ordinating,  34,  594;  elastic,  102 
eliminative,  32;  excretory,  32;  ir 
ritable,  33;  jelly-like  connective 
103,  lymphoid,  49;  metabolic,  33 
288  ;  motor,  35,  121  ;  muscular 
35,  127;  nervous,  176;  nutritive 
32 ;  plain  muscular,  123,  142 
protective,  36;  receptive,  32;  re 
productive,  36;  respiratory,  33 
retiform  or  adenoid,  103  ;  secre 
tory,  32,  283;  storage,  33,  464 
supporting,  32;  undifferentiated, 
32;  white  fibrous,  100. 

Tissue-forming  foods,  314,  452. 

Tone,  sensations  of,  564,  568. 

Tone  color  (timbre),  564. 

Tongue,  332. 

Tonsil,  335. 

Touch-organs,  576. 

Touch,  sensations  of,  578. 

Trachea,  3S2. 

Tracts  of  defeneration  in  spinal 
cord,  596. 

Training,  157. 

Transm'lata,  283. 

Trigeminal  nerve,  173. 

Trophic  nerves,  192,  295.  ♦ 


Trypsin,  289,  369. 
Tunica  ait  vent  ilia.  225. 
Turbinate  bones,  70. 
Tympanic  bones,  "157,  571. 
Tympanic  membrane,  557,  571. 
Tvrein,  10. 
Tyrosin,  308,  462. 

Ulna,  77. 

Ulnar  artery,  219. 

Undifferentiated  tissues,  32. 

Upper  maxilla,  75. 

Urea.  11,  434,  460. 

Ureter,  427. 

Uric  acid,  11,  434. 

Urinary  organs,  427. 

Urine,  432. 

Uterus,  652. 

Utilization  of  energy  in  Body,  310 

Utriculus,  561. 

Uvula,  329. 

Vagus  nerve,  174,  264,  420. 

Valve,  ileocolic,  342. 

Valves,  auriculo-ventricular,  217  ; 
of  veins,  226;  semilunar,  217. 

Valvulae  conniventes,  339. 

Vasoconstrictor  centre,  276. 

Vaso-dilator  centre,  280. 

Vaso-dilator  nerves,  273,  279,  293. 

Vaso-motor  nerves,  273,  280. 

Vegetable  foods,  322. 

Veins,  220,  226;  cephalic,  222; 
coronary,  215;  hepatic,  345;  in- 
nominate, 223;  jugular,  223;  long 
saphenous,  222;  portal,  224,  345; 
pulmonary,  216. 

Velum  interpositum,  169. 

Vena  cava,  215. 

Venous  blood,  225. 

Ventilation,  401. 

Ventral  cavity,  3,  6, 

Ventricles  of  brain,  168;  of  larvnx 
635. 

Vermicular  (peristaltic)  move- 
ments, 365,  378. 

Vermiform  appendix,  342. 

Vertebra^,  64;  cervical,  68;  coccy- 
geal, 70;  dorsal  or  thoracic,  66; 
lumbar,  69;  sacral,  69. 

Vertebral  artery,  219. 

Vertebral  column,  3,  64,  71. 

Vertebral  foramen,  68. 

Vertebrata,  3. 

Vestibule,  560,  561,  574. 

Vibrations,  analysis  of,  568;  com- 
position of,  567;  pendular,  565; 
sonorous,  564. 

Villi  of  intestine,  341. 


INDEX. 


685 


Vision,  color,  541. 
Vision,  purple,  511,  535. 
Vision,  stereoscopic,  555. 
Visual  axis,  541. 
Visual  contrasts,  547. 
Visual  perceptions,  551. 
Visual  sensations.  530,  536;  dura- 
tion of,  539;  intensity  of,  536. 
Vital  capacity,  392. 
Vitreous  humor,  516. 
Vocal  chords,  635. 
Vocal  chords,  false,  635. 
Voice,  633. 

Voluntary  movements,  622. 
Vomer,  75. 
Vowels,  640. 

Walking,  151. 
Wallerian  method,  210,  596. 
Wandering  cells,  103. 
Warm-blooded  animals,  477. 
Water,  constituent,  27. 


Water,  percentage  of,  in  body,  13. 
Weber's  law,  499. 
Weber's  schema,  240, 
Weissman's    theory    of    heredity, 

661. 
Wheat,  323. 
Whipped  blood,  52. 
Whispering,  643. 
White  blood-corpuscles.  17,  47,  60. 
White  fibrous  tissue,  101. 
Windpipe,  382. 
Work,  muscular,  135. 
Wrisberg,  cartilage  of,  635. 
Wrist,  77. 

Xantho-proteic  test,  9. 

Yawning,  425. 

Yellow  spot,  511 

Young's    theory   of    color    vision, 

542. 
Zoological  position  of  man,  2. 


SCIENCE 
REFERENCE  AND  TEXT-BOOKS 


PUBLISHED    BY 


HENRY  HOLT  &  COMPANY,  29SY2iSL 


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Laboratory  Manual  (to  Elementary  Course).     196  pp.     121110.     40  cents  net. 
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nia.    Advanced  Course.     902  pp.     8vo.     $3.50  net. 

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in  the  University  of  Chicago.     (In  Preparation.) 

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Hi,  I9OO  (,) 


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Allen's    Laboratory  Exercises   in    Elementary  Physics.       By  Chas.    R. 

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Howell's  Dissection  of  the  Dog.  As  a  Basis  for  the  Study  of  Physi- 
ology. By  W.  H.  Howell,  Professor  in  the  Johns  Hopkins 
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Jackman's  Nature  Study  for  the  Common  Schools.  (Arranged  by  the 
Months.)  By  Wilbur  Jackman,  of  the  Cook  County  Normal 
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Kerner  &  Oliver's  Natural  History  of  Plants.  Translated  by  Prof.  F. 
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Kingsley's  Vertebrate  Zoology.  By  Prof.  J.  S.  Kingsley,  of  Tufts 
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Macloskie'S  Elementary  Botany.  With  Students'  Guide  to  the  Exam- 
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McMurrich's  Text-book  of  Invertebrate  Morphology.  By  J.  Playfair 
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McNab's  Botany.  Outlines  of  Morphology,  Physiology,  and  Classi- 
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Martin's  The  Human  Body.     See  American  Science  Series. 

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hi,  1700 


HENRY  HOLT  &    CO.'S    WORKS   ON  SCIENCE. 

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hi,  1900 


Two  Masterpieces  on  Education 


5th  Impression  of  a  fascinating  book 

JAMES'S    TALKS    ON    PSYCHOLOGY 

TALKS  TO  TEACHERS  ON  PSYCHOLOGY  AND  TO 
STUDENTS  ON  SOME  OF  LIFE'S  IDEALS.  By  Wil- 
liam James,  Professor  in  Harvard  University,  Author  of 
"The  Principles  of  Psychology,"  etc.  xi  -\-  301  pp.,  i2mo, 
gilt  top.     $1.50,  net. 

In  writing  these  "  Talks  "  out,  the  author  has  gradually  weeded  out  as  much 
as  possible  of  the  analytical  technicalities  of  the  science.  In  their  present 
form,  they  contain  a  minimum  of  what  is  deemed  "scientific"  in  psychology 
and  are  practical  and  popular  in  the  extreme. 

The  Nation  :  "  His  style  has  the  quality  of  a  communicable  fervor,  a  clear, 
grave  passion  of  sincerity  and  conviction,  from  which  some  vibration  detaches 
itself  and  passes  into  the  reader,  and  forms  him  to  the  writer's  mood." 
Contents  :  Psychology  and  the  Teaching  Art ;  The  Stream  of  Consciousness  .; 
The  Child  asa  Behaving  Organism  ;  Education  and  Behavior  ;  The  Neces- 
sity of  Reactions  ;  Native  and  Acquired  Reactions  ;  What  the  Native  Reac- 
tions Are  ;  The  Laws  of  Habit  ;  The  Association  of  Ideas  ;  Interest  ;  Atten- 
tion ;  Memory  ;  The  Acquisition  of  Ideas  ;   Apperception  ;  The  Will  ;  The 
Gospel  of  Relaxation  ;  On  a  Certain  Blindness  in  Human  Beings  ;  What 
Makes  Life  Significant. 

WALKER'S   DISCUSSIONS   IN 
EDUCATION 

By  the  late  Francis  A.  Walker,  President  of  the  Massachu- 
setts Institute  of  Technology.      Edited  by  James   Phinney 
Munroe.     342  pp.,  8vo.     $3.00,  net. 
The  author  had  hoped  himself  to  collect  these  papers  in  a  volume. 

The  Dial:  "A  fitting  memorial  to  its  author.  .  .  .  The  breadth  of  his 
experience,  as  well  as  the  natural  range  of  his  mind,  are  here  reflected.  The 
subjects  dealt  with  are  all  live  and  practical.  .  .  .  He  never  deals  with  them 
in  a  narrow  or  so-called  'practical  '  way.'1 

Literature  :  "  The  distinguishing  traits  of  these  papers  are  open-minded- 
ness,  breadth,  and  sanity.  .  .  .  No  capable  student  of  education  will  overlook 
General  Walker's  book;  no  serious  collection  of  books  on  education  will  be 
without  it.  The  distinguished  author's  honesty,  sagacity,  and  courage  shine 
on  every  page." 

The  Boston  Transcript  :  "  Two  of  his  conspicuous  merits  characterize  these 
papers,  the  peculiar  power  he  possessed  of  enlisting  and  retaining  the  attention 
for  what  are  commonly  supposed  to  be  dry  and  difficult  subjects,  and  the  ca- 
pacity he  had  for  controversy,  sharp  and  incisive,  but  so  candid  and  generous 
that  it  left  no  festering  wound." 

HFNRY   HOI  T  &  CH        29  West  23d  St.,  New  York 
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"Should  find  a  place  in  every  college  and  public  library."— Boston  Transcript. 

KERNER'S  NATURAL  HISTORY 
OF  PLANTS. 

Translated  by  Professor  F.  \V.  Oliver,  of  University  College, 
London.  A  work  for  reference  or  continuous  reading,  at  once 
popular  and,  in  the  ;-odern  sense,  thoroughly  scientific.  With 
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Cloth.     %\z>.oo  net. 

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For  this  purpose  he  has  skillfully  employed  a  brilliant  style  of  exposition,  and  he  has  not 
hesitated  to  use  illustrations  in  black  and  in  color  with  the  freest  hand.  The  purpose  has 
been  attained.  He  has  succeeded  in  constructing  a  popular  work  on  the  phenomena  of 
vegetation  which  is  practically  without  any  rival.  The  German  edition  has  been  accepted 
from  the  first  as  a  useful  treatise  for  the  instruction  of  the  public  ;  in  fact,  some  of  its  illus- 
trations have  been  taken  bodily  from  the  volumes  by  museum  curators,  to  enrich  exhibi- 
tion cases  designed  for  the  people.  With  two  exceptions,  the  full-page  colored  plates 
leave  little  to  be  desired,  and  might  well  find  a  place  in  every  public  museum  in  which 
botany  has  a  share.  Most  of  the  minor  engravings  are  unexceptionable.  They  are_  clear, 
and  almost  wholly  free  from  distracting  details  which  render  worthless  so  many  illustra- 
tions in  popular  works  on  natural  history.  Professor  Kerner's  style  in  German  is  seldom 
obscure — it  is  what  one  might  fairly  call  easy  reading;  but  it  is  no  disparagement  to  him 
and  his  style  tostate  thatthe  translation  is  clearer  than  the  original  throughout.  .  .  In  the 
first  two  issues  the  author  was  engaged  chiefly  with  the  study  of  the  structure  of  the  plant, 
and  its  adaptation  to  its  surroundings.  In  this  concluding  volume  he  considers  the  plant 
from  the  point  of  view  of  its  relation  toothers.  Therefore  he  begins  with  a  full  and  ab- 
sorbingly interesting  account  of  reproduction  in  the  vegetable  kingdom,  and  then  passes  to 
an  examination  of  species.  .  .  With  this  book,  there  is  no  excuse  for  even  busy  people  to 
be  ignorant  of  how  the  other  half,  the  plant-half,  lives." 

Botanical  Gazette  :  "  Kerner's  work  in  English  will  do  much  toward  bringing  modern 
botany  before  the  intelligent  public.  We  need  more  of  this  kind  of  teaching  that  will 
bring  those  not  professionally  interested  in  botany  to  some  realization  of  its  scope  and 
great  interest." 

Professor  J.  E.  Humphrey  :  "  It  ought  to  sell  largely  hereto  colleges  and  public  libra* 
ries,  as  well  as  to  individuals,  and  lean  heartily  commend  it." 

John  M.  Macfarlane,  Professorin  University  of  Pennsylvania  :  "  It  is  a  work  that 
deserves  a  wide  circulation." 

Professor  John  M.  Coulter  in  The  Dial ':  "  It  is  such  books  as  this  that  will  bring 
botany  fairly  before  the  public  as  a  subject  of  absorbing  interest  ;  that  will  illuminate  the 
botanical  lecture-room  ;  that  will  convert  the  Gradgrind  of  our  modern  laboratory  into  a 
student  of  nature." 

New  York  Times  :  "  A  magnificent  work,  with  Its  careful  text  and  superb  illustrations 
The  whole  processof  plant  life  is  explained,  and  all  the  wonders  of  it." 

The  Critic  :  "  In  wonderfully  accurate  but  easily  comprehended  descriptions,  it  open 
to  the  ordinary  reader  the  results  of  botanical  research  down  to  the  present  time." 

The  Outlook  :  ".  .  .  For  the  first  time  we  have  in  the  English  language  a  great  work 
upon  the  living  plant,  profound,  in  a  sense  exhaustive,  thoroughly  reliable,  but  in  language 
simple  and  beautiful  enough  to  attract  a  child.  .  .  The  plates  are  most  of  them  of  unusual 
beauty.  Author,  translator,  illustrators,  publishers,  have  united  to  make  the  work  a 
success." 

HENRY  HOLT  &  CO.,  29  West  23d  Street,  New  York. 


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